Clay Surfaces: Fundamentals and Applications

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INTERFACE SCIENCE AND TECHNOLOGY - VOLUME 1

Clay Surfaces Fundamentals and Applications Edited by

Fernando Wypych and Kestur Gundappa Satyanarayana* Universidade Federal do Paraná Curitiba, Brazil * Visiting Professor

2004

ELSEVIER ACADEMIC PRESS

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Clay Surfaces Fundamentals and Applications

INTERFACE SCIENCE AND TECHNOLOGY Series Editor: ARTHUR HUBBARD In this series: Vol. 1: Clay Surfaces: Fundamentals and Applications Edited by F. Wypych and K.G. Satyanarayana

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First edition 2004

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ISBN: 0-12-088439-9 ISSN (series): 1573-4285 The paper used in this publication meets the requirements of ANSI/NISO Z39.48-1992 (Permanence of Paper). Printed in The Netherlands.

Preface The purpose of this book is to introduce the reader to the fascinating world of the chemistry and physics of the clay mineral surfaces/interfaces and to demonstrate that, in spite of lot of information that has been accumulated over the decades, a vast field still exists to be explored. This book will not only be useful for the specialists directly involved with clay science and related areas but also as a text or supplementary reading for undergraduate and graduate students. When a crystalline or amorphous solid is brought in contact with a liquid, a gas or another solid, the possible reactions and interactions take place via their surfaces. The study of the chemistry and physics of the surfaces plays a fundamental role in an attempt to understand those phenomena in a general sense. Apart from the structure, the composition is usually very different from the bulk of the crystal, mainly due to the sorption of ions and/or neutral molecules on the surface of the crystal. Another fundamental point in the reactivity of materials is related to the defects on the crystals' surface and surface reconstruction, the first originating in the preparation procedure and the second in a mechanism of compensation of the unsaturated bonds when a surface is generated by growth or fracture. All these phenomena are of fundamental importance for several branches of science. However, many of them remain slightly misunderstood, mainly based on the fact that only in the last few decades has appropriate equipment been developed and/or improved for this kind of studies. Usually, the information generated by the atoms of the surface is infinitely weaker than that by the atoms of the bulk, which makes it difficult or hinders the studies of the interfaces of the crystals. Considering the clay minerals and the possible interactions with multi components of the soil (that can be obtained by the degradation of organic matter of vegetable or animal origin, deliberately introduced through agricultural activities or via the decomposition cycle of rocks and minerals, apart from the activity of microorganisms), it is easy to imagine the extreme complexity of the phenomena on the surfaces involved in those systems. In this book, we have made an attempt to describe the clay surface/interfaces phenomena with the main purpose to try to understand the interrelations among the minerals, living organisms and human activities on our planet. The above aspects of clays are presented in this volume. This book contains 17 chapters, which have been classified under two main headings, viz., Natural clays: theoretical aspects and applications and Synthetic clays: Synthesis and applications with an introductory chapter by Prof. Fernando Wypych. This chapter defines layered materials and deals with derivatives of simple hydroxides, hydroxysalts, synthetic clays (Layered Double Hydroxide (LDH)) and natural clays. Herein various reactions such as adsorption, solvation, grafting, exfoliation, thermal, mechanochemical modifications and intercalation have been explained in respect of the mentioned derivatives. The Section under Natural clays: theoretical aspects and applications includes 10 chapters. Chapters 2—5 deal with the theoretical aspects of clay surfaces. Knowing that the electrokinetic properties of a substance are used to explain the mechanism of dispersion and agglomeration in a liquid phase and identification of the adsorption mechanisms of ions or molecules at a solid-liquid interface, the second chapter presents an overview of the electrokinetic properties of clay minerals, elucidating the electrokinetic behavior of clay surfaces and the mechanism of particle-particle interactions in aqueous systems. On the other hand, considering various forces that operate at the interface where the two surfaces interact and the importance of such

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studies on colloidal materials such as clays, the third chapter presents the recent thermodynamic study of clay surfaces, both swelling and non-swelling type, and relates it to the data obtained by measurements of surface tension, immersion enthalpy and sorption. The properties of any material near surfaces or interfaces are different to the properties of the same material in the bulk because of the different coordination environment of surface atoms than that of those in the bulk. This special character of surfaces has some thermodynamic implications, especially in terms of interaction with molecules of other substances in close contact. This aspect is described in the fourth chapter. Taking into account the vital role played by the adsorption/desorption processes in determining the efficacy and environmental behavior of pollutants or nutrients in soil, and most of the adsorption in natural systems occurs in the dispersed phase that consists predominantly of inorganic colloids such as clays, the fifth chapter presents a model for these processes and also other mechanisms and reactions for predicting the behavior of reactive solutes in complex systems in both single and multi-component systems with several interacting species. Characterization of clay surfaces, including modification of kaolinite surfaces, intercalation by mechanochemical method as studied by Raman and IR spectroscopic techniques are described in Chapter 6. It is well known that NMR studies provide structural and dynamic information at a local and sometimes mesoscopic scale. Hence a brief outline of the relevant NMR theory, starting with the solid state and simplification brought about by the mobility increase in the liquid state and studies of natural, synthetic, or modified clay suspensions along with some NMR results on solid systems are summarized in Chapter 7. Some of the important applications of clays include ceramics, paper, paint, plastics, drilling fluids, foundry bondants, chemical carriers, liquid barriers, decolorization, and catalysis. Further, pesticides, toxic organic chemicals, greenhouse gases, heavy metals, undesirable inorganic substances are as much targeted molecules to be controlled for the preservation of flora and fauna and safety of the earth. Accordingly, Chapters 8-10 deal with various application areas of natural clays mentioned, including removal of a wide range of contaminants from industrial effluents or wastewater by anion exchange and adsorption processes or catalytic remediation, using LDHs, modified LDHs or calcined LDHs, pharmaceutical & cosmetics, environmental remediation, waste treatment including nuclear wastes, heavy and toxic metals removal, biological applications, etc. Removal of metals, particularly of chromium in wastewater, by natural and modified clays is described in Chapter 10. The catalytic and adsorption properties of modified clays is given in Chapter 11 covering some general aspects of structure of clays, cation exchange capacity and swelling capacity of clays, clay-organic cation- interaction and acid-activated organo clays with a view to understand better the relationship between the clays and their catalytic uses. The section on Synthetic clays: Synthesis and applications has six chapters. Over the last two decades, interest has been growing in the availability for the intercalation of various organic anions having flexible or rigid molecular frameworks into LDH, not only from the scientific but also from industrial viewpoints. This includes synthetic anionic clay materials. Accordingly, the state of the art for the materials composed of the assembly between Layered Double Hydroxides (LDH) and Polymer are described in these chapters and new trends in term of applications are identified. It is pointed out here that the polymer/LDH assembly, not yet extensively studied, constitutes an appealing new class of nanocomposites in numerous topical applications. Chapter 12 is related to the synthetic methods of layered double hydroxides and mechanism of decomposition studied by in-situ techniques. Chapter 13 concentrates

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on the newest research progress in the compositions, structures, synthesis methods and photocatalytic and oxidation-catalytic functions of the POM-LDHs complexes. The preparation and applications of a new emerging class of bio-clay hybrids are presented and discussed in Chapter 14. A wide range of contaminants can be removed from industrial effluents or wastewater by anion exchange and adsorption processes or catalytic remediation, using LDHs, modified LDHs or calcined LDHs. With this background, Chapter 15 describes the investigations on the potential uses of LDHs for decontamination of environmental sites or prevention of pollutant dispersion in Nature. Chapter 16, dealing with Layered Double Hydroxide/Polymer nanocomposites, gives a description of LDH materials including their natural occurrence, chemical composition and aspects of the stacking sequence, the building of inorganic-organic assemblies including synthetic pathways for the LDH/polymer assembly along with the layer charge density and the colloidal and exfoliation properties. Chapter 17 describes a broad spectrum of catalytic applications of layered double hydroxides and possibilities of designing catalysts tailored for specific reactions and/or substrates. With the accumulation of the above knowledge, we expect to understand in the future the mechanisms of the interactions of the soils when in contact with the wastes originated from human activities. We also hope to understand what action should be taken in the case of an accidental spilling of chemicals, how to improve the production of food without affecting the ecological balance significantly, how to increase the fixation of carbon in the soil to increase the production of cereal grains and reduce carbon dioxide emissions to the atmosphere, and safely predict the effects of a certain activity on the soil, waters and atmosphere of our fragile planet. We hope this book will be good reading material for all concerned with all aspects of mother Earth. It is not an exaggeration to say that this volume represents the expertise, time, efforts and opinions of 27 specialists in their subject areas. The statements, views and recommendations made by each of the contributors are their own and should be considered to be made with appropriate responsibility. The editors express their sincere and heartfelt gratitude to all these contributors for their devotion and providing the chapters, for carrying out alterations/modifications as and when called for to suit the overall uniformity of this volume. They also express their gratitude to Louise Morris and Derek Coleman of Elsevier for their guidance and help from time to time and last but not the least, to Dr. Arthur Hubbard for the invitation, encouragement and the first table of contents of the book. Fernando Wypych Kestur Gundappa Satyanarayana

Contributors ALBERTO LOPEZ-GALINDO *' and CESAR VISERAS 2 1 Instituto Andaluz de Ciencias de la Tierra (CSIC-UGR). Facultad de Ciencias, Campus Fuentenueva. 18071 - Granada - SPAIN. 2 Departamento de Farmacia y Tecnologia Farmaceutica, Facultad de Farmacia. Universidad de Granada. 18071, Granada - SPAIN. E-mail: [email protected] * E-mail: [email protected] ALEXANDER MORONTA Centro de Superficies y Catalisis, Facultad de Ingenieria, Universidad del Zulia, Maracaibo 4003-A - VENEZUELA. E-mail: [email protected] B.S. JAI PRAKASH Department of Chemistry, Bangalore Institute of Technology, k.r. road, Bangalore 560 004, INDIA. E-mail: [email protected] CHANGWEN HU '* and DANFENG LI 2 1 Department of Chemistry, Beijing Institute of Technology, Beijing, P.R. CHINA, 100081 2 Institute of Polyoxometalate Chemistry, Northeast Normal University, Changchun, P.R. CHINA, 130024 * E-mail: [email protected] CLAUDE FORANO Laboratoire des Materiaux Inorganiques, UMRCNRS 6002, Universite Blaise Pascal, 63177, Aubiere Cedex - FRANCE E-mail: [email protected] CRISTINA VOLZONE Centro de Tecnologia de Recursos Minerales y Ceramica - CETMIC (CIC-CONICET-UNLP) - CC 49, Cno. Centenario y 506, (1897) M.B. Gonnet Provincia de Buenos Aires - ARGENTINA E-mail: [email protected] / [email protected] EIJI KANEZAKI Department of Chemical Science and Technology, Faculty of Engineering, The University of Tokushima, 2-1 Minamijosanjima, Tokushima 770-8506 - JAPAN E-mail: [email protected] FABRICE LEROUX * and JEAN-PIERRE BESSE Laboratoire des Materiaux Inorganiques, UMR 6002-CNRS, Universite Blaise Pascal, 24 av. des Landais, 63177 Aubiere cedex, FRANCE. * E-mail: [email protected] FERNANDO WYPYCH Centro de Pesquisas em Quimica Aplicada - CEPESQ. Universidade Federal do Parana - UFPR - Departamento de Quimica CP 19081 - Centro Politecnico - 81531-990 - Curitiba - PR - BRAZIL. E-mail: [email protected]

Contributors

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GIORA RYTWO School of Environmental Sciences and Technology - Tel Hai Academic College, Upper Galilee 12210, ISRAEL. MIGAL, Galilee Technological Center, Kiryat Shmona, ISRAEL. E-mail: [email protected] JEAN GRANDJEAN Universite de Liege - Institut de Chimie B6a - COSM Sart Tilman - B-4000 Liege BELGIUM E-mail.: [email protected] JEAN MARC DOUILLARD* and FABRICE SALLES University of Sciences, L.A.M.M.I., CC015, Universite Montpellier 2. Sciences et Techniques du Languedoc. PL Eugene Bataillon, Montpellier Cedex 05 - FRANCE * E-mail: [email protected] JIN-HO CHOY* and MAN PARK National Nanohybrid Materials Laboratory (NNML) -School of Chemistry and Molecular Engineering - Seoul National University, Seoul, 151-747 - KOREA * E-mail: [email protected] JUAN CORNEJO*, RAFAEL CELIS, LUCIA COX and M. CARMEN HERMOSIN Instituto de Recursos Naturales y Agrobiologia de Sevilla, CSIC. P.O. Box 1052. 41080 Sevilla - SPAIN. * E-mail: [email protected] MEHMET SABRI CELIK Istanbul Technical University - Mining Engineering Dept, Mineral Processing Section Ayazaga 34469 Istanbul - TURKEY E-mail: [email protected] RAY L. FROST * ] and JANOS KRISTOF 2 1 Inorganic Materials Research Program, Queensland University of Technology, GPO Box 2434, Brisbane Queensland 4001 - AUSTRALIA. 2 Department of Analytical Chemistry, University of Veszprem, H8201 Veszprem, PO Box 158-HUNGARY. * E-mail: [email protected] SIMONE ALBERTAZZI, FRANCESCO BASILE and ANGELO VACCARI* Dipartimento di Chimica Industriale e dei Materiali, Alma Mater Studiorum - Universita di Bologna, INSTM-UdR di Bologna, Viale del Risorgimento 4, 40136 Bologna ITALY. * E-mail: [email protected]

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Contents Preface List of contributors Introduction 1 - Chemical Modification of Clay Surfaces Fernando Wypych

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Section I - Natural clays: theoretical aspects and applications 2 - Electrokinetic Behavior of Clay Surfaces 57 Mehmet Sabri Qelik 3 - Surface Thermodynamics of Clays 90 B.S. Jai Prakash 4 - Phenomenology of Water Adsorption at Clay Surfaces 118 Jean Marc Douillard and Fabrice Salles 5 - A Worksheet Model for Adsorption/desorption of Ions on Clay Surfaces 153 Giora Rytwo 6 - Raman and Infrared Spectroscopic Studies of kaoUnite Surfaces Modified by Intercalation 184 Ray L. Frost andJanos Kristof I - Nuclear Magnetic Resonance Spectroscopy of Molecules and Ions at Clay Surfaces Jean Grandjean 216 8 - Pesticide-clay interactions and formulations 247 Juan Cornejo, Rafael Celis, Lucia Cox andM.Carmen Hermosin 9 - Pharmaceutical and Cosmetic Applications of Clays 267 Alberto Lopez- Galindo and Cesar Viseras 10 - Removal of Metals by Natural and Modified Clays 290 Cristina Volzone I1 - Catalytic and Adsorption Properties of Modified Clay Surfaces 321 Alexander Moronta Section II - Synthetic clays: Synthesis and applications 12 - Preparation of Layered Double Hydroxides Eiji Kanezaki 13 - Polyoxometalate Complexes of Layered Double Hydroxides Changwen Hu and Danfeng Li 14 - Cationic and anionic clays for biological applications Jin-Ho Choy and Man Park 15 - Environmental Remediation Involving Layered Double Hydroxides Claude Forano 16 - Layered double hydroxide/polymer nanocomposites Fabrice Leroux and Jean-Pierre Besse 17 - Catalytic properties of Hydrotalcite-type Anionic Clays Simone Albertazzi, Francesco Basile andAngelo Vaccari Index

345 374 403 425 459 496

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CHEMICAL MODIFICATION OF CLAY SURFACES FERNANDO WYPYCH Centra de Pesquisas em Quimica Aplicada - CEPESQ. Universidade Federal do Parana - UFPR - Departamento de Quimica CP 19081 - Centra Politecnico - 81531 -990 - Curitiba - PR - BRAZIL. E-mail: [email protected]

Clay Surfaces: Fundamentals and Applications F. Wypych and K. G. Satyanarayana (editors) © 2004 Elsevier Ltd. All rights reserved.

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F.Wypych

1 - Introduction Layered materials belong to a special class of compounds in which the crystals are built by the stacking of "two-dimensional" units known as layers that are bond to each other through weak forces [1,2]. Intercalation reactions occur by the topotactic insertion of mobile guest species (neutral molecules, anhydrous or solvated ions) into the accessible crystallographic defined vacant sites located between the layers (interlayer spacings) in the layered host structure. In this intercalation compounds, strong covalent bonds occur in the layers and weak interactions, between host lattice and guest species or co-intercalated solvents. Ionic and solvent exchange reactions are related to the replacement of solvated guest species (cations and anions) located into the interlayer spacings. In this case, only the solvent, the cations or the solvated cation can be replaced, depending on the reaction conditions. Grafting reactions occur by establishing covalent bonds between the reactive groups of the layer and an adequate reactant molecule, which ensures higher chemical, structural and thermal stability for the compound. These reactions can be restricted to the crystal surface (the basal spacing remains unchanged) or layer surface (in this case an interlayer expansion occurs). These compounds can be collectively defined as hybrid materials, or more specifically, surface-modified inorganic layered materials. One of the simplest families in the compounds with layered structures belongs to the alkaline earth or transition metal hydroxides. Common examples involve the structure of brucite (Mg(OH)2) [3,4], gibbsite, bayerite, nordstrandite and doyleite (polymorphic modifications of A1(OH)3), among others. Brucite has the most representative structure that is adopted by several simple hydroxides. In brucite structure [3], atoms of magnesium are octahedrically bonded to six hydroxyl groups, being these units linked to each other through the edges, producing charge neutral two-dimensional layers. Both sides of the layers are covered in hydroxyl groups, being potentially susceptible to be grafted by adequate organic/inorganic molecules, producing grafted or pillared derivatives. Brucite layers are the fundamental building units of a great variety of geologically important hydrous phyllosilicates, including micas, clays and layered double hydroxides (LDH). Figure l(a) displays the structure of brucite, Figure l(b), the schematic representation of a single layer and Figure l(c), the layer top view

Figure 1 - (a) Structure of brucite, (b) the schematic representation of a single layer and (c), the layer top view [5].

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Another class of compounds with a little more complex structures, is the layered hydroxysalts, being typical structures of hydrozincite (Zn5(OH)6(CO3)2) [6], zinc hydroxide nitrate (Zn5(OH)g(NO3)2.2H2O) [7] or copper (II) hydroxide acetate (Cu2(OH)3CH3COOH.H2O) [8]. In these compounds, part of the hydroxyl anions is exchanged by other anions. Water molecules may also be incorporated in the interlayer region for stability. The typical formulation can be written as: Mx+(OH)x_yBn"y/n.zH2O (Mx+ = metal cation and Bn"= anion). Normally, the substituting ion neither needs to have similar hydroxyl ion chemical characteristics nor the same size. In this case, an interlayer expansion is expected in order to accommodate the solvated ion. In hydroxysalts, the process of grafting of specific molecules to the hydroxylated side of the layer (as in the case of the simple hydroxides) and processes of anionic exchange, are also possible. The synthesis of the copper(II) hydroxide acetate is presented in [Eq. 1]. 2 Cu+2 + 3 OH" + CH3COOH" -» Cu2(OH)3CH3eOO.H2O

(Eq. 1)

Controlling the conditions, copper(II) hydroxide or copper(II) hydroxide acetate can be precipitated. Figure 2(a) presents the structure of the zinc hydroxide nitrate, Figure 2(b), the layer top view and Figure l(c), the schematic representation of a single layer.

Figure 2 - (a) Structure of the zinc hydroxide nitrate, (b) the layer top view and (c), the schematic representation of a single layer [5,8]. Other most typical examples of minerals belonging to clay minerals class are hydrotalcite (Mg6Al2(OH)16CO3.4H2O) [9] and pyroaurite (Mg6Fe2(OH)16CO3.4.5H2O) [10], which have the layered double hydroxides (LDH) structure, also known as anionic clays [11-14]. In these materials of variable compositions and mainly of synthetic origin, the layered structure is intimately related to the structure of brucite, where hydroxyl ions are hexagonally close packed and magnesium (or aluminum) ions occupy octahedral sites. Both sides of the layer are covered in hydroxyl groups. LDHs have a generic formulation [M+21.xM+3x(OH)2]x+(Am")x/m.nH2O, where +3 M and M+2 represent metal ions in octahedral sites and Am" represents the interlayer

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anion. In these compounds, the trivalent metal substitutes isomorphically a metal in the divalent state of oxidation of the hydroxide structure, generating charges that are compensated by the intercalation of hydrated anions (Eq. 2). LDHs can also be prepared with a single metal in two different oxidation states (Ex.: [Fe+21.xFe+3x(OH)2]x+(Am" )x/m.nH2O) producing the "green rusts". 0,67Mg+2 + 0,33 Al+3 + 2OH" + CO3"2 ^

-> Mg0 67Al0 33(OH)2(C03)o i65.nH2O

(Eq. 2) When another interlayer ion is needed instead of carbonate, the reactions should be performed under protective gas and using boiled and degassed solutions. In the case of using alkaline metal hydroxides as precipitation agent, the excess carbonate should be removed. The salts should preferably contain the anions to be intercalated or an excess of the anion should also be added to the reaction media to compensate the undesirable species (Eq. 2). These hydrated anions are free to move, as they are located in the interlayer spacings. They are exchangeable, being attributed to these compounds the characteristic anionic exchange capacity (AEC). The anions class to be exchanged is wide, going from organic [15], inorganic [16] and even complexes [14,17], with varied oxidation states. Figure 3 (a) presents the structure of the layered double hydroxides and Figure 3(b), the schematic representation of a single layer.

Figure 3 — (a) Structure of the layered double hydroxides and (b) the schematic representation of a single layer [5]. Another class of similar compounds and of natural or synthetic origin involves the clay minerals of the phyllosilicate group [18,19]. In these compounds, generically two structural units are normally involved in the construction of their crystalline lattices. The first is constituted of octahedrons of oxygen atoms and hydroxyl groups at the corners with an aluminum atom in the center or of the gibbsite type. The other is constituted of tetrahedrons with oxygen atoms at the corners and with a silicon atom in the center or of the silica (tridymite) type. If we consider the connection of those isolated units along the plane (octahedrons or tetrahedrons) we will have superposed atomic planes that constitute a sheet. The superposition of the sheets build the layers, the ones which were stacked and separated by the interlayer spacings constitute the structural units [19]. Clay minerals are essentially hydrated crystalline aluminosilicates


5

containing several main elements as iron, alkaline and alkaline earth metals. The crystalline lattices are classified as three-dimensional, two-dimensional (layered) or mono-dimensional (fibrous structures) [18,19]. The hydrated aluminosilicates can be neutral or ionic exchangers and the groups treated in this work involve basically the phyllosilicates of the smectite group (type 1:2- derived from the idealized formula of pyrophyllite (Al2Si4Oi0(OH)2 dioctahedral = only 2/3 of the octahedral sites are occupied) and kaolin group (type 1:1). More specifically montmorillonite (typical formulation: 0.33M+(Al167Mgo33)Si4Oio (OH)2) and kaolinite (ideal formulation: Al2Si2O5(OH)4). The composition of the montmorillonites is variable and depends on its own genesis, which is attributed to the characteristic of different cationic exchange capacity. However, in general, it is very superior to that of kaolinite. The low cationic exchange capacity of kaolinite is justified by the low isomorphic structural substitution (usually Al+3 for Fe+2 or Fe+3). In other words, its composition can be considered fixed. The nomenclature 1:2 and 1:1 refers to the construction of the layers: in the case of the smectites, the layer is built by two sheets of silicon atoms tetrahedrically bonded to oxygen atoms that involve a sheet of aluminum atoms octahedrically bonded to oxygen atoms and hydroxyl groups. Both sides of the layer expose planes of oxygen atoms (siloxane surface), having distorted hexagonal cavity formed by six-corner sharing silica tetrahedron (Fig. 4(c)). The low interaction between the layers provides the material easy cleavage and consequently, anisotropic properties. In nature, aluminum atoms are isomorphically replaced by atoms of lower oxidation states, thus producing negatively charged layers, in higher or lower degree. The negative layers are charge balanced through the process of intercalation of hydrated cations in interlayer vacant sites, which is being attributed to these clay minerals for their high cationic exchange capacity characteristics (measured in meq/lOOg). Figure 4(a) presents the structure of montmorillonite, Figure 4(b), the schematic representation of a single layer and Figure 4(c), the silicate sheet top view.

Figure 4 — (a) Structure of the montmorillonite, (b) the schematic representation of a single layer and (c), the silicate sheet top view [5].

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F.Wypych

In kaolinite, the layers are built of only a sheet of silicon, tetrahedrically bonded to oxygen atoms and a sheet of aluminum octahedrically bonded to oxygen atoms and hydroxyl groups. As a consequence, the aluminum side of the layer is covered in hydroxyl groups (aluminol) and one third of the octahedrons are vacant in order to maintain a neutral sheet (dioctahedral) (Fig. 5(c)). The silicon side is covered in oxygen atoms (siloxane surface) and the same distorted hexagonal cavity present in the 1:2 group, is observed (Fig. 4(c)). These characteristics turn kaolinite structure into an unique matrix in which the confined molecule will be subjected to an asymmetric chemical environment, producing materials with interesting properties suitable for applications in the non-linear optics as recently reported [20,21]. Adjacent layers of kaolinite are linked to each other by hydrogen bonds, which involve the aluminol and siloxane groups. A high cohesion between the layers results from this type of bonding that hinders intercalation, grafting and exfoliation reactions. Although kaolinite has a fixed composition, depending on its genesis, different degrees of crystallinities can be obtained (low, medium and high) with decisive factors in its chemical reactivity. Figure 5(a) presents the kaolinite structure, Figure 5(b), the schematic representation of a single layer and Figure 5(c), the hydroxide sheet top view.

Figure 5 — (a) Structure of kaolinite, (b) the schematic representation of a single layer and (c), the hydroxide sheet top view [5]. The surface reactions and the corresponding surface complexes play a fundamental role in the behavior and properties of the materials with layered structures. A specific example can be found in the acid catalytic properties of the clay minerals, which can promote processes of polymerization of organic matter residues of the animal/vegetable origin, contributing to the humification processes [22]. Being clay minerals the main components of the colloidal fraction of soils, the above mechanism promotes the formation of organo-mineral compounds, which have a large capacity of ionic exchange and the ability of complexing metals and nutrients. Thus, this system becomes important for the agricultural activities, mainly in tropical countries [23]. Another interesting characteristic of the clays in soils is the possible modification (or destruction) of pesticides through oxidation/reduction reactions, being isomorphic substituting iron the key factor for these mechanisms.


7

One of the most important examples of a layered material application involves the structure of molybdenum disulfide doped with metals (normally Ni or Co), as an hydrotreatment catalyst. Although these materials have been used for many decades, one of the main doubts have been persisting for years is related to the positioning of the dopant in the structure of the sulfide. Only recently, results have demonstrated that the active phase in this catalyst is generated when the dopant is bonded to the layered crystal edges producing a phase of the type Ni(or Co)-Mo-S [24,25]. As this phase is preferably generated on the edges of the crystals, it is difficult to characterize and still lead to divergent views among the researchers. Being predominantly surface processes, the understanding of these phenomena in compounds with layered structures is of fundamental importance for several branches of science, mainly those devoted to soils science; environment pollution control, special materials, catalysts design are among others. Due to the similar structural characteristics of the five groups of layered compounds described above, the intercalation and grafting processes are similar, differing basically in the particularities of the layers' surfaces in each system and accessibility of the reactants to the interlayer spacings. We start in the following sections, the description of generic and specific reactions involved for each group of layered materials mentioned above. 2 - Simple hydroxides derivatives The reactions with hydroxides are quite limited, mainly based on the fact that a great difficulty exists in penetrating in the interlayer spacings. Although the layers are linked to each other through weak forces, only small molecules can potentially be inserted and grafted to the layers. The five potentially possible reactions in simple hydroxides are related with the process of surface adsorption (basal and edge planes), substitution of the hydroxyl groups for other anions transforming the materials in hydroxysalts and processes of grafting of the interlayer (and surface) hydroxyl groups as well as the processes of interlayer hydroxyl groups' solvation and intralayer metal oxidation/reduction reactions. Only a few examples are reported in the literature about the reactions described above [26-29]. In the case of gibbsite, which has octahedral vacant sites in the layer (Figure 5(c)), reactions with lithium salts producing compounds similar to the layered double hydroxides were also reported [30,31]. This reaction is very rare as both cation and anion are simultaneously intercalated into the layered structure, being the layer octahedral vacant site directly involved in the process. 2.1 - Surface adsorption Based on the fact that the unit cell is electrically neutral, the simplest reactions for the hydroxides involve the processes of reversible surface adsorption, through neutral or charged organic (org.) and inorganic (inorg.) molecules (Eq. 3) Mg(OH)2 + x inorg/org Mg(OH)2(inorg/org)x

(Eq. 3)

As the adsorbed species are located mainly in the unsaturated bonds present on the edges and basal defects of the crystallites as single (or multiple) layers, and does not develop a three dimensional structure, the structure of the original hydroxide is retained. Depending on the type and energy of interaction involved, chemical or physical processes can be considered and the surface complexes can acquire important characteristics depending on the involved constituents. When a single layer of a specific

8

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dye can be adsorbed preferably on the edges or basal surface of the crystals, a special procedure for the surface area determination can be developed. 2.2 - Exchange of hydroxyl ions Potentially the hydroxyl anions bonded to the layer can be exchanged for other anions of the same charge, where non-stoichiometric compounds similar to the hydroxysalts can be produced (Eq. 4). If the exchanged anions are of different charges, compositions similar to the layered double hydroxides (although being cationic exchangers) can also potentially be obtained. (Eq.5) Mg(OH)2 + x CH3COO" -> Mg(OH)2_x(CH3COO)x + x OH" (Eq. 4) Mg(OH)2 + x CO3-2 H> [Mg(OH)2.x(CO3)x]-1 + x OH" (Eq. 5) These types of compounds are obtained easily when a nickel salt is precipitated with sodium hydroxide. If the conditions are not very well established, besides Ni(OH)2 (a phase) the non-stoichiometric (3 phases can also be observed (Ni(OH)2.x(An" X/n.yH2O; An"= NO 3 \ Cl", SO4"2, CO3"2) [32]. Obviously, if the anion charge is larger than 1, the layers are negatively charged and cations must be intercalated between the layers (See Section 3). 2.3 - Interlayer (or surface) grafting The process occurs through the grafting of the layer surfaces with organic or inorganic molecules, in the same way as in glass, silica or other materials with modified surfaces, used mainly in chromatographic purposes (Eq. 6). The compounds display stronger interactions (through covalent bonds) which ensure higher chemical, thermal and structural stability. When both sides of the layer can be grafted and an appropriate molecule is used (with two reactive terminal functions), organic pillared compounds can be prepared. (Eq. 7). Mg(OH)2 + x CH3-OH -> Mg(OH)2.x(O-CH3)x + x H2O

(Eq. 6)

Mg(OH)2 + x/2 HO-(CH2)2-OH -> Mg(OH)2.x(O-(CH2)2-O)x/2 + x H2O (Eq. 7) In this case it is important to emphasize that only small molecules of appropriate acidity that can have access to the interlayer spacings and consequently, to the hydroxyl ions, can be grafted. The resultant non-stoichiometric compounds have organic functions in an inorganic layer matrix. This type of compound has unique characteristics since the grafting process will preserve the structure of the layered compound, being obtained only through this kind of procedure. By controlling the size, the distribution, and the nature of the organic pillars, interesting materials with special properties can be engineered and synthesized. 2.3.1 - Grafting of ethylene glycol and glycerol into brucite [33] In this case the "esterification" or "acid/base" reaction with ethylene glycol is presented (Eq. 7). The reaction concentrates on the insertion of ethylene glycol molecules between the layers and the reaction with the interlayer hydroxyl groups. In


9

the case of the grafting reaction we should imagine that similar processes could happen in unsaturated bonds on the edges or basal surface defects of the crystals. The X-ray powder diffraction patterns of brucite and the reacted composites are shown in Figure 6. Brucite has a basal spacing of 4.78A, in perfect agreement with the literature value (Fig. 6(a)) [3]. After the reaction with ethylene glycol, a new phase was obtained with a basal spacing of 8.3A (Mg-EG), as shown in Figure 6(b). Only one broad basal diffraction peak was observed, showing that this phase has low crystallinity with low stacking order. The increase of the basal spacing was of 3.5 A compared to that of the brucite. Small diffraction peaks of brucite were still observed even after reaction times of 72 hours. In the reaction of brucite with ethylene glycol for 24 hours (Fig. 6(b)), the concentration of brucite is higher. This can be observed in Figure 6(c) where the reaction time was increased to 72 hours.

Figure 6 - X-ray powder diffraction patterns (a) of brucite and (b) the ethylene glycol derivative obtained after reaction for 24 hours (Mg-EG), (c) 72 hours and (d) the glycerol derivative (Mg-GL). Powder of silicon was used as internal standard (*). [Reprinted by kind permission ofJ. Coll. Interface Sci., (253, 180, 2002)] [33]. The TG/DSC curves can be observed in Figure 7. In brucite (Fig. 7(a)), an initial mass loss of 3.1% is associated with the elimination of absorbed/adsorbed water molecules (weak endothermic peak centered at 92°C) followed by a large endothermic peak centered at 393°C, which is attributed to the dehydroxylation of the layered structure, producing MgO. The mass loss of 31.5% after water evaporation due to dehydroxylation is very close to the theoretically expected value. The final product was additionally characterized by XRD (not shown) and consists of MgO crystals with the periclase structure, as expected [34]. The Mg-EG sample (Fig. 7(b)) shows two endothermic peaks at 67 and 139°C, attributed to elimination of water molecules, which amounts to a 19.7% mass loss between room temperature and 160°C. The exothermic peaks at 204, 371 and 403°C are collectively attributed to the combustion of organic matter. The 50.9% mass loss between 160 and 1000°C leads us to the following empirical formula from the experimental data: Mg(C2H402)o,95(OH)o,o5.1,12H20 (mixture of 95% Mg(C2H4O2) and 5% of Mg(OH)2). The Mg-GL phase (Fig. 7(c)) presented a slightly different profile,

10

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where only one endothermic peak at 70°C was observed. A 9.4% mass loss was found up to 160°C, attributed to water removal. Three exothermic peaks centered at 272, 364 and 420°C, are also clearly associated to the combustion of organic material combustion. Here we find an empirical formula of Mg(C3H6O3).0,66H2O considering the 67.2% experimental mass loss above 160°C for the Mg-GL original sample.

Figure 7 - TG/DSC measurements of (a) brucite, (b) Mg-EG and (c) Mg-GL grafted phases. [Reprinted with kind permission of J. Coll. Interface ScL, (253, 180, 2002)] [33]. Figure 8 present the FTIR spectra of (a) brucite, (b) Mg-EG and (c) neat ethylene glycol. Brucite displays the typical hydroxyl stretch bands at 3698 and 3643cm"1 and an extended band centered at 3428cm"1 relative to hydroxyl stretching of water molecules in various states [35]. We note a small shoulder at 3275cm"1, which corresponds to strongly bonded water. Small bands were also observed at 1637cm"1 (surface adsorbed/co-intercalated water), 1425 and 1118cm"1. The Mg-EG sample shows a small brucite contamination


11

(band at 3698cm" ) as already detected in the X-ray powder diffraction patterns. The 3700 and 2700cm"1 region of the FTIR spectra provides interesting information about the structure of the brucite grafted derivatives. Two important bands were found within these spectral regions: the out-of-plane stretching vibrations of C-H bonds at 31002700cm"1 and the stretching vibrations of O-H groups at 3700-3100cm"1. After the covalent grafting of ethylene glycol into brucite, two C-H stretching bands of ethylene glycol, originally centered at 2946cm"1 (antisymmetric) and 2879 (symmetric) were either displaced or converted to at least four new absorption bands at 2954, 2919, 2852 and 2704cm'1.

Figure 8 - FT1R spectra of (a) brucite, (b) Mg-EG and (c) neat ethylene glycol. [Reprinted by kind permission ofJ. Coll. Interface Sci., (253, 180, 2002)] [33], This FTIR observation confirms the successful insertion of ethylene glycol into the layer structure of the host matrix, because their C-H groups are now vibrating in a distinct chemical environment. Two small bands observed at 1325cm"1 and 1359cm"1, which may suggest that oxyethylene units (O-CH2-CH2-O) are in trans conformation [36,37]. The ISOO-nOOcm"1 spectral region of the grafted material reveals also a series of peaks with low intensity, collectively attributed to CH2 twisting, wagging and scissoring vibrations, in ethylene glycol [38]. The absorption bands at 1030-1100cm"1, typically attributed to Al-O-C and CC-0 bonds in kaolinite and 1100cm"1 in barium aluminate glycolate [39], have been observed at 1040, 1076, 1109 and 113 lcm"1 for Mg-EG. Similar bands were observed in 1043, 1072 and 1081cm"1 in the compound obtained by the grafting of ethylene glycol in layered double hydroxide [40]. Rocking vibrations of the CH2 groups and C-C stretching vibrations, generally centered at 865 and 882cm"1 for ethylene glycol, were observed at 858 and 880cm"1. A similar effect over the CH2 and C-C vibrations was also observed when boehmite was used as the host matrix, where the original bands were displaced to 868 e 900cm"1 after the monodentade grafting of ethylene glycol [41]. Therefore, larger shifts to higher frequencies are expected when ethylene glycol is grafted into the host matrix under a bidentade conformation [41]. The absorption bands at 447 and 558cm"1, which can be assigned to Mg-0 lattice vibrations in brucite now,

12

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appear enlarged at 561, 608 and 655cm"1. In the case of the functionalization of brucite with glycerol (Fig. 9), the spectra are very similar mainly based on the similarity of the structures of ethylene glycol and glycerol.

Figure 9 - FTIR spectra of (a) brucite, (b) Mg-GL and (c) neat glycerol. [Reprinted by kind permission ofJ. Coll. Interface Sci., (253, 180, 2002)] [33]. 2.4 - Solvation reaction of the interlayer hydroxyl groups Potentially, after breaking the weak forces that maintain the layers together, the interlayer hydroxyl groups can be solvated with small polar molecules. In this case experimental evidences of this reaction type do not exist. However, similar results were accomplished in the process of interlayer groups' solvation of the kaolinite structure [42-46] (described in section 5.1.) (Eq. 8). Mg(OH)2 + x solv. Mg(OH)2(solv.)x

(Eq.8)

2.5 - Oxidation-reduction reactions If we consider that layered double hydroxides can be obtained with the same metal in two oxidation states, this type of compounds can be synthesized starting from the oxidation of iron(II) hydroxides (Eq. 9). These processes are potentially reversible. This reaction (or alternatively for the co-precipitation of Fe+2/Fe+3 salts in alkaline pH conditions) originates the family of the green rusts [47-50]. These rusts are very important materials in the solubility and transport of iron in soils and underground water, inactivation of pesticides and maybe in pollution control due to its oxidation/reduction potential [50]. The reduction reaction of a +3 metal can potentially generate cationic exchangers, being those processes also reversible (Eq. 10). It is important to emphasize that this kind of reaction is only possible if the metals in the two oxidation states have compatible sizes and can occupy sites of identical geometry and coordination numbers. The diameter of Fe+2 being larger than Fe+3, the replacement of Fe+2 for Fe+3 is expected ( Eq. 9). However, the opposite reaction is quite unlikely (Eq. 10). Fe(OH)2 - xe + A" « [Fe+3xFe+2,.x(OH)2]x+(A"n)x/n.yH2O

(Eq.9)


Fe(OH) 3 + xe" + B + " » [Fe +2 x Fe +3 ,. x (OH) 2 ] x -(B +n ) x/n .yH 2 O

13

(Eq. 10)

2.6 - Reaction of Gibbsite with lithium chloride In the structure of gibbsite only 2/3 of the layer octahedrons are occupied by Al+3 ions, being the structure classified as dioctahedral (Fig. 5(c)). As the third position of the aluminum ion in the layer is empty, it can be filled out with a alkaline metal cation with reduced dimensions leaving the layer positively charged, which is balanced with a hydrated anion intercalation (Eq. 11) [30,31]. 2 A1(OH)3 + LiCl -> [LiAl2(OH)6]Cl.nH2O

(Eq. 11)

In this case the intercalated ions can potentially be exchanged as described in the session 3.1 for the hydroxysalts and 4.2 for the layered double hydroxides. This reaction type is rare and, as far as we know, it has been described only for the derivatives of aluminum hydroxide with lithium salts. The size and the stereochemistry of the cavity in the gibbsite layer can explain this limitation. This kind of reaction can also potentially be applied to the kaolinite structure, where a hexagonal siloxane cavity and empty octahedral sites can be found in both sites of the building layers. The product of the reaction similar to layered double hydroxides can be obtained with well-ordered structure. Computer calculations can be performed to find the correct position of the intercalated molecules or ion. This procedure is important when good quality single crystals are not available and the powder X-ray powder diffraction patterns are of poor quality. 3 - Hydroxysalts derivatives The hydroxysalts reactions supply new alternatives, mainly for the previous expansion of the layers for ions of larger dimensions, located in the crystallographic positions of the hydroxyl ions [7,8,51-53]. The reactions can be classified as surface adsorption [similar to the case of the simple hydroxides (Eq. 3), ionic exchange reactions, processes of replacement of the hydroxyl ions (Eq. 04, 05) and probable simultaneous ionic exchange and grafting reactions (Eq. 6). Exfoliation reactions (also this kind of reaction can be potentially applied to other layered structure compounds) can be performed and new classes of materials can be obtained as the nanocomposites involving polymers. In this class of compounds, only the reactions of ionic exchange have been described in the literature. However, a large research field still remains to be explored, which may produce very interesting materials. 3.1 - Ionic and solvent exchange reactions The ionic exchange reactions take place, when the solvated interlayer ions are replaced (Eq. 12). Cu2(OH)3CH3COO.H2O + NO3" - Cu2(OH)3NO3.H2O + CH3COC>- (Eq. 12) Depending on the used solvent, not only the replacement of the cation but also of the solvent can take place (Eq. 13,14). The solvent can be any polar molecule, including polymers of synthetic or natural origin. The mechanism involved in the exchange reactions occur with the shift of the basal spacing to higher values, being regulated by the size of the ion (in anhydrous conditions) or by the solvation energy of

14

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the ion. Normally the size of the solvent is bigger than the one of the ion and determines the value of the basal spacings. Cu2(OH)3CH3COO.H2O + CH3-OH -> Cu2(OH)3CH3COO.CH3-OH + H2O (Eq. 13) Cu2(OH)3CH3COO.H2O + NO3" + CH3-OH -> Cu2(OH)3NO3.CH3-OH + CH3COO" + H2O (Eq. 14) Depending on the time and the concentration of the ions in the solution, an equilibrium can be established where cations and solvents mixtures will be present in the final composition. The ionic exchange capacity of a compound is measured in meq/lOOg of the material. 3.2 - Hydroxyl ions substitution + ionic exchange reactions Depending on the reaction conditions, not only the ionic exchange as well as reactions of hydroxyl replacement for other ions can take place simultaneously. This reaction can also include the replacement of solvents (not shown, however similar to the one described in the ionic exchange reactions) (Eq. 15). These possibilities have shown that exchange reaction can be very complex, effects that are rarely described in the specialized literature. Cu2(OH)3CH3COO + 1+y NO3" - Cu2(OH)3.y(NO3)yNO3 + yOH" + CH3COO" (Eq. 15) 3.3 - Grafting reaction [54-57] Although the grafting reaction with simple hydroxides is difficult to happen, the same kind of reactions with the hydroxysalts is very much facilitated, mainly based on the fact that the layers had already been moved away previously by the presence of the exchangeable ion (Eq. 16). The possibility that the exchangeable ion can be simultaneously replaced during the grafting reaction should also be considered (Eq. 17). Cu2(OH)3CH3COO + y CH3-OH -> Cu2(OH)3_y(CH3-O)yCH3COO + y H2O (Eq. 16) Cu2(OH)3CH3COO + 2 CH3-OH -+ Cu2(OH)2(CH3-O)CH3O + H2O + CH3COOH (Eq. 17) 3.3.1 - Grafting of copper(II) hydroxide acetate with benzoic acid [54] The hydrated copper(ll) hydroxide acetate (Cu2(OH)3CH3COO.H2O) was reacted with benzoic acid in water, under magnetic stirrer agitation at 60° C for 36 hours. Figure 10 shows the X-ray powder diffraction patterns for the described samples. Pure copper (II) hydroxide acetate (Fig. 10(a)) is clearly layered with basal plane diffraction peaks and their multiples corresponding to basal spacing of 9.3 A. The reacted material, independent from the reacted amount shows a basal spacing of 15.6A. The line width of the X-ray powder diffraction patterns shows that the crystallinity of the reacted compounds reduces as more benzoic acid is incorporated between the layers. The stoichiometric 1/0.75 ratio (Fig. 10(c)) produced the highest quality crystals.


15

Figure 10 — X-ray powder diffraction patterns (a) for copper (II) hydroxide acetate and CuOHAc reacted with benzole acid in the proportions: (b) 1/0.5; (c) 1/0.75; (d) 1/1 and (e) 1/4. *= Si internal standard. Reprinted by kind permission of [J. Coll. Interface Set, (240, 245, 2001)] [54]. Figure 11 shows the TG/DSC measurements for copper (II) hydroxide acetate (Fig. 1 l(a)) and the reacted materials ((b) 1/0.75 and (c) 1/4). In pure hydroxide acetate, two endothermic peaks observed at 128 and 188°C can be attributed to water elimination and fragmentation followed by oxidation, respectively. The 7.2% mass loss up to 142°C due to water removal and the 33% mass loss up to 1000°C leads to the Cu2(OH)3(CH3CO2).(H2O)1>03 stoichiometry. The theoretical values for the Cu2(OH)3(CH3CO2).(H2O)ii(, formula would be of 7.06% water content and 32.92%, considering CuO as the final oxidation product of this experiment. Figure ll(b) shows the TG/DSC results for the 1/0.75 stoichiometry reacted material with a more complex profile. The 0.5% moisture is eliminated up to 100°C. Three endothermic peaks are observed at 180, 208 and 220°C. These peaks are attributed to fragmentation and oxidation steps respectively. The three exothermic peaks at 275, 295 and 315°C are attributed to organic matter oxidation. At 322°C part of the material is ejected from the crucible. Considering the mass loss of 54.9% up to 322°C we obtain a stoichiometry for the reacted material of Cu2(OH)2j4(C6H5CO2)i,6 while the predicted theoretical stoichiometry, considering the reaction proportions, would be Cu2(OH)3 25(C6H5C02)o,75. Considering the % stoichiometry material, whose thermal decomposition and reaction behavior is displayed in Figure ll(c), we observe a broad endothermic peak centered at 232°C followed by three exothermic peaks at 254, 299 and 313°C respectively. The mass losses are of 0.3% up to 100°C and 65.8% from 100 to 325CC. The 2% mass gain from 325 to 1000°C is related to copper oxidation. The calculated stoichiometry of this material would be Cu2(OH)! 6(C6H5CO2)2i4, while the theoretical expected stoichiometry for this reacted material should be Cu(C6H5C02)2,oThe general findings of these thermal experiments are that the ideal organic functionalization occurs for the 1/4 proportion of reaction, where clearly an excess of benzoic acid is present while the maximum incorporation never exceeds 60%, even after

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36h of reaction time. We synthesized copper benzoate in hydrated and anhydrous form and none of these compounds could be identified in the grafted samples.

Figure 11 - TG/DSC measurements for (a) copper (II) hydroxide acetate and the reacted materials ((b) 1/0.75 and (c) 1/4). [Reprinted by kind permission of J. Coll. Interface ScL, (240, 245, 2001)] [54]. Figure 12 shows the FTIR spectra for the different reacted materials in comparison to (a) pure copper (II) hydroxide acetate and (f) pure benzoic acid. For comparison we also show FTIR spectra for the 1/4 proportion of sample (Fig. 12(e)) as well as for benzoic acid (Fig. 12(f)), obtained by pressing the material with KBr powder (circa 1% of material relative to KBr). Since the spectra for the reacted materials are very similar, we concentrate our discussion on the 1/2 proportion of reaction (Fig. 12(d)). The region between 3000 and 3600cm'1, which is attributed to hydroxyl groups' vibrations, demonstrates that water molecules are absent in the grafted materials and that hydroxyl groups are still present in the samples even after functionalization by excess benzoic acid. This interpretation is


17

in accordance with the proposed stoichiometries and partial grafting reaction. Low intensity bands observed at 3609, 3576, 3520, 3449, 3403 and 3263cm"1 attributed to water and matrix hydroxyl groups have been substituted by very well defined absorption bands at 3613, 3601 and 3586cm"1. In the grafted phase, the bands can collectively be attributed to free hydroxide vibrations. Absorption peaks at 3089, 3066, 3056, 3025 and 2932cm"1 demonstrates that the organic molecule has been added to the host due to the characteristic C-H vibrations.

Figure 12 - FTIR spectra for (a) copper (II) hydroxide acetate, (f) benzoic acid and their reacted products in the reaction proportions: (b) 1/0.75; (c) 1/1; (d) 1/2 and (e) 1/4. [Reprinted with kind permission ofJ. Coll. Interface Sci., (240, 245, 2001)] [54]. The absorption bands in the 3000-3100cm"1 region grow in intensity in correlation with benzoic acid proportion. These bands are practically absent in CuOHAc. The large benzoic acid band in this region does not contribute to the grafted samples. The IR absorption in the 1200 and 400 cm"1 region of the functionalized samples differs markedly from pure CuOHAc. In this region benzoic acid group absorption is relevant. The most important bands in the functionalized material occur at 683, 703, 871, 928, 1029, 1066 and 1310cm"1 (667, 685, 709, 936, 1006, 1026, 1073, 1101, 1127, 1182cm"1 in benzoic acid) and 651, 799, 947, 1022 and 1047cm"1 in copper (II) hydroxide acetate. The most interesting characteristic of the IR spectra in this region is the maintenance of absorption bands of carboxyl groups. Peaks observed at 1409cm"1 attributed to symmetric C=O vibrations and 1549cm"1 attributed to asymmetric C=O in CuOHAc are seen at 1404, 1429, 1551 and 1595cm"1. Since the two bands in the original compound transform to four bands after functionalization, it is not clear which type of grafting reaction occurs for the carboxylate groups to the layers [54,57]. The unidentate grafting probably dominates the process, but simple intercalation and bidentate grafting cannot be excluded [58]. The absorption band relative to the asymmetric C=O vibration is more affected by the chemical environment and to ligand fields than the symmetric band, an effect clearly confirmed here [59]. But the absorption band of the C=O symmetric vibration also shows a splitting into two new bands. The band at 1429cm"1 grows proportionally to the benzoic acid content in the

18

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reacting medium. We observe that, while the use of KBr pellets is a common procedure to prepare samples for IR analyses, that this preparation in our case has led to moisture absorption (and other unwanted reactions), masking important features of the spectra in the range from 3000 to 3500cm"1 (Fig. 12(e)). The spectral region between 2000 and 500cm"1 show only small effects besides slight dislocations and intensity shifts. Figure 13 shows the SEM micrographs of the samples starting with pure CuOHAc.

(d)

Figure 13 - Scanning electron microscopy micrographs for (a) copper (II) hydroxide acetate and CuOHAc reacted with benzoic acid in the proportions: (b) 1/0.75, (c) 1/4 and (d) 1/2 reacted for 96h. [Reprinted with kind permission ofJ. Coll. Interface Sci., (240, 245, 2001)] [54]. Bar = 2îm. We observe that the original hydroxide acetate (Fig. 13 (a)), is composed of platelets of approximately 5um diameter. Upon grafting with benzoic acid (1/0.75 proportion), the highly crystalline platelets are degraded (Fig. 13(b)), while the higher addition of benzoic acid (1/4 proportion - Fig. 13(c)) leads to a fibrous compound. For the 1/2 proportion (Fig. 13 (d)) we have also tried a longer reaction time of 96 h, and observed a complete separation of the sample into submicrometric fibers of about 5000A diameter. This longer reaction time has shown otherwise no effects on all the physicochemical characterizations. Several visual evidences in the micrographs point to the fact that the pellets are fragmented according to preferential easy directions, originating from the fibers.


19

3.4 - Exfoliation reactions Exfoliation reactions refer to the process of separation of individual layers in an appropriate solvent [60-66]. Literally, it is a process of rupture of a layered crystal in such a way that stacked single layers are removed of the crystal and taken to suspension (Eq. 18). Cu2(OH)3CH3COO -> Cu2(OH)3CH3COO (single layers)

(Eq. 18)

Potentially, the process of separation of individual layers can be used for reactions of direct functionalization since in those conditions, the bonds do not exist among the layers that hinder the access of the reactants to the interlayer spacings (Eq. 6,16,17). Similarly, this can be used in the transition metal dichalcogenides coprecipitation of polymers (charged or not) or other species to the surfaces of the layers followed by the rearrangement of the layers in a crystal [67-70]. That can lead to a range of interesting materials, from a technological point of view, including lightemitting diodes and chemical sensors (Eq. 19,20). Cu2(OH)3CH3COO(single layers) Cu2(OH)3CH3COO(polymer)x (Eq. 19)

+

x

polymer

-»

Cu2(OH)3CH3COO(single layers) + x polymer" -> Cu2(OH)3(CH3COO)!.x (polymer)x + x CH3COO" (Eq. 20) During the process of restacking of the layers, they are free to rearrange in a way to minimize the energy (being influenced by the adsorbed molecules). This can produce new phases, which are normally impossible to be prepared for the conventional intercalation reactions. The nanocomposites are singular material, in which, at least one of the phases is in the nanometer scale range. In the case of layered materials as only one dimension of the layer is in the nanometer range (perpendicular to the layer), the materials are denominated as polymer-layered crystal nanocomposites [71]. Depending on the chosen system, each component can contribute to the properties in a synergistic way, being produced materials with special properties. Although in this system the matrix is a hydroxysalt, after the intercalation process, the nanocomposite can be incorporated in a polymer. The nanocomposite can facilitate the interaction and the compatibilization of the filler in the polymer, giving the material any desired property. In that specific case, we can mention the possibility of incorporation single layers of clay minerals as fillers in the reinforcement of polymers (from synthetic or natural origin), resins or rubbers of special uses [71-76]. Alternatively, those suspensions of single layers (or the nanocomposites) can be used in the production of thin films with similar characteristics to those of the Langmuir-Blodgett films and with potential applications such as: sensors, electric devices, materials with enhanced mechanical properties, resistant to fire, etc. Special attention should be focused on the production of interstratified thin films obtained by restacking mixtures of single layers with opposite charges (or even with the same charge). An interesting example is the mineral lithiophorite (Al2LiMn+2o.5Mn+42.506(OH)6 = [Al2Li(OH)6][Mn+20.5Mn+425O6], obtained by alternate positive layers of aluminum/lithium hydroxide and negative manganese oxide layers [19] (fig. 14) (ideal formulation Al 2 Mn 3 0 9 .3H 2 0 = 2Al(OH)3.3MnO2).

20

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Figure 14 - (a) Structure of lithiophorite and (b) the schematic representation of the corresponding single layers [5]. 4 - Layered double hydroxide derivatives Layered double hydroxides react similarly to the hydroxysalts (Eq. 12-20). However, two metals with different oxidation states can produce interesting materials in several branches of chemistry, physics and engineering [11-17]. If only one metal with two different oxidation states is involved, similar reactions proposed for the simple hydroxides can be observed (Eq. 9,10). In these systems, surface adsorption, ionic exchange reactions (involving the exchange of solvents or not), replacement of hydroxyls simultaneously with ionic exchange reactions, grafting and exfoliation reactions (nanocomposites obtaining) are perfectly possible. Another reaction, still not explored, refers to the process of solvation of the hydroxyl groups, as reported in the case of kaolinite (section 5.1). 4.1 — Surface adsorption reactions Based on the fact that the unit cell is positively charged, the processes of surface adsorption of charged species involve the ionic interactions as well as the charge residues in the crystal edges, similarly to those described in the case of the simple hydroxides (Eq. 3) and hydroxysalts [51-53]. These extensively studied processes, involve the first layer of adsorption quite organized (usually constituted of hydrated anions), followed by other less organized layers until the interaction with the external aqueous solution happens. 4.2 - Exchange + grafting reactions 4.2.1 - Grafting of layered double hydroxide (LDH) with ethylene glycol [40] The layered double hydroxide with the nominal composition Zno.66Alo.34(OH)2(C03)o.i7.nH20 was dispersed in 15cm3 of ethylene glycol (Merck) or glycerol, in a 50 cm3 flat bottomed reaction flask connected to a reflux condenser. The reaction mixture was heated up to 80°C and kept under magnetic stirrer agitation for 5 days. The X-ray powder diffraction patterns of both phases, (a) Zn-Al-CO3 and (b) Zn-


21

Al-EG, are shown in Figure 14. The Zn-Al-CO3 compound showed a basal spacing of 7.78A and this data was in perfect agreement with that found in the literature. This typical basal spacing was readily attributed to the summation of the basal lattice parameters of brucite (4.78A) [3] and the diameter of the intercalated anion (CO3"2, 3A)[11]. After reaction of the Zn-Al-CO3 phase with ethylene glycol, a new pure phase was generated with a basal lattice parameter of 9.78A (Zn-Al-EG). The main reflections of these phases, labeled as "Hn" and "En" in Figure 14, were respectively associated with the sequence of basal reflections of pure Zn-Al-CO3 and Zn-Al-EG, where "n" is integral number. The "n" values could not be used to represent normal indexing because structural transitions of the original hydroxide could not be clearly excluded after intercalation. This was due to the tendency of the layered crystallites in getting organized on the surface of the glass sample-holder, therefore intensifying the basal reflections from the background (in contrast to other existing signals). The basal spacing, calculated in relation to the basal reflection of highest order, was normally 6 or 7 depending on the type of material under analysis. Variations in basal spacing were obtained by subtracting the basal spacing of the intercalated LDH derivative (9.78A) from that of both Zn-Al-CO3 (7.78A) and pure brucite (4.78A). Compared to brucite, the variation of 5.0A in the basal spacing of the Zn-Al-EG compound was consistent with the establishment of either two EG loops on the surface of each of the adjacent layers (resulting in two opposite bidentade forms in cis) or one EG bridge between two adjacent layers (bidentade form in trans linking both layers). As both opposite layer surfaces are susceptible to grafting and the grafted molecule contains two carbon atoms, grafting between two layers forming a bridge provides an equivalent basal expansion as the independent double-grafting of each layer (EG loop).

Figure 14 - X-ray powder diffraction patterns of (a) the Zn-Al-CO3 phase before and (b) after reaction with ethylene glycol. Powdered silicon, identified by an asterisk (*), was used as the internal standard. [Reprinted with kind permission ofj. Coll. Interface Sci., (227, 445, 2000)] [40]. Theoretically, the latter case {trans conformation) seems to be preferable over the former case (cis conformation), since the independent grafting of each layer would require a perfect and conserved orientation and/or spacing of the two hydroxyl groups

22

F.Wypych

involved in grafting. Of course, being a topotactic reaction, it is perfectly conceivable that both linkage types are present in the resulting grafted Zn-Al-EG derivative. When kaolinite was grafted with ethylene glycol, two distinct basal expansions of 2.2A and 3.6A were observed [77,78]. Since these variations are considerably smaller than 4.2A, which is the nominal diameter of the isolated ethylene glycol molecule [41,79], the former seems to be related to the direct grafting of ethylene glycol (Ac= 2.2A), whereas the latter would indicate its simple intercalation within the layer structure (Ac= 3.6A)[77,78]. Variations lower than the molecular diameter of ethylene glycol are justified by the interpenetration or keying of the grafted molecules into the siloxane hexagonal cavity (Fig. 4(c)). As reported in the literature, grafting of boehmite with ethylene glycol generated a single basal expansion of 5.5A [41,80,81] of the starting material. This variation was attributed to the double-layer monodentade grafting of ethylene glycol, in which the remaining unreacted hydroxyl groups contributed to the larger expansion in the basal spacing [79,80], compared to the isolated molecule (4.2A in size). The TG/DSC measurements of both (a) Zn-Al-CO3 and (b) Zn-Al-EG phases are shown in Figure 15. The temperatures indicated there were determined at every minimum and maximum of the DSC profiles. In the Zn-Al-CO3 phase (Fig. 15(a)), a large endothermic peak centered at 216°C was attributed to both the removal of water (6% in mass) and dehydroxylation of the layered structure. This peak was followed by an exothermic band centered at 814°C, which was attributed to the combustion of the residual organic matter or partial crystallization of the oxide mixture. Considering that the residue was composed of a 0.17:0.66 ratio of A12O3 and ZnO, a mass loss of 31.5% was observed until temperatures of 550°C were reached and this experimental observation was in perfect agreement with the theoretical prediction. Between 550°C and 950°C, there was an additional mass loss of 3.5% and this was presumably attributable to complementary reactions involving oxides. The empiric formulae derived from the experimental data was calculated as Zn0.66Alo.34(OH)2(C03)o i?.0.4H2O, which is in perfect correlation with the Al:Zn ratio (1:2) used for sample preparation (Table 1).

Figure J5 - TG/DSC measurements of both (a) original Zn-Al-CO} and (b) the chemically modified Zn-Al-EG phase. Reprinted by kind permission of [J. Coll. Interface Sci., (227, 445, 2000)] [40].


23

The Zn-Al-EG phase (Fig. 15(b)) presented a much more complex TG profile in which several simultaneous (therefore superimposed) processes were observed within the experimental temperature range (room temperature to 950°C). The corresponding DSC curve was characterized by a small endothermic peak at 60°C, which could be readily attributed to the removal of water from the crystal structure (nearly 6% in weight until 100°C). This peak was followed by the elimination and burning of ethylene glycol from the sample, characterized by an intense exothermic peak at 218°C. Smaller exothermic peaks at 340 and 399°C were attributed to the elimination of the remaining organic matter present within the sample. A 44% mass loss was observed between room temperature and 500°C. Between 500 and 950°C, an additional 3% mass loss was observed and this was again attributed to complementary reactions involving oxides. An exothermic peak, attributed to the partial crystallization of oxides, was also observed and centered at 752°C. Considering that the reaction procedure did not impair any changes to both Zn and Al contents, a theoretical mass loss of 43.3% could be predicted from the [Zno66Al034(OCH2CH20)](OH)o34.0.4H20 empirical formulae. Therefore, a slight deviation was observed between the theoretical and experimental data and this was probably a result of the residual ethylene glycol still present within the grafted material. Nevertheless, this small contamination was not enough to impair changes in the C:H ratio of the Zn-Al-EG phase (see Table 1 and FTIR data). Table 1 shows the chemical characterization of both Zn-Al-CO3 and Zn-Al-EG phases in relation to their wet mass for a typical moisture content of 6%. These results are in close agreement with the grafting of all hydroxyl groups present within the structure of the original LDH phase, giving the following empirical formulae, [Zno.66Alo.34(0-(CH2)2-0)](X"n)o.34/n.0.4H20, where OH" is the probable resident counterion. Table 1 - Chemical composition of both AI-Z11-CO3 and Al-Zn-EG phases, as determined by elemental analysis (C.H.N) and atomic absorption spectroscopy (Al,Zn) (wet basis for an average moisture content of 6%). Element Al-Zn-CO3 Al-Zn-EG T. (%, m/m) Exp. Exp. T. Carbon 19.2 2.0 2.7 19.7 Hydrogen 2.7 4.1 3.4 2.7 Nitrogen ND ND Aluminum 7.3 7.2 8.8 8.9 34.4 Zinc 41.6 34.1 41.7 ND = not detected. T. = theoretical; Exp. = experimental [Reprinted with kind permission from J. Coll. Interface Set, (227, 445, 2000)] [40]. To investigate whether the experimental procedure could cause any loss of Al and Zn by leaching, the original matrix was also subjected to the same experimental conditions. As a result, there were no observable changes in the Al/Zn ratio of the matrix. Therefore, this experimental control clearly demonstrated that no leaching of Al and Zn had occurred during sample preparation and that the assumptions made for the calculation of the empirical formulae were correct. Higher C contents in both samples were probably due to the occurrence of some residual ethylene glycol and contaminating solvent, even though this was not corroborated by the corresponding change in H content.

24

F.Wypych

The possibility of having carbonate as the counter-ion of the intercalated ZnAl-EG phase was eliminated by the complete absence of any FTIR band that could be attributed to its presence. However, the exchange of OH" for CO3"2 does not result in a significant variation in C, H, Al and Zn contents. Hence, the presence of CO3"2 could not be eliminated by elemental analysis and the theoretical yields of the empirical formulae, containing this counter-ion [Zn0.66Alo.34(0-(CH2)2-0)](C03)o n.0.4H2O, lie perfectly within the acceptable range depicted in the experimental data of Table 1 (C= 20.0%; H= 3.7%, Zn= 33.6%, Al= 7.0%). The FTIR spectra of (a ) Zn-Al-CO3,(b) Zn-Al-EG and (c) pure ethylene glycol are shown in Figure 16 with two distinct spectral ranges. The 2700 and 3700cm"1 region of the FTIR spectra provided interesting information about the structure of the LDH grafted composites. Two important bands were found within this spectral region: the out-of-plane stretching vibrations of C-H bonds at 2700-3100cm"1 and the stretching vibrations of O-H groups at 3100-3700cm"'. After the covalent grafting of ethylene glycol into the Zn-Al-CO3 phase, it was observed that two C-H stretching of ethylene glycol originally centered at 2879 (symmetric) and 2946cm"1 (antisymmetric) were either displaced or converted to at least three new absorption bands at 2856, 2896 and 2923cm"1. This observation confirmed the successful grafting of ethylene glycol within the layer structure of the host matrix because their C-H groups were then vibrating in a distinct chemical environment. Band displacements such as these can be used to characterize the higher rigidity of the grafted composite since the out-of-plane stretching of C-H bonds were naturally shifted to higher wavenumbers. However, other chemical interactions such as those with residual water and/or unreacted hydroxyl groups might have also contributed to the band displacements discussed above.

Figure 16 - FTIR spectra of (a) Zn-Al-COs, (b) Zn-Al-EG and (c) pure ethylene glycol (EG). [Reprinted with kind permission ofJ. Coll. Interface Sci., (227, 445, 2000)] [40]. Similar effects were also observed when both kaolinite and gibbsite were successfully grafted with ethylene glycol. The out-of-plane C-H stretching vibrations in kaolinite were shifted to 2969, 2945 and 2895cm'1 after grafting, whereas these same bands were displaced to 2920 and 2870cm"1 when gibbsite was used as the host matrix.


25

For the simple intercalation of ethylene glycol into the kaolinite layer structure, there was no considerable change in the absorption profile at this spectral range and the observed bands at 2890 and 2945cm"1 were similar to those present in the FTIR spectra of the pure EG (2879 and 2946cm"1). Vibration frequencies other than those strictly related to the grafting of ethylene glycol into Zn-Al-CO3 were also observed in the FTIR spectra of the Zn-Al-EG phase. Hence, the occurrence of a number of relatively weak absorption bands, such as those centered at 2705, 2713, 3026, 3061 and 3082cm"1, were probably associated with minor contamination that might have been incorporated within the structure of the covalently grafted material. Sample preparation for FTIR was carried out after drying at 50°C to avoid exposure of the grafted composite to exceedingly higher temperatures. Therefore, complete removal of water could not be easily achieved and this was detrimental to the interpretation of absorption bands occurring around 3431cm"1 (water O-H stretching vibrations). The occurrence of a band at 1635cm"1 was the strongest evidence that some adsorbed water had remained within the sample. Likewise, this spectral region (1630cm" 1 and 1650cm"1) has also been used to detect water in other similar compounds such as kaolinite grafted with ethylene glycol [77,78]. The 1500-1200cm"1 spectral region of the grafted material revealed a series of peaks with low intensity, collectively attributed to CH2 stretching vibrations. This was an additional evidence for the strong rigidification of the ethylene glycol backbone after grafting [77]. The broad absorption band attributed to the carbonate counter-ion (1365cm1) (see the FTIR spectra of Zn-Al-CO3 in Fig. 16(a)) was completely removed from the LDH after grafting. In fact, this band was replaced by a sharp peak of low intensity, probably attributed to CH2 deformations of the ethylene glycol backbone. Therefore, as stated above, the occurrence of carbonate as the counter-ion for the Zn-AlEG phase was completely discarded and OH" was considered the actual counter-ion that was intercalated within the grafted material. Ethylene glycol is a very hygroscopic compound and any small amount of water present within the reaction mixture may trigger the gradual displacement of carbonate from the layered structure. This proposed exchange of counter-ions can also partly explain the broad association band (at 343 lcm"1) found in the FTIR spectra of the grafted material. Even though the absence of an absorption band at 1325cm"1 may be used to suggest that the conformations of oxyethylene units (O-CH2-CH2-O) are not in a trans conformation, it is possible that this same absorption band was slightly displaced to 1362cm"1 in the LDH-EG compound, thus characterizing a shift that has been already observed in other systems. For instance, absorption bands at 1030-1100cm"1, typically attributed to Al-O-C and CC-0 bonds in kaolinite, have been observed at 1043, 1072, 1081 and 1124cm"1 for ZnAl-EG. Rocking vibrations of the CH2 groups, generally centered at 864 and 882cm"1 for ethylene glycol, were almost completely absent from the FTIR spectra of the LDHEG compound. In fact, after grafting, these bands were converted into three new bands at the higher wavenumbers of 903, 911 and 919cm"1. A similar effect over the CH2 rocking vibrations was also observed when boehmite was used as the host matrix, where the original bands at 865 and 885cm"1 were displaced to 868 and 900cm"1 after the monodentade grafting of ethylene glycol [79]. Therefore, larger shifts of these rocking vibrations are expected when ethylene glycol is grafted into the host matrix under a bidentade conformation [79]. The relatively weak bands observed at 865, 882, 1041 and 1085cm"1 may be an additional evidence for the existence of small amounts of adsorbed

26

F.Wypych

ethylene glycol within the LDH-EG matrix. Nevertheless, the same observation led to the conclusion that this organic compound was successfully grafted onto the matrix from both ends (bidentade conformation), since the FTIR spectra brought little evidence for the persistence of any considerable amount of free ethylene glycol hydroxyl groups after grafting. 4.3 - Thermal reactions When hydrated ions intercalated compounds are heated to higher temperature, the solvent is released and a contraction of the basal spacing occurs. In most of the cases only the reversible dehydration/rehydration process occur (Eq. 21). [AlxMgl.x(OH)2]Clx.nH2O AlxMgl.x(OH)2Clx + nH2O

(Eq. 21)

In some cases, the structure of the layered double hydroxide is preserved and intercalated ions can undergo chemical transformations that allow grafting to the layers (Eq. 22) [82,83]. Potentially this kind of reactions can also be applied to the hydroxysalts. [AlxMg1.x(OH)2](A")x(H2O)y + x/n B"n -> grafted derivatives

(Eq. 22)

By increasing the temperature, a mixture of amorphous oxides or ternary compounds (spinel like) can be obtained. One of the most interesting features of LDHs is the memory effect or reconstruction of the structure. This reconstruction is totally reversible (Eq. 23) when moderate temperature calcination temperatures are employed (ca. 300 - 500°C depending on the metals of the structure) and the amorphous material is put in contact with a solution or water vapor. The amorphous basic mixed oxides with a high surface area, high porosity, homogeneous dispersion of metallic particles, have many practical applications like catalysts, catalysts supports, ions exchangers, stabilizers in polymers, adsorbents, etc. [AlxMgl.x(OH)2]Clx.nH2O -> x/2 A12O3 + 1-x MgO o

[Al x M gl . x (0H) 2 ]-

(Eq. 23) In the case of ion intercalation containing a metallic atom, the thermal treatment under controlled conditions (inert or hydrogen atmosphere or vacuum), metallic particles can be obtained in mixtures of oxides matrix (Eq. 24) or still metal alloys [84,85,86]. The metallic particles will eventually be able to be used in the production of devices of the most varied species, catalysts, etc. [AlxMg1.x(OH)2](Fe(CN)6)x/3.nH2O -> x/2 A12O3 + 1-x MgO + x/3 Fe° (Eq. 24) 4.3.1 - Iron nanoparticles embedded in Al2O3-ZnO matrix [86]. Nitrate ions from a layered double hydroxide were exchange by hexacyano Fe(III) complex. The decomposition of the hexacyano Fe(III) complex and subsequent dehydroxilation of the LDH matrix was achieved by thermal treatment in high vacuum at 450°C during 2 hours, generating nanoparticles of Fe in a A12O3 and ZnO matrix. Figure 17 presents the FTIR spectra of the (a) original LDH and (b) after the exchange reaction with the hexacyano Fe(III) complex. The original LDH presents characteristic bands at 619, 1111, 1175, and 1384cm"1 attributed to nitrate and sulfate ion bands, respectively.


27

After the hexacyano Fe(III) complex exchange reaction, bands attributed to carbonate ion bands were observed at 1357 cm"1 [40], nitrate bands at 1384 cm"1, sulfate bands at 1109 cm"1 and cyanide bands at 2097 cm"1 and 2110 cm"1 [14], respectively. The position of the cyanide bands depends on the composition of the LDH and possible processes of the iron oxidation/reduction of the hexacyano Fe(III) complex ion [14]. The cyanide bands are very strong, demonstrating that in spite of the presence of the other ions, the hexacyano Fe(III) complex ion is in a large concentration in the sample. The broad band centered at 3450 cm*1 and a narrow band in the 1630 cm"1 region, are attributed to adsorbed/absorbed/coordinated water molecules. Figure 18 shows the X-ray powder diffraction patterns of the (a) LDH as prepared, (b) after the hexacyano Fe(III) complex exchange reaction and (c) after thermal treatment of the LDH-FeCN at 450 °C under vacuum. Firstly, a compound of low crystal quality with a basal spacing of 10.7A, is observed. Although sulfate, nitrate and chloride salts have been used in the synthesis, it is expected that the LDH present a basal spacing of the larger diameter ion, i.e., sulfate. The basal spacing of 10.7A is usually observed when besides the sulfate ion, a neutral salt is also co-intercalated.

Figure 17 - FTIR spectra of (a) the original LDH and (b) after the exchange reaction with the hexacyano Fe(IH) complex ion. [Reprinted with kind permission ofJ. Phys. D: Appl. Phys., (36, 428, 2003)] [86]. The compounds intercalated exclusively with carbonate and nitrate ions would have the basal spacings of 7.8A [40] and 8.8A, respectively. After the exchange reaction, a compound with basal spacing of 10.9A was obtained. This basal spacing corresponds to the intercalation of the hexacyano Fe(III) complex ion, with the three-fold axis perpendicular to the LDH layers [17]. Obviously, in this case, the exchange reaction does not necessarily processed entirely. However, the basal expansion corresponds to those of the ion with larger diameter or hexacyano Fe(III) complex. After thermal treatment, a polycrystalline material with preponderant double-oxide Al2O3-ZnO composition was obtained. Some Bragg reflection peaks are identified in the powder X-ray powder diffraction pattern, whose detailed investigation was performed using transmission electron microcopy.

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F.Wypych

Figure 18 - X-ray powder diffraction patterns of the (a) original LDH, (b) LDH-FeCN, and (c) after thermal treatment of the LDH-FeCN at 450°C under vacuum. The asterisk denotes the internal standard peak of Si. [Reprinted with kind permission ofJ. Phys. D: Appl. Phys., (36, 428, 2003)] [86].

Figure 19 - (a) Bright-field image obtained of the sample after thermal annealing under vacuum obtained with a transmission electron microscope operating at 120 kV, (b) SAED pattern of a large sample area showing diffraction rings associated with A12O3, ZnO and Fe, and (c) particle size distribution obtained by computational method. [Reprinted with kind permission ofJ. Phys. D: Appl. Phys., (36, 428, 2003)] [86].


29

Figure 19(a) shows the bright-field image obtained with transmission electron microscope of the sample after thermal annealing under vacuum. The dark regions in the image are associated with Fe-rich particles. The Fe particles have rounded shapes with clear boundaries and small connectivity. Figure 19(b) shows the selected-area electron diffraction (SAED) pattern of a 30 um-diameter area of the same sample. The polycrystalline character of the double-oxide Al2O3-ZnO matrix is clearly observed from the diffraction rings. Since several interplanar spacings of the A12O3 bulk are superimposed to those of the ZnO bulk, an unambiguous identification of the diffraction rings could not be obtained. The presence of diffraction rings related to strained or unstrained Fe oxides cannot also be discarded. SAED patterns obtained from the electron beam focalization on the largest dark-regions reveal a predominance of the diffraction rings, which are unambiguously associated with metallic Fe. The doubleoxide diffraction pattern of the matrix appears rather uniform among distinct analyzed regions in the sample. Figure 19(c) exhibits a particle size distribution with a predominant maximum around 1.5 nm2 and several secondary peaks at 13, 29, 40, and 51 nm2. A dedicated software was used to obtain particle size area from the image computation. This procedure described the production of metallic Fe nanoparticles in a double-oxide of aluminum and zinc. The route is very attractive technologically since it involves simple chemical procedures with low cost resources. In the case of the doubleAl-Zn-hydroxide intercalate with the Fe(CN)6"3 complex anion, the annealing and subsequent dehydroxylation in vacuum leads predominantly to 7A-radius metallic Fe particles embedded in a double Al2O3/ZnO oxide host. 4.4 - Exfoliation reactions and preparation of nanocomposites Although the process of separation of individual layers can be potentially applied to any layered compound, transition metal dichalcogenides of the V and VIB groups were the first ones to be subjected to this kind of reaction [60-66]. The literature reports also two specific examples for the layered double hydroxides exfoliation [87,88] and probably none in simple hydroxides or hydroxysalts. 4.4.1 — Exfoliation of a layered double hydroxide and reaction with poly(ethylene oxide) [89] Sulfate ions from the layered double hydroxide were exchanged with dodecylsulfate, using sodium dodecylsufate (SDS). The supernatant suspension containing an excess of SDS was separated and reacted with an aqueous solution of poly(ethylene oxide) in 50ml of water. Figure 20 shows the X-ray powder diffraction results for the entire sample preparation sequence. The sequence of diffraction results shows layered compounds with increasing crystal quality and increasing basal plane separation. Figure 20(a) shows the typical powder X-ray diffraction pattern of the sulfate intercalated layered double hydroxide (LDH-SO4) with a 11.1 A layer separation in accordance with the literature results [90]. The replacement of SO4"2 for the anion dodecylsulfate leads to the LDH-DDS sample shown in Figure 20(b). This sample has a basal spacing of 26.2A in good agreement with the literature [91,92]. The nanocomposite formed after the LDH-DDS reaction with PEO, which we call LDHDDS-PEO, has an expanded basal spacing of 35.9A, as shown in Figure 20(c). This separation grows to 38.2A for the 110°C heat-treated nanocomposite, as shown in Figure 20(d). The X-ray diffraction results of the LDH-DDS-PEO composite show clear modulation of the high order diffraction peaks. In one case (Fig. 20(d)), these peaks go up to 15 orders. The high diffraction orders indicate very well crystallized samples with

30

F.Wypych

long order correlation lengths, while the modulation reveals ordered substructures within layers. The transmission electron microscopy results were obtained by using a JEOL 1200 EX-II instrument operating at 40 kV. The measurements were extremely difficult, since the interaction of the electron beam with the sample caused rapid deterioration of the latter. The selected area electron diffraction (SAED) pattern shows some welldefined spots together with a strong diffuse ring for the LDH-DDS-PEO nanocomposite (not shown). The ring is due to the already mentioned deterioration processes. In contrast, the well-defined spots reveal a hexagonal structure with (4.7J6 0.248)A and (2.715 + 0.082 )A for the interplanar spacing of the [ 1 100] and [2110] directions, respectively. No distortion in the basal plane projection could be observed, indicating that the lattice distortion occurs only in the "c " axis direction. The composite dried at 110°C could not be observed due to the above mentioned beam-sample interactions. The TG results are shown in Figure 21 for (a) LDH-SO4, (b) LDH-DDS and (c) LDH-DDSPEO. DSC measurement for LDH-DDS-PEO was also included (Fig. 18(d)). Clearly, all results are distinct in each case, indicating the presence of different layered materials. In the case of LDH-SO4 the TG data permit us to calculate the formula for the compound: Al0,33Mgo,67(OH)2(S04)o,i7.0,61H20. After the exchange of the sulfate ions by DDS ions, the TG data are consistent with the following formula: Alo,33Mgo,67(OH)2(Ci2H25S04)o,33.0,64H20. Here, a mass loss of 64,5% was observed between 150 and 1000°C. In the HDL-DDS-PEO case, an additional mass loss was observed, that is consistent with PEO incorporation into the sample. We observed a 71.5% sample mass loss between 150 and 1000°C.

Figure 20 ~ X-ray powder diffraction patterns (a) for LDH-SO4, (b) LDH-DDS, (c) LDH-DDS-PEO at room temperature and (d) heated at JOO°C for 1 hour. The asterisk indicates the silicon standard (111) peak. Copyright - Langmuir, (18, 5967, 2002) [89]. Also distinct are the Fourier Transform Infrared (FTIR) results for the samples as shown in Figure 22. Here again we find that the intercalation always lead to new features and characteristic absorption bands. With the exception of HDL-SO4, all other samples display the characteristic absorption bands of DDS at 2850, 2919 and 2957cm"1 [91,92]. Finally, some bands related to PEO [93] are superimposed on the strong DDS


31

bands in the same region. Additionally, we observe that the sulfate ion in HDL-DDSPEO is coordinated in a different form as compared to HDL-SO4 (449, 619, 991, 1115 and 1190cm"1) [90,94] and SDS (1221 and 1247cm"1), since the bands have been shifted to 1214, 1249 and 1270cm"1. Absorption bands relative to KNO3 (695, 828 e 1370cm"1) [94] or nitrate ions (1380cm"1) [94,95] as well as carbonate ions (1365cm"1) were not observed [11]. The intercalation of layered double hydroxides with long chain surfactants yields hydrophobic surface properties. These systems are potential adsorbents for the removal of charged and neutral organic molecules from aqueous systems, being important from environmental pollution control [96-99].

Figure 21 - Thermogravimetry (TG) curves for (a) LDH-SO 4> (b) LDH-DDS and (c) LDH-DDS-PEO. (d) Differential scanning calorimetry (DSC) curve for LDH-DDSPEO. Copyright - Langmuir, (18, 5967, 2002) [89].

Figure 22 - Fourier transform infrared spectra (FTIR) for (a) LDH-DDS, (b) LDH-SO4 and(c) LDH-DDS-PEO. Copyright - Langmuir, (18, 5967, 2002) [89].

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F.Wypych

5 - Kaolinite derivatives In kaolinite, due to presence of hydroxyl groups on the aluminum side of the layer (Fig. 5), the process of solvation of those groups may occur (including polymers) [100-104], as well as the grafting reactions [77,78,105,106]. An important characteristic is the possibility to bond covalently specific molecules to the layer (through interlayer aluminol groups) or to modify them after the grafting process, with an intention of attributing their own characteristics to the matrix. Through this procedure interesting materials can be obtained, such as those obtained by the intercalation of organic or inorganic compounds (dyes or pigments, catalysts, precursors of catalysts, ionic exchangers, materials with controlled surface area, with controlled micro and macroporosity, etc.). Apart from these possibilities, the confinement (through grafting or intercalation) of molecules in an asymmetrical chemical nanoenvironment can generate materials with differentiated physical properties of from those that are observed with the free molecules or in the crystalline state [20,21]. The intercalated molecules can also be located into the hexagonal siloxane cavity (Fig. 4(c)) or occupy the octahedral aluminum vacant site (Fig. 5(c)). Despite such infinite possibilities, there have been very few attempts made for the kaolinite as host matrix for such reactions. The most important reactions involve the solvation of the hydroxyl groups (intercalation) and aluminol groups' functionalization (grafting). However, reactions of surface adsorption, exfoliation and synthesis of nanocomposites should also be considered [107,108]. 5.1 - Direct solvation (intercalation) Considering that appropriate polar molecules can have access to the interlayer hydroxyl ions linked to aluminum atoms (aluminol), a solvation process is perfectly possible [42-46]. Apart from organic molecules, simple salts as potassium acetate can be intercalated directly by the simple milling process with pure kaolinite [109-111], or its contact with a solution containing the molecule to be intercalated or the simple contact with the solvent to be intercalated. Usually the intercalation processes are carried out at temperatures slightly higher than the room temperature. After several days of reaction, the solid material is separated by centrifugation and washed with an appropriate solvent or dried at a controlled temperature, to avoid the removal of the intercalated molecule (when the intercalated molecule is sufficiently volatile) (Eq. 25). Al2Si2O5(OH)4 + x (CH3)2SO Al2Si2O5(OH)4((CH3)2SO)x

(Eq. 25)

5.1.1 - Intercalation of Dimethylsulfoxide (DMSO) 9.0g of kaolinite were dispersed in a mixture composed of 60mL of (DMSO) and 5.5mL of distilled water. The reaction was carried out at room temperature for a period of 10 days in a 50mL plane-bottom glass flask equipped with magnetic stirrer agitation. The resulting material was centrifuged at 4000 rpm and dried at 50°C for 24 h, to eliminate the excess of DMSO. A light brow expanded kaolinite-DMSO complex was obtained with an intercalation ratio of about 85% and the basal spacing of 11.21 A, which represent an expansion of 4.04A in relation to the basal spacing of the raw kaolinite (7.16A) (Fig. 23). The basal spacings were obtained from the powder X-ray powder diffraction patterns, by using the reflection of a higher possible order (normally 5). The results of the TG/DSC analysis are presented in Figure 24. For pure kaolinite (results not shown) two endothermic peaks could be observed. The first one, was related


33

to elimination of adsorbed/absorbed water, centered at 51°C while the second centered at 529°C, was attributed to the dehydroxylation process to metakaolinite.

Figure 23 -X-ray powder diffraction of (a) kaolinite and (b) the K-DMSO (*= Si).

Figure 24 - TG/DSC of kaolinite reacted with DMSO. The K(DMSO)X phase showed two endothermic peaks, one centered at 175°C, which could be attributed to DMSO elimination, and other centered at 509°C that corresponded to the kaolinite dehydroxylation process. Considering the concentration of non-reacted kaolinite (near of 16.5%) and the moisture of the material (0.7%), the measured loss of organic matter (8.8%) was in accordance with the theoretical value obtained from the proposed formula (8.8%). The concentration of the final residue (79%) was also in accordance with the decomposition of non-reacted kaolinite and the intercalated material (theoretical value: 78.5%). Due to the relatively low temperature of DMSO elimination (175°C), it could be taken for granted that the process involved integral elimination of the molecule instead of its burning. This fact has been

34

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corroborated by the absence of any exothermic peak that could be attributed to DMSO combustion, even when the experiments were carried out in an air atmosphere. Considering the results of the thermal analysis it has been possible to estimate that the stoichiometry is K(DMSO)0 5.2 - Intercalation by displacement [112-117] Some specific molecules cannot be intercalated directly into kaolinite. However, they can displace molecules previously intercalated (Eq. 26), mainly based on the fact that hydrogen bonds that maintain the structure have been already partially broken. Al2Si2O5(OH)4((CH3)2SO)x + y C6H5CONH2 -> Al2Si2O5(OH)4(C6H5CONH2)y + x(CH 3 ) 2 SO (Eq.26) Sometimes the exchange process is incomplete and compounds containing mixtures of intercalated molecules in varied proportions can be obtained. Several are the replacement procedures of previously intercalated molecules. If the exchanging process involves a solvent, it is enough to keep the intercalation compound in contact with the new solvent. In the case of intercalation of a solid, a solution in a non-reactive solvent is used or through the fusion of the solid to be inserted [112]. In the case of polymers, the procedure involves the intercalation of a monomer and subsequent polymerization by chemical, thermal treatment [101,102] or fusion of the polymer in contact with the previously intercalated compound [100]. 5.2.1 -Displacement of DMSO by benzamide [112] The apparent intercalation ratio (I.R.) of both K-DMSO and K-BZ was determined in their X-ray powder diffraction patterns using the Eq. 27 [117]. I.R. = Intensity (first peak) Intercalate / Intensity (first peak) Intercalate + Intensity (first peak) kaolinite (Eq. 27) The DMSO-intercalated kaolinite (K-DMSO fraction) has shown to be paleyellow powder with an intercalation ratio (I.R.) of 81.5% and basal spacing of 11.21 A. The benzamide-intercalated kaolinite (K-BZ fraction), obtained from K-DMSO, was also shown to be pale-yellow powder with an I.R. of 73% and a basal spacing of 14.29A, which represents an expansion of 7.14A in relation to raw kaolinite. In this case, the I.R. could not be directly calculated from the X-ray powder diffraction pattern because the second reflection of the K-BZ fraction (d=7.14A) and the first reflection of the raw kaolinite (d=7.16A) were almost perfectly superimposed. Therefore, measurements for I.R. calculations were only performed in K-BZ after its normalized X-ray powder diffraction pattern was subtracted from the K-DMSO normalized X-ray diffraction background. For the experimental control in which raw kaolinite was used in the absence of DMSO, there was no evidence that benzamide could be intercalated within the kaolinite host, suggesting that pre-treatment with DMSO (displacement method) was a requirement for the successful inclusion of this intercalation compound. The X-ray powder diffraction patterns of (a) pure benzamide, (b) raw kaolinite, (c) both K-DMSO and (d) K-BZ composites are collectively shown in Figure 22, where the diffraction pattern of the internal standard (powdered silicon) is labeled with an asterisk


35

(*). Figure 25 clearly indicates that there is no evidence for crystalline benzamide in the K-BZ X-ray powder diffraction pattern, suggesting that benzamide substitution has been fully accomplished and that the only crystalline materials found within the matrix are kaolinite and the resulting K-BZ intercalation compound. To facilitate interpretation of Figure 22, the basal reflections of pure kaolinite, K-DMSO and K-BZ have been respectively labeled as Kn, Dn and Bn. However the "n" values observed in this figure do not represent normal indexations because it was not possible to determine if kaolinite has undergone any structural transition after intercalation. As stated above, there was no evidence that any DMSO had remained co-intercalated within the K-BZ matrix, unless that occurs at a relatively low level, which would not interfere with its basal spacing. Pure benzamide was also apparently absent from the X-ray powder diffraction pattern of K-BZ. Variations in basal spacing were determined by subtracting the basal spacing of the intercalated kaolinite from the basal spacing of raw kaolinite (7.16A). The molecular diameter of benzamide, measured between the /^-substituted aromatic hydrogen and the oxygen atom of the carbonyl group, was determined as 7.7A by using a modeling software [118]. Therefore, based on the 7.14A variation in the kaolinite basal spacing after intercalation, it seems that only one type of hydroxyl group is interacting directly with a single intercalated benzamide molecule and that each intercalated molecule is displaced at an angle of 68° in relation to the plane of the kaolinite layer. In fact, this assumption is consistent with the 50 to 75° orientation angle that is normally assumed by hydroxyl groups on the surface of the kaolinite layer [119,120].

Figure 25 - X-ray powder diffraction patterns of (a) pure benzamide, (b) raw kaolinite, (c) K-DMSO and (d) K-BZ composites. [Reprinted with permission from J. Coll. Interface Set, (221, 284, 2000)] [112]. Figure 26 shows the TG/DSC/DTG measurements made on (a) raw kaolinite and (b) on the K-BZ intercalation compound. For raw kaolinite, the 0.7% mass loss observed at temperatures below 250°C was attributed to the loss of moisture. After that, the dehydroxylation of kaolinite into metakaolinite was observed as an endothermic peak centered at 532°C. This process generated a mass loss of 14.1% and this was in good agreement with the 13.96% value predicted from the theoretical formula of pure kaolinite (Al2Si2O5(OH)4). Therefore, the kaolinite sample used in this study was of a

36

F.Wypych

very good quality, a property that could be even improved by appropriate chemical treatments to remove iron (e.g., the dithionite/citrate/bicarbonate method). As the same amount of impurities was detected after several attempts to further purify the untreated kaolinite, these elements appear to be present within the kaolinite matrix as isomorphic/non-isomorphic substitutions. The exothermic peak centered at 985°C was attributed to the crystallization of both Si and Al oxides. The K-BZ phase showed one small endothermic peak at 60°C, readily attributed to the loss of adsorbed water (0.5% mass loss), and one broad endothermic band centered at 225°C, followed by two endothermic peaks with their average intensities centered at 314 and 341°C, respectively. These measurements were collectively attributed to the loss of organic matter from the kaolinite host. As both processes were characteristically endothermic, benzamide molecules appeared to be displaced from their interlayer spacing without being burnt. The endothermic peak centered at 514°C was associated with dehydroxylation of the lattice matrix, whereas crystallization was observed at 988°C as the last exothermic event of the DSC profile.

Figure 26 - TG/DSC measurements made on (a) raw kaolinite and (b) on the K-BZ intercalation compound. Reprinted with permission, from J. Coll. Interface Sci., (221, 284,2000)] [112]. These two processes were also observed in raw kaolinite at 532 and 985°C, respectively. The complete absence of benzamide melting peaks (130°C) in the K-BZ thermal curves demonstrated that there was no excess of this compound within the intercalated derivative. Assuming that K-BZ has nearly 9.6% organic matter in its chemical composition (dry basis), the overall mass loss for a K(BZ)o 2 stoichiometry up to 350°C would theoretically correspond to approximately 9.57%. Indeed, there was a perfect agreement between this theoretical value and the data determined experimentally. Likewise, the amount of residues recovered after the experiment at 1000°C (77.6%) also revealed a perfect agreement with the expected theoretical value of 77.8% in relation to the K-BZ dry weight. After 300°C, no more organic matter was expected to be present in K-BZ and assuming that the K-BZ has an I.R. of 73%, the experimental mass loss of 13.9% was in perfect agreement with both theoretical and


37

experimental values obtained previously for raw kaolinite (13.96% and 14.1%, respectively). Considering that at temperatures beyond 35O°C, all of the benzamide molecules had been completely removed from the kaolinite matrix, it is possible to conclude from the TG/DSC measurements that both intercalation derivatives have very similar stoichiometries of K(BZ)o 2 and K(DMSO)0! - This observation suggests that the intercalation of benzamide molecules was dependent upon the substitution of DMSO molecules from the host matrix. As the I.R. ratio is not precisely known, it is possible that the actual K-BZ stoichiometry is slightly different from that proposed above. The TG/DSC measurements of the K-BZ phase also suggested that DMSO was completely substituted by benzamide during intercalation of the K-DMSO derivative. This was supported by the complete absence of an endothermic peak at 189°C, which corresponds to the elimination of DMSO from the kaolinite matrix. Figure 27 shows the FTIR spectra of (a) raw kaolinite, (b) the K-BZ phase and (c) pure benzamide.

Figure 27 - FTIR spectra from (a) raw kaolinite, (b) the K-BZ phase and (c) pure benzamide. Reprinted with permission, from J. Coll. Interface Sci., (221, 284, 2000)] [112]. A tentative interpretation of the FTIR spectra is given in Table 2, on the basis of FTIR data available in the literature for raw kaolinite, K-DMSO, benzamide and others [121-123]. The FTIR spectrum of the K-BZ derivative contained all the major FTIR bands attributed to kaolinite and benzamide. However, there was no evidence for bands associated with DMSO and this confirmed the absence of any co-intercalated DMSO within the benzamide-intercalated kaolinite. Compared to kaolinite, the FTIR spectrum of the K-BZ derivative showed variations within the region characteristically attributed to O-H axial deformations (3400-3800cm"'). There was a considerable increase in the absorption intensities at the 3647cm"1 region with the concomitant appearance of a shoulder at higher wavenumbers while both 3666cm"1 and 3696cm"1 bands remained relatively constant or even decreased in their relative intensities. This observation led to the hypothesis that, out of the two (or three) distinct hydroxyl groups found on the surface of the kaolinite layered structure [123], only one contributed the most to the hydrogen bonding established directly with the intercalation compound. The

38

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FTIR absorption band at 3619cm"1 has been previously attributed to hydroxyl groups that are embedded within the kaolinite matrix [124,125]. Figure 27 shows that the intensity of this band was not influenced by the intercalation process and this was apparent from the FTIR spectra of both K-DMSO and K-BZ derivatives. Therefore, it seemed that hydroxyl groups that are distributed internally are not directly affected by intercalation of either DMSO or benzamide into kaolinite. The intercalation process of K-DMSO with benzamide also generated three new absorption bands in the FTIR spectra of the resulting K-BZ intercalation compound. These bands, located at 3598cm" ', 3549cm"1 and 3472cm"1, were tentatively attributed to the axial deformations of hydroxyl groups that are involved in hydrogen bonding with the carbonyl group of benzamide. Considering that FTIR could be used to characterize the nature and strength of hydrogen bonds and that the weaker the hydrogen bonding, the lower the wavenumber in which the associated O-H stretching occurs, it seemed that the new absorption band at 3549cm"1 could indicate the occurrence of hydrogen bonding between the carbonyl group of benzamide and co-intercalated water molecules. Nevertheless, the amount of co-intercalated water molecules must be very low because there was no evidence for considerable mass loss in the TG/DSC experiments. .data of K-BZ. Table 2 Attribution Peak Wavenumber 1 K:3694 - O-H surface 3696 2 K:3666 - O-H surface 3670 3 3647 K:3650 - O-H surface 3619 4 K:3619 - O-H inner 3598 5 K:O-H O=C 6 K:O-H O=C or H-O-H 3549 7 K:O-H O=C 3472 8 3391 B:3370-N-H 9 3372 B:3370-N-H 3180 10 B:3176-N-H 11 B:1625 - H-O-H; B:1660 - C O 1638 B:1603, 1617 - N-H and/or H-O-H 12 1606 B:1578-N-H 13 1574 B:1449 14 1447 B:1404 15 1407 B:1298 16 1300 K:1107-Si-O-Si 17 1108 B:1073 18 1083 B:1073 1057 19 B: 1026;K:1033-Si-O 1034 20 B: 1001;K:1006-Si-O 21 1007 K:936 - O-H inner 23 938 24 914 B: 919; K:913 - O-H surface 879 25 K:877 790 26 B:792; K:791 - Si-O-Si 27 K:752 - Si-O-Si 754 28 K:697 - Si-O-Si 692 29 548 K:538 - Al-O-Si 30 472 K:467 - Si-O 431 31 K:431-Si-O B: 414; K:411-Si-0 32 411 [Reprinted by permission from J. Coll. Interface Sci., (221, 284, 2000)] [112].


39

The external hydroxyl groups are also responsible for an absorption band at 936cm"1 in the FTIR spectrum of kaolinite, whereas internal hydroxyl groups contribute with a band located at 913cm"1. Figure 27 shows that the intensity of the former band (internal hydroxyl groups) in K-BZ decreases in relation to the latter (external hydroxyl groups) after intercalation. Therefore, this was another strong evidence that the intercalated molecule was directly associated with the kaolinite matrix through hydrogen bonding. The FTIR spectrum of pure benzamide showed a sharp, single absorption band at 919cm"1. Therefore, if any free benzamide were present in K-BZ, this band would have partially contributed to the relative intensity of the broader K-BZ band at 913cm"1 (internal hydroxyl groups). However, no other spectral evidence for free benzamide was found in K-BZ, particularly within the 1000-4000cm"' region, suggesting that benzamide was indeed absent from the K-BZ composite. Additional variations in the FTIR spectra were observed within the 1500-1700cm"1, which corresponds to N-H and C=O deformation modes in amides. Based on the benzamide FTIR spectrum, the C=O stretching at 1660cm"1 was shifted to a band centered at 1638cm"1. Likewise, both N-H deformation modes located at 1578 and 1625cm"1 were detected as a single peak at 1574cm"1 with a shoulder at a slightly higher wavenumber. This is an additional evidence that, besides the C=O bond, the N-H bond in benzamide is also affected by the intercalation process. 5.3 - Direct grafting Apart from the processes of simple intercalation, processes of direct grafting can be achieved with kaolinite (Eq. 28) [105,106]. Al 2 Si 2 0 5 (0H) 4 + x C6H5PO(OH)2 -+ Al2Si2O5(OH)4.x(C6H5PO3H) + x H2O (Eq. 28) Usually only part of the interlayer hydroxyl groups are reacted, in which a mixed composition containing both Al-O-H and Al-O-C chemical bonds (for the specific case of kaolinite reaction with an alcohol) are obtained. In this kind of reaction, kaolinite is suspended in a non-reactive solvent containing the molecule to de grafted and kept in refluxing conditions by several days or in a pressurized container. After reaction, the solid material is separated by centrifugation and washed with an appropriate solvent or dried in a controlled temperature, to avoid the removal of the grafted molecule. Usually, those compounds are more stable than those obtained by simple intercalation, but in some cases the grafted molecules can be removed by hydrolysis. The range of possible reactions for this case involves all those available in the classic organic chemistry or others, characteristic of the system. 5.3.1 — Reaction with phenylphosphonic acid [105] The reaction of phenylphosphonic acid with kaolinite in aqueous/acetone solution was carried out at C for a period of 20 days. The X-ray powder diffraction patterns (Fig. 28) showed that all the reflections of the raw kaolinite could be indexed, which implied that no crystalline impurities were present. A basal spacing of 15.02A (KPPA15) was observed on the final compound, which represented an interlayer expansion of 7.88A. This value is coherent with the grafting of phenylphosphonate groups into the layer of kaolinite. For a better observation of the non-basal reflections, a ten-fold expanded X-ray powder diffraction pattern of the final product is shown in Figure 29. In

40

F.Wypych

this sequence, a single number denotes the basal reflections of the modified material. K denotes the signal of the residual kaolinite, while an asterisk (*) represents silicon as an internal standard. One spurious phase was identified as a splitting of the first basal reflection of the phase K-PPA-15, occasionally observed during the synthesis process. With the assumption that the basal spacing of this compound was of 16.45A, it was denominated K-PPA-16. Considering the possibility of formation of a hydrated species, the K-PPA-15 phase was subjected to a hydration process. For this purpose, 0.5g of the dry material was reacted with 50 mL of distilled water for a period of 48 hours. In this experiment (results not shown), a partial decomposition of K-PPA-15 phase to kaolinite was observed (about 20%). The hydrated form was observed as a small shoulder on the left side of the first basal reflection of K-PPA-15.

Figure 28 - X-ray powder diffraction pattern of the (a) kaolinite and product of the reaction (K-PPA) with different reaction times. [Reprinted by permission from J. Coll. Interface Set, (206, 281, 1998)] [105].

Figure 29 - X-ray powder diffraction patterns of the final product with the expansion of the powder diffraction pattern (lOx) to better observe the non-basal reflections. Reprinted, by permission, from [J. Coll. Interface Sci., (206, 281, 1998)] [105].


41

Although the material was in contact with water for several days, the isolation of a pure K-PPA-16 phase was not possible. If we consider that the observed interlayer expansion is larger than those observed for water molecules intercalated in zinc and cobalt phenylphosphonate and methylphosphonate, it is possible to assume that the KPPA-16 phase corresponds to a product of partial hydrolyses, instead of a hydrated form. 5.4 - Grafting by displacement Apart from the substitution of the intercalated molecules, processes of functionalization can be also accomplished (Eq. 29).

+ y H2O

Al 2 Si 2 O 5 (OH)4(DMSO) x + y CH 3 -OH -> Al2Si2O5(OH)4_y(O-CH3)y + x DMSO (Eq. 29)

Depending on the organic molecule, bonds through one or more bridges can be established. In the case of one bridge, reactions between kaolinite and primary alcohols can be used as examples, being the resulting material consisted of a composition similar to an ester, releasing a water molecule. In the case of two bridges, reactions with ethylene glycol or glycerol can be used as examples. The grafted compounds can be chosen in such a way to produce very interesting compounds for potential industrial applications. Examples of such applications could be obtained with molecules containing ionic exchange groups (cationic or anionic) that are positioned between the layers, reactions that produce colored materials (having some specific physical properties) or even those containing catalysts or precursors of catalysts. The process of grafting of long linear molecules can produce materials with high porosity with potential applications in filtration, chromatographic or environmental pollution control [96-99]. 5.4.1 - Grafting of ethylene glycol by the displacing of dimethylsulphoxide

Figure 30 - X-ray powder diffraction patterns of (a) raw kaolinite, (b) K-EG and (c) KDMSO.

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Figure 30 shows the powder X-ray powder diffraction patterns of (a) raw kaolinite, (b) kaolinite grafted ethylene glycol (K-EG) and (c) kaolinite intercalated with dimethylsulphoxide (K-DMSO). The main reflections of these phases, labeled as "Dn", "En" and "Kn" in Figure 30, were respectively associated with the sequence of basal reflections of K-DMSO, K-EG and K, where "n" is an integral number. In the K-EG phase, a basal spacing of 9.5 A was observed, which is consistent with the grafting of an ethylene glycol single layer to the aluminol side of the kaolinite layer. The basal expansion of 2.3A is similar to those described previously for the same compound [77,78]. Variations lower than the molecular diameter of ethylene glycol are justified by the interpenetration of the grafted molecules into the hexagonal siloxane cavity of the silicate sheet (Fig. 4(c)). 5.5 — Mechanochemical modifications [109-111] Intercalation reactions, chemical or morphological modifications of layered crystals, can be obtained by milling specific chemicals with the layered materials. Using kaolinite structure as an example, basically the process consists of dry milling kaolinite with appropriate chemicals (urea, potassium acetate, etc.). Apart from the intercalation reaction as described in Eq. 25, the crystals can change the morphology from layered to cylindrical or tubular, being this change is more pronounced when pressurized vessels are employed after the mechanochemical activation. The real positioning of the intercalated molecules is still under discussion. However, as described previously, part of them (the cation or the anions) can be inserted into the hexagonal cavity of the siloxane surface sheet (Fig. 4(c)). 5.5.1 - Intercalation of urea and preparation of hydrated kaolinite [111] The X-ray powder diffraction patterns of the resulting material from the milling of kaolinite with urea, in the proportion of 20% in mass are shown in Figure 3 l(b).

Figure 31 - X-ray powder diffraction patterns of (a) neat urea, (b) kaolinite intercalated urea in the proportion of 20%, (c) 30% and (d) raw kaolinite. [Reprinted from Quimica Nova, (24, 6, 761, 2001), with Permission from Sociedade Brasileira de Quimica] [111].


43

It can be observed that practically all kaolinite was intercalated, generating a compound with basal spacing of 10.76A (expansion of 3.6A in relation to kaolinite; d = 7.16A [109,110]. In the case of the intercalation with 30% of urea (Fig. 31(c)), all kaolinite was reacted, however, through an intense X-ray diffraction reflection, urea was detected in the region of 26=26°(Fig. 31 (a)). The second basal reflection of kaolinite (28 =28.9°) coincides with the third basal reflection of the urea-intercalated compound. A priori this could indicate a compound that did not undergo reaction with the urea. However, that is not the case as evidenced below. It is observed that after washing the intercalated sample with 20% of urea (the same happens with the 30% sample), urea is totally eliminated from the interlayer spacings. While the sample is still wet (Fig. 32(b)) few X-ray diffraction peaks were observed, which demonstrate that the coherency of stacking of layers is poor. Apart from raw kaolinite at 7.2A, two small reflections with basal spacings around 20.1 A and 8.4A were identified.

Figure 32 — X-ray powder diffraction patterns of the intercalated sample (a) with 20% of urea after washing at 90°C in an ultrasound bath and air drying, (b) still wet and (c) after the milling of the air dried product (hydrated kaolinite) with 30% of urea as described for the pure kaolinite and (d) hydrated kaolinite produced through the methanol washing of the dimethylsulfoxide intercalated kaolinite. [Reprinted from Quimica Nova, (24, 6, 761, 2001), with Permission from Sociedade Brasileira de Quimica] [111]. The first refection could be attributed to an intermediary hydrated phase or formation of ordered heterostructures and the second, to the stable hydrated phase. After the air drying process (Fig. 32(a)), hydrated kaolinite was observed, which was characterized by a broad reflection with an basal spacing of 8.4A [126], apart from a small concentration of raw kaolinite. After milling of the dry hydrated kaolinite with 30% of urea, the urea intercalated kaolinite phase has reappeared, with an excess of urea (Fig. 32(c)), demonstrating that the hydrated kaolinite was chemically similar to raw kaolinite. Keeping in mind that water plays an important role in breaking the hydrogen bonds that hold the layers together, it is expected, that with a bigger basal spacing, this phase is

44

F.Wypych

more appropriate for the intercalation of molecules, which is normally not directly intercalated with raw kaolinite. TG/DSC measurements were performed with raw kaolinite, pure urea, urea intercalated kaolinite in the proportion of 30% and 20% and hydrated kaolinite, as can be seen in Figure 33. TG curve of raw kaolinite (Fig. 33(a)) presents a mass loss of 1.05% up to 200°C attributed to moisture, followed by a process of dehydroxylation of the matrix (endothermic peak in the DSC curve at 527°C) and the crystallization of the oxides (exothermic peak in the DSC curve at 987°C). The mass loss up to 1000°C (in dry matter basis) of 14.1% is in a very close agreement with the expected value of 13.96% for the ideal composition proposed for the kaolinite (Al2Si2O5(OH)4) and formation of oxides at the end of the thermal treatment: A12O3 and SiO2. Pure urea (Fig. 33(b)) presents a complex decomposition profile under air, beginning with the fusion process associated with an endothermic peak centered at 142°C observed in the DSC curve. An endothermic peak is observed at 213°C following by endothermic peaks at 252, 343, 374 and 396°C. At that sweeping speed, at least 4 decomposition steps were observed till the complete elimination of the sample from temperatures up to 420°C. The decomposition process of the 30% urea intercalated kaolinite (Fig. 33(c)) presents a typical profile of a mixture of urea and urea intercalated kaolinite. In the TG curve up to 120°C, the elimination of the sample moisture was observed (1.36%), associated with an endothermic peak at 54°C in the DSC curve. At 141°C in the DSC curve, the fusion of the excess urea was observed along with a complex decomposition process of the residual urea and the destruction of the intercalation compound (endothermic peaks at 189, 199, 219, 236 and 308°C). The matrix dehydroxylation was observed at a slightly lower temperature (513°C) than in raw kaolinite (527°C), demonstrating that the intercalation process produced crystal delamination (an effect that facilitates the structure dehydroxylation process). The exothermic peak relative to the crystallization of the oxides was observed at 991°C in the DSC curve. The TG/DSC curves of the intercalated sample with a composition of 20% of urea (Fig. 33(d)) present a quite simplified profile in relation to the 30% proportion. One endothermic peak was observed at 59°C, being associated with the dehydration of the sample (mass loss of 2.48% between room temperature and 120°C) followed by a step of organic matter removal (endothermic peak at 231°C in the DSC curve and a mass loss of 15.87% between 120°C and 380°C). The dehydroxylation of the matrix was observed in 517°C (loss of mass of 12.37% between 370°C and 1000°C) apart from one exothermic peak at 988°C. An interesting characteristic in this system is associated with the elimination of the whole organic matter of the sample without the destruction of the matrix. Apart from the simplification of the urea decomposition process that takes place in only one step, the absence of urea fusion peak in the DSC curve, demonstrates that the intercalated phase with 20% does not present any nonintercalated urea as also evidenced by powder X-ray diffraction. Another interesting fact is the stabilization of the decomposition of the urea after the intercalation process that takes place at 231°C in comparison with 213°C in the pure urea. The generated stoichiometry starting from the obtained data, Al2Si205(OH)4(N2H4CO)o,84 is quite close to the predicted stoichiometry starting from the mixture of the chemicals (Al2Si20;5(OH)4(N2H4CO)o 86 ). The hydrated phase (Fig. 33(e)) presents a quite different decomposition profile. The process of moisture elimination between room temperature and 100cC in the TG curve (1.03%) is accompanied by two endothermic peaks at 40°C and 59°C in the DSC curve.


45

Figure 33 — TG/DSC curves of (a) raw kaolinite, (b) pure urea, (c) urea intercalated kaolinite in the proportion of 30%, (d) 20% and (e) hydrated kaolinite. [Reprinted from Quimica Nova, (24, 6, 761, 2001), with Permission from Sociedade Brasileira de Quimica] [111].

46

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The process of intercalated water removal between 100 and 370°C (3.27%) is accompanied by two endothermic peaks of low intensity centered at 113°C and 192°C, in the DSC curve. Based on these data, the stoichiometry can be obtained (Al2Si205(OH)4(H20)o.64), this being identical to the phase observed through water washing of the ethylene glycol intercalated kaolinite and through methanol washing of the dimethylsulfoxide intercalated kaolinite. Between the temperatures 370 to 1000°C, a mass loss of 13.72% is observed in the TG curve to which one endothermic peak related do the dehydroxylation process at 524°C and one characteristic exothermic peak at 989°C, were observed. The total residue of 82.8% is close to the expected value of 83.2% for the proposed stoichiometry. As the organic molecules of the described phases are eliminated before the beginning of the process of the matrix decomposition, being heated until a certain temperature, kaolinite can be fully recovered, although it looses crystallinity as a consequence of the intercalation reaction.

Figure 34 - FTIR measurements of (a) the raw kaolinite, (b) urea intercalated kaolinite in the proportion of 20%, (c) hydrated kaolinite and (d) pure urea. [Reprinted from Quimica Nova, (24, 6, 761, 2001), with Permission from Sociedade Brasileira de Quimica] [111]. After the intercalation reaction (Fig. 34(b)), the region related to the hydroxyl groups in the FTIR spectra (between 3200 and 3800cm"1) was well affected with the disappearance of the external hydroxyl groups' bands at 3651 and 3669cm"1 and a decrease of the intensity of the band at 3698cm"1. This was slightly moved to larger wavenumbers in relation to the raw kaolinite. Even then, the relative bands for internal hydroxyl groups were maintained at 3619cm"1 and the appearance of new bands at 3504, 3411 and 3388cm*1. Those new bands are associated with the bonding of the urea molecule to the external hydroxyl groups of the kaolinite layer. This was another evidence for the urea interaction with the interlayer hydroxyl groups. The bands observed at 1467, 1617 and 1686cm"1 in the pure urea have moved up to 1475, 1590, 1622 and 1673cm"1 in the intercalated compound. The band attributed to the surface hydroxyl groups has moved from 914cm"1 in the kaolinite to 903cm"1 in the urea


47

intercalated sample. The other bands did not undergo significant changes. In the hydrated phase (Fig. 34(c)) a characteristic profile was observed with the maintenance of the relative band for internal hydroxyl groups (3618cm"1), a shoulder at 3649 and a band at 3692cm"1 (relative to the external hydroxyl groups) apart from the appearance of new bands at 3600 and 3556cm"1. The small band at 1655cm"1 is attributed to the deformation band of the water molecule. The band attributed to the surface hydroxyl groups returned practically to the original position at 912cm"1. However, a new band was observed at 965cm"1, which demonstrated the interaction between the intercalated water molecules and the external hydroxyl groups. The possibility that some product from the decomposition of the urea could remain between the layers cannot be excluded. However, after washing the sample with hot water, the hydrated kaolinite appeared as a final product. This phase was identical to the previously reported one, as confirmed by powder X-ray diffraction, FTIR and thermal analysis. The hydrated kaolinite should not be confused with halloysite, since the two differ in some aspects, although some coincidences can be cited. The most outstanding difference is related with the habit of the crystals. Hydrated kaolinite presents itself in the form of layered crystals while halloysite (with two molecules of hydration water also known as Endellite and anhydrous) is tubular. In spite of this difference, halloysite can show an uncoiling of the crystals after dehydration, turning into layered structure. The stoichiometries between halloysite (Al2Si205(OH)4(H20)2,o and hydrated kaolinite (Al2Si2O5(OH)4(H2O)0)64) are very different, although variable amounts of water can be observed during the dehydration process of halloysite to anhydrous halloysite. The same happens to the basal spacings that are very different [(halloysite=10.lA [127] and hydrated kaolinite (unstable 10.lA and stable 8.4A)] [126,128-130], although intermediate spacings between the two extremes can be identified by "in-situ" X-ray diffraction measurements during the process of dehydration of halloysite. The hydrated kaolinite and halloysite regenerate the kaolinite (or anhydrous tubular halloysite) after total dehydration. The observed bands in the FTIR spectra in the region between 3200 and 3800cm"1 differ only in the intensity, although that characteristic can be explained by the variable amount of interlayer water (halloysite with two molecules of hydration 3695, 3620, 3602 and 3550cm"1 and hydrated kaolinite - 3692, 3649, 3619, 3600 and 3556cm"1). In the region between 2000 and 400cm"1, although the differences are very subtle, the spectra are closer to kaolinite than to halloysite. The bands in the FTIR spectrum of raw kaolinite (Fig. 34(a)) attributed to the external hydroxyl groups were observed at 3694, 3669 and 3651cm"1 and those attributed to the internal hydroxyl groups at 3619cm"1 [131-133]. Although hydrated kaolinite was never reported to occur in nature, this appearance could be predicted mainly in mixed deposits of halloysite/kaolinite. Hydrated kaolinite can be used to prepare long chain amine derivatives [115] and aminoacids [134] that are similar to hydrophobic organo layered double hydroxides [96-99]. These compounds are very easy to prepare and can be an interesting alternative for the environmental cleanup and remediation of contaminated soils, groundwater and industrial effluents that are resistant to biological degradation. 6 - Smectite derivatives In the smectites, the reactions that are more different from kaolinite are related with the isomorphic substitution, which attribute to smectite a higher capacity of cationic exchange. Due to presence of planes of oxygen atoms on both sides of the layer

48

F.Wypych

(siloxane surface), structural modifications through grafting reaction that are quite important for kaolinite do not play an important role for the preparation of new smectite derivatives (structure type 1:2). On the other hand, similar materials can be obtained through the thermal treatment of intercalated smectites with some specific compounds producing pillared clays, with several applications as catalysts and adsorbent materials. Apart from the reactions of ionic exchange, many of the reactions described for the other systems can be applied to the smectites as well. 6.1 - Cationic exchange reactions The importance of the cationic exchange reactions will not be described in this work in full detail, but it can be mentioned that those processes are extremely important sources of essential cations for the growth of plants, apart from many important industrial applications [23,135]. The reactions proceed through the unsaturated bonds on the crystal edges (silanol and aluminol bonds), replacement of the cations in the crystalline lattice, replacement of the hydrogen ions of the hydroxyl groups with other cations or through the replacement of the interlayer cations. Although the crystallite size can influence the capacity of ionic exchange attributed to the edges of the crystals, the process of interlayer cations exchange can be considered predominant. Apart from the process of ionic exchange, other solvents can replace the layers of solvation of the intercalated ions in a similar way to those described for the hydroxysalts (Eq. 13,14), including natural and synthetic polymers (Eq. 30). When the hybrid materials are used as fillers in polymers [136-144], those phases allow a larger interaction among the phases and in some cases, a total exfoliation inside the matrix of the polymer can take place. Those processes allow a great improvement of the mechanical properties of the reinforced polymers and help save polymer consumption and reduce costs as well. Clay(A+)x(H2O)y + polymer —> nanocomposites

(Eq. 30)

6.2 - Thermal reactions Depending on the thermal treatment, the dehydration processes or a reversible removal of solvents of the intercalated cations can take place (Eq. 31,32). After increasing the temperature, the dehydration is followed by a process of dehydroxylation of the clay mineral matrix, producing a mixture of amorphous or crystalline oxides, depending on the involved temperature (Eq. 33). Clay(A+)x(H2O)y Clay(A+)x + y H2O (-» heating and heating and oxides mixtures (high temperatures)

(Eq. 33)

Adopting an appropriate strategy of the intercalation of key cationic complexes (Ex.: salts of Al, Ti, etc) and depending on the involved thermal treatment, a process of decomposition of the intercalated compound can take place, leaving the structure of the


49

clay mineral almost intact. A reaction between the clay mineral layer with the product of decomposition of the complex material occur, which produces pillars between the layers, generating a class of extremely important catalyst known as "pillared clays" (Eq. 34) [145,146]. Clay(A+)x(H2O)y -> pillared clays (presence of adequate cationic complexes and controlled temperatures) (Eq. 34). 6.4 — Grafting reactions Clays from the smectite groups are used as fillers in polymers [136-144]. As the interaction between the hydrophilic clay and hydrophobic polymer is poor, it is difficult to disperse and the reinforcing property is not maximized. In order to improve the interaction between the phases, hydrophilic phyllosilicate surfaces can be modified through grafting of organic groups, silanes being the most widely employed material [147-150]. The reaction is similar to kaolinite surface (Section 5.3), but restricted to crystal edges and basal defects, as both sides of the smectite layers are covered in oxygen atoms. In some cases the clays are subjected to acidic activation that leaches octahedral aluminum from the structure, producing more available sites to be reacted with the silane [147]. The surface modification with a silane coupling agent is shown in Eq. 35 (R denotes an organic group that can interact (or react) with the polymer and improve the interactions between the phases. Clay/-3OH + RSi(OH)3 -> Clay/-O3Si-R + 3 H2O

(Eq. 35).

Modifying the terminal group R, this kind of surface modified material can be used to immobilize several interesting compounds going from enzymes, catalysts (oxidation catalysts based on metalloporphyrins is one example), pigments, surfactants, etc. 6.3 - Exfoliation and preparation of nanocomposites One of the most important characteristics of the smectites is that, depending on the energy of hydration of the intercalated cations, those can be intercalated with one, two, three or more layers of hydration. The distribution of the layers of water in the smectites can be regular or a randomic mixture of different hydrate forms and depends on the relative humidity that the sample was exposed to and the solvation energy of the cation. As the interaction between layers are through the intercalated cations, the tendency of those materials are to be presented in low ordinate structures which hinders the processes of characterization especially through X-ray diffraction. The structure determination is normally obtained by X-ray powder diffraction patterns using specific programs, when single crystals are usually not available. When a colloidal particle of the mineral clay of the smectite group is put in contact with water (through a dispersing agent or otherwise), that is associated to the hydrated exchangeable cations which interact strongly with water molecules and counter-ions of the solution. The ions are not directly bound to the surfaces but build a diffuse layer of ions around the colloidal particles. In special conditions (usually in the presence of ions of phosphate, silicate, hydroxide, etc.), a sol or gel can be reversibly stabilized. As attractive forces act between the particles, the colloidal dispersion can be destabilized specially by the influence of salts or pH. The coagulated particles can

50

F.Wypych

interact in different ways (surface-surface, surface-edge or edge-edge), producing morphologies that depend on the method employed in the drying procedure. The smectites derived organophyllic gels have wide application as components of drilling muds for the perforation of oil wells and in cosmetics, toiletries, lubricants, adhesives, paint and other related industries (Eq. 35) [151-157]. Clay(A+)x(H2O)y -+ sols or gels

(Eq. 35)

Although the single-layer suspensions can potentially be used for the nanocomposites preparation or dispersion in polymers, few reports are described in the specialized literature [158,159]. Most of the examples that involve clay gels and polymers are centered around rheological behavior and studies related with the interactions of the soluble polymer molecules and the clay single layers [160-165]. Recently, hectorite [76] and other layered compounds [158] were used as fillers in order to improve the mechanical properties of glycerol plasticized starch films. The films were characterized by several techniques. Dynamic mechanical analyses have shown that the composite films present three relaxation processes, attributed to glass transition of glycerol rich phase; water loss including the interlayer water molecules from the clay structure and the starch rich phase. The film with 30% in mass of hectorite showed an increase of more than 70% in Young's modulus compared to non-reinforced plasticized starch. Both X-ray diffraction and infrared spectroscopy have shown that glycerol can be intercalated into the clay galleries and there is a possible conformational change of starch molecule in the plasticized starch/clay composite films. In the unplasticized clay/starch mixtures, the clay is almost totally exfoliated in the starch matrix. These data show that the glycerol/starch interactions, hinders the exfoliation process of the clay. 7 - Concluding remarks Although clay minerals represent a small fraction in the layered compounds family, their chemistry and physics are very rich and fascinating. Despite the efforts that have been made during the last few decades, many aspects related to their reactivity and properties are still obscure. Studies centered about those compounds, be they of synthetic or natural origin have been exciting to the researchers due to their applications flexibility. It can be safely predicted with certainty that clay minerals will reveal in the near future their tremendous potential, especially in the formulation of new and exciting compounds having bearing on important industrial applications. Apart from clay minerals, mono dimensional materials like natural phyllosilicates from the serpentine sub-group (serpentine-kaolin group) will be used to prepare nanocomposites with unthinkable properties. These materials will achieve improved properties specially when functionalized surfaces will be employed to help in the compatibility of the polymeric matrix with the nanoparticles surfaces and also to increase the strength of the interface. Oriented nanotubes within the matrix will produce anisotropic mechanical properties and researches will resurrect banned materials like chrysotile, as ideal material for this purpose. Aknowledgements The author gratefully acknowledges CNPq, PRONEX/MCT, Fundacao Araucaria (Brazilian agencies), his undergraduate and graduate students as well as research co-workers.


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[126] J.J. Tunney and C. Detellier, Clays Clay Miner., 42 (1994) 473. [127] I. Bobos, J. Duplay, J. Rocha, and C. Gomes, Portugal, Clays Clay Miner., 49, 6 (2001) 596. [128] P.M. Costanzo, R.F. Giese and C.V. Clemency, Clays Clay Miner., 32, 1(1984) 29. [129] R. Raythatha and M. Lipsicas, Clays Clay Min., 33 (1985) 333. [130] P.M. Costanzo, C.V. Clemency and R.F. Giese Jr., Clays Clay Miner., 28 (1980) 155. [131] S. Shoval, S. Yariv, K.H. Michaelian, I. Lapides, M. Boudeuille and G. Panczer, J. Colloid Interface Sci., 212 (1999) 523. [132] R.L. Frost, Clays Clay Miner., 46, 3 (1998) 280. [133] R.L. Frost and A.M. Vassallo, Clays Clay Miner., 44, 5 (1996) 635. [134] M. Sato, Clays Clay Miner., 47, 6 (1999) 793. [135] P.W. Faguy, W. Ma, J.A. Lowe, W. Pan, and T. Brown, J. Mat. Chem., 4 (1994) 771. [136] K.A. Carrado and L. Xu, Microp. Mesop. Mat., 27 (1999) 87. [137] D.C. Lee and L.W. Jang, J. Appl. Pol. Sci., 68 (1998) 1997. [138] M.S. Wang and T.J. Pinnavaia, Chem. Mat., 6 (1994) 468. [139] T. Lan and T.J. Pinnavaia, Chem. Mat., 6 (1994) 2216. [140] T. Lan, P.D. Kaviratna and T.J. Pinnavaia, Chem. Mat., 7 (1995) 2144. [141] K.A. Carrado and L. Xu, Chem. Mat., 10, 5 (1998) 1440. [142] A. Okada and A. Usuki, Mat. Sci. Eng. C3 (1995) 109. [143] Y. Kurokawa, H. Yasuda, M. Kashiwagi and A. Oyo, J. Mat. Sci. Lett., 16 (1997) 1670. [144] M. Biswas and S.S. Ray, Adv. Polym. Sci., 155 (2001) 167. [145] D.E.W. Vaughan, Cat. Today, 2 (1988) 187. [146] K. Sapag and S. Mendioroz, Coll. Surf., A, 187 (2001) 141. [147] J. C. Dai and J. T. Huang, Applied Clay Sci., 15 (1999) 51. [148] P.K. Pal and S.K. DE, Rubber Chem. Tech., 56, 4 (1983) 737. [149] T. Seckin, A. Gultek, M.G. Icduygu and Y. Onal, J. Appl. Polym. Sci., 84, 1 (2002) 164. [150] K. Song and G. Sandi, Clays Clay Miner., 49, 2 (2001) 119. [151] L. Bailey, M. Keall, A. Audibert and J. Lecourtier, Langmuir, 10, 5 (1994) 1544. [152] B.J. Briscoe, P.F. Luckham and S.R. Ren, Phil. Trans. Royal Soc. London, AMat. Phys. Eng. Sci., 348, 1686 (1994) 179. [153] S. Abend and G. Lagaly, Appl. Clay Sci., 16, 3-4 (2000) 201. [154] P.F. Luckham and S. Rossi, Adv. Coll. Interf, 82, 1-3 (1999) 43. [155] P.K. Singh and V.P. Sharma, Energ. Source, 13, 3 (1991) 369. [156] V.P. Sharma, S. Laik and S. Srinivasan, Res. Ind., 31, 3 (1986) 230. [157] B. Bloys, N. Davis, B. Smolen, et al, Oilfield Ver., 6, 2 (1994) 33. [158] H.M. Wilhelm, M.R. Sierakowski, G.P. Souza and F. Wypych, Polym. Int., 52 (2003) 1035. [159] K.A. Carrado, P. Thiyagarajan and K. Song, Clay Miner., 32, 1 (1997) 29. [160] J. Lai and L. Auvray, Mol. Cryst. Liq. Cryst, 356 (2001) 503. [161] J. Lai and L. Auvray, J. Appl. Cryst., 33, 1 (2000) 673. [162] O.I. Ece, N. Gungor and A. Alemdar, J. Incl. Phen. Macr. Chem., 33, 2 (1999) 155.

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[163] D.Y. Gao, R.B. Heimann, M.C. Williams, L.T. Wardhaugh and M. Muhammad, J. Mat. Sci., 34, 7(1999)1543. [164] J. Dau and G. Lagaly, Croat. Chem. Acta, 71,4 (1998) 983. [165] M.V. Smalley, H. Jinnai, T. Hashimoto and S. Koizumi, Clays Clay Miner., 45, 5 (1997)745.

ELECTROKINETIC BEHAVIOR OF CLAY SURFACES MEHMET SABRI CELIK Istanbul Technical University Mining Engineering Dept., Mineral Processing Section Ayazaga 34469 Istanbul - TURKEY E-mail: [email protected]


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M.S. Çelik

1 - Introduction Clay minerals are hydrous silicates or alumina silicates and constitute a major proportion of soils, sediments, rocks and waters [1]. Suspended particulate matter, an important component of natural waters and ore dewatering systems, is largely composed of various clay particles which often control the kinetics and deposition of different organic and inorganic species. Interfacial phenomena at clay/water interface are usually controlled by electrokinetics properties including zeta potential (zp), the structure of electrical double layer (EDL), surface potential, and isoelectric point (iep). The electrokinetic properties of a substance, inorganic or organic, are used to explain the mechanism of dispersion and agglomeration in a liquid phase and to identify the adsorption mechanisms of ions or molecules at a solid-liquid interface. They, therefore, play an important role in a spectrum of applications including ceramics, mining, paper, medicine, water and wastewater treatment and emulsions. A larger quantity of literature on electrokinetics of mineral particles is available [2-6]. This chapter presents an overview of the electrokinetic properties of clay minerals, to elucidate the electrokinetic behavior of clay surfaces and the mechanism of particle-particle interactions in aqueous systems. 2 - Electrokinetic properties 2.1 - Zeta Potential Zeta (Q potential is an intrinsic property of a mineral particle in a liquid. It determines the strength of the EDL repulsive forces between particles and identifies the stability of a colloidal system. The zeta potential (zp) is known as the measurable surface potential of a particle viz., the potential at the shear plane. There is no direct experimental method for determining both the surface potential ((p0) and Stern layer potential (q>5) [7]. So far the exact position of the shear plane within the diffuse layer of the EDL could not be determined, but it is assumed that the position of the shear plane is very close to the outer Helmholtz plane (OHP) [8]. The C, potential is fairly close to the Stern potential, cp5, in magnitude, and definitely less than the potential at the surface, cp0. The conventional position of the shear plane is usually thought to be two to three water molecule diameters, or about 5 A from the surface of particle. However, a recent study by Li et al [9] particularly for swelling type clay minerals has shown that the shear plane is closer to the Gouy plane as illustrated in Figure 1. Accordingly, the distance between the shear plane and Stern plane may be about 200-300 A in the presence of 10"4 M of 1:1 electrolyte. Application of this concept using the Gouy-Chapman theory provides a reasonable prediction to the swelling of clay minerals. More importantly, calculation of the Stern potential (cp6) from known values of crs (the surface charge density) using the Gouy-Chapman theory revealed that the zeta potential cannot be approximated to the surface potential [9]. jlRTecx, , ,

CT5

fZFnX

= V —^— smhV^^7^.J

0)

where c0 is the concentration of the electrolyte in bulk solution, s is the dielectric constant of the medium, Z and F are the valency and Faraday constant, respectively. Li et al [9] have also shown that the swelling pressure (P) of the electrostatic repulsion in the diffuse double layer between two adjacent particles can be calculated from the following equation:

Electrokinetic Behavior of Clay Surfaces

F = 2*r,{co sh (^fi)-i]

59

(2)

where cpd is the potential at the overlapping point of the diffuse double layer. The relation between swelling pressure and interlayer distance is presented in Fig. 2

Figure 1 - A diagram illustrating the position of the shear plane in a diffuse layer [9].

Figure 2 - The relationship between the swelling pressure (P) and the interlayer distance (X) as determined experimentally and by double layer theory using different values of(q>^: (—) theoretical curve with cps = - 270 mV; (0) theoretical curve with cps ~ £, = - 55.6 mV; (%) experimental data [9J.

60

M.S. Çelik

2.2 - Potential Determining Ions The interface between the solid and solution may be treated as a semipermeable membrane which allows only the charged species common to both the solid and the solution to pass through. These species are called potential determining ions (pdi). They are the major ions responsible for the establishment of the surface charge of particle [7]. Their activities in the liquid play a crucial role in the generation of potential difference across a solid-liquid interface. They are also able to reverse the sign of zp of the solid. As a simple recipe, for a cation to be the pdi, it must make the surface more positive upon increasing the cation concentration. Similarly, for an anion to be the pdi it must impart the surface more negative charges with increasing the anion concentration. The pdi for ionic solids such as Agl, BaSO4, and CaCO3 etc. are the lattice constituent ions, i.e. Ag+, I", Ba2+, SO42", Ca2+, CO32; whereas H+ and OH" ions are for metal oxides and hydroxides, silicate or clay minerals, some hydrophobic minerals (e.g., coal) and some synthetic polymers with sulfate groups [2,10]. 2.3 - Surface Potential Surface potential is theoretically the potential at zero distance from the surface. It is the highest potential and exhibits an exponential decay with distance. For oxides and silicates, for example, the surface potential is determined by the activities of pdis and expressed by Nernst-like equation q)0 = (RT/F)ln(a H+ /a 0 H+ )

(3) +

where F is the Faraday constant, aH+ is the activitiy of H and a°H+ refers to the point of zero charge (pzc). The operational formula for aqueous solutions at 25 °C is (p0 = 0.059 (pHc - pH) volts where pH° refers to the pzc [11]. Although the isoelectric point (iep) and the point of zero charge (pzc) are identical by definition, there are some differences between them. While the pzc denotes the state in which the net surface charge of the solid is zero, the iep describes the condition at which the potential at the shear plane, i.e., the zp obtained from electrokinetic measurements is zero; the iep and pzc are the same in the absence of the specific adsorption. But the pzc of a mineral need not coincide with the iep in most cases [3]. 2.4 - Origin of Surface Charge Each mineral particle in a liquid whether colloidal (< 1 um) or nanoparticle (4.00 1.4-1.5 Na 1.16 K 1.4-1.5 Ca >4.00 Montmorillonite 0.67 Li >4.00 Na 1.55 and >4.00 K 1.91 Ca >4.00 Baidellite 0.25-0.6 Li 1.52 Na 1.27 K 1.54vw, 1.89s Ca >4.00 Saponite 0.25-0.6 Li 1.52 Na 1.26 K 1.54s, 1.87vw Ca 3.2 - Structure of clay minerals Classification of clay minerals is not within the context of this paper. However, in order to present the data available in the literature in some order, it is necessary to follow one of the accepted classifications. Grim [21] has grouped crystalline clay minerals based on their structural arrangement of layers as two-layer, three-layer, mixed layer and chain structure types. Since there is very little literature on the mixed layer, for the sake of simplicity, the electrokinetic properties of clay minerals will be presented in the order of the most common names, i.e. kaolinite, smectite and palygorskite. Literature data on other specific minerals will be also separately elaborated. The most important types of clay minerals are: kaolinites, smectites, attapulgites, illites and chlorites. Hikes and kaolinites exhibit plate-like particles without expanding lattice due to strong interlayer bonding and strong hydrogen bonding, respectively [22]. Chlorites show positive charge on one layer balanced by an


63

additional negative charge practically with no interlayer water. The composition of clay mineral and the structural arrangement of octahedral and tetrahedral sheets and minerals account for the differences in the electrokinetics properties. Structural arrangements of tetrahedral and octahedral layers and their combinations constitute the layer charge in clay minerals. Some of the important properties of clay minerals pertinent to electrokinetics are presented in Table 2 [23]. Table 2 - Important properties of clay minerals related to electrokinetics Kaolin 1:1 layer Little substitution

Smectite 2:1 layer Octahedral and tetrahedral substitution Minimal layer charge High layer charge Low exchange capacity High exchange capacity capacity

Palygorskite 2:1 layer inverted Octahedral substitution Moderate layer charge Moderate exchange

3.3 - Kaolin Kaolin with an ideal formula of Al4Si4O10(OH)8 exhibits very little substitution in the structural lattice resulting in a minimal layer charge and low base exchange capacity. Linkage of a tetrahedral siloxane layer to one dioctahedral (gibbsite) or trioctahedral layer (brucite) forms the structure of 1:1 layer silicate. While in trioctahedral clay minerals all the three sites are occupied by magnesium ions to achieve charge balance, in dioctahedral layer two out of three positions are occupied by trivalent aluminum ions leading to practically uncharged layers which are held together through van der Waals' forces and partly by hydrogen bonding; kaolinite is a dioctahedral non-expendable clay mineral [4]. Published data on the electrokinetic properties of kaolin minerals are relatively few [24-30]. The iep of kaolinite is reported by several researchers [24-27,31]. Smith and Narimatsu [24] have used both microelectrophoresis and streaming potential techniques and found an iep of 2.2 for the former and no iep for the latter technique (Fig. 3). Cases et al [25] reported an iep of kaolinite at pH 3 and found that the sample to spontaneously flocculate at pH < 3.5 and stabilized at pH >5. The surface charge of kaolinite at pH= 3 did agree with the zeta potential vs. pH profile due to the electronegative character of the (001) faces characterized by OH" or O2" ions; this induced the particles to move under an applied field even at zero surface charge. Five commercial deposits from Georgia yielded iep values in the range of 1.5 to 3.5. The variation in zeta potential values were ascribed to the differences in ionic composition of the clay samples particularly those of exchangeable or soluble calcium ions [27]. Similarly, among the three commercial ball clays (kaolinite rich clay, smectitic kaolinite rich clay and illitic kaolinite rich clay) only kaolinite rich clay displayed a variation as a function of pH whereas the other two clays showed practically constant zp profiles against pH. Zeta potential curves versus pH with 10 mM Mg(NO3)2 and Ca(NO3)2 showed negative values in the entire pH range, the lower zp being about - 25 mV in the neutral and alkaline pH region. Dependence of zp on pH for ripidolite and kaolinite is presented in Fig. 4. Both minerals are characterized by two regions of distinctly different slopes, the steeper slope being below pH= 5.

64

M.S. Çelik

Figure 3 - Zeta potential ofkaolinite and a-Al2O3 as a function of pH; sp-streaming potential, mep-microelectrophoretic mobility [24].

Figure 4 - Zeta potential ofripidolite (U) and kaolinite (M) in 10'3 MNaCl solution as a function ofpH[19].


65

When an alumino- or magnesium silicate layer is disrupted, the valences of the exposed crystal atoms are not completely compensated as they are in the interior of the crystal. These surfaces are called broken-bond surfaces or edge surfaces. The exposed functional groups are very active and may act as electron pair donors or acceptors. The free charges may be balanced by the uptake of cations or anions through either chemisorption or electrostatic attraction [20]. The zeta potential of clay particles inferred from the electrophoretic mobility using the Schmoluchowski equation has been criticized because of the heterogeneous nature of the particle charge. Kaolinite particles are not spherical but exist as hexagonal platelets. Calculation of zp and surface charge densities for non-spherical kaolinite assuming an equivalent sphere may result in quite misleading values. The zp calculated from such mobilities does not reflect the potential at the shear plane because of the screening effect of positive charges on the edges relative to those of negative charges at the faces resulting in a lower negative mobility [32]. Williams and Williams [26] have developed a method of calculating the zp of edge surfaces assuming a linear combination of the zeta potentials of quartz and alumina (Fig. 5).

Figure 5 - Estimation of edge potential by linear combination of quartz and a-alumina [26].

66

M.S. Çelik

They also investigated the effect of preparation techniques, pH and NaCl concentration on the electrophoretic mobility of sodium kaolinite. It is shown that above pH 4 the kaolinite platelets carry a net negative charge at different salt concentrations. A method based on the edge zero point charge is proposed to calculate face zp values. The Gouy-Stern-Graham model was shown to reasonably fit experimental data after making some unrealistic assumption of model parameters. A number of evidences suggest that the edge is composed of silica and alumina layers with a net positive charge at low pH and negative at high pH; the iep value of the edge varies in the range of 5-8. The face, on the other hand, is negatively charged at all pH values due to isomorphous substitutions within the lattice [25]. Delgado at al. [33] using the concept of Williams and Williams [26] together with the formulation of O'Brien and White [34], which considers double layer polarization and surface conductance, predicted the zp of sodium montmorillonite particles reasonably well. The electrokinetic properties of montmorillonite suspensions investigated by Callaghan and Ottewill [35] and Rioche and Siffert [36] indicate a strong pHindependent negative charge on the clay surfaces and a much weaker positively charged double layer on the edges. Kaolinite is composed of pH-independent (permanent) and pH-dependent surface charges [1,37-40]. The kaolinite basal surfaces carry a negative permanent charge ascribed to replacement of Si4+ by Al3+ in the tetrahedral layer of kaolinite and to isomorphic substitution. Because kaolinite has very small CEC (1-5 mequiv/100 g) [41-43], the permanent negative charge on the tetrahedral basal plane is often neglected in model calculations [37]. The pH-dependent surface charge of kaolinite may stem from the following [44]: (i) protonation and deprotonation of aluminol (> A1OH) on the edge; (ii) deprotonation of silanol (> SiOH) on the edge; and (iii) protonation and deprotonation of basal plane hydroxyl groups that are coordinated to two underlying aluminum atoms (> A12OH). The last site which protonates at lower pH is less reactive than the aluminol at the edge. Surface charge models presented on kaolinite tend not to include contributions from basal plane [37]. As most of the basal plane aluminum surface sites (> A12OH) undergo neither protonation nor deprotonation [45], the pHdependent surface charge of kaolinite in the pH range of 3 to 9 is dominated by the surface charge of edges which is equivalent to the difference of the amount of protonated Al edges (> A1OH+) and that of deprotonated Si edges (> SiO"). A plausible evidence for the existence of edge-face interactions in kaolinite suspensions has been observed [46] owing to increased edge surface area of kaolinite platelets relative to their lateral extent [21,47]. The ratio of total aluminol to total silanol edge sites is governed by the kaolinite structure and the electrostatic valence principle of Pauling and appears constant in various kaolinite samples. Ganor et al [44] have shown that the absolute PHPZNPO which is reported in the range of 3 to 7.5 in different kaolinites [1], is about constant. Since at the pHpZNPC only the edge contributes to the charge, changes in the total edge surface area should not appreciably change the pHPZNPC. The discrepancies observed in the 6 titration curves were attributed to the differences in the calculation of PHPZNPC- The authors further emphasized that incorrect assumption of PHPZNPC will introduce error in the calculation of surface potential and of the electrostatic correction factor [44]. Van Olphen [48] put forward that the edge potential is a combination of the potential on the silica and alumina surfaces. A simplified model of the edge utilizing a linear combination of quartz and alumina in the presence of electrolyte predicted a value of 7.2 close to that of actual zpc value. The potentials at low pH were found to


67

agree well with those of Johansen and Buchanan for aluminum [49] silicate composed of about equal proportions of silica and alumina. Several studies have considered the isolated contribution of SiO2 and various aluminum hydroxyl species [1,19,50-51]. The absolute value of negative charge generally depends upon the Si4+/Al3+ substitution and adsorption of charged species onto the clay surfaces.

Figure 6 - The electroakustic zeta potential behavior of kaolinite particles as a function ofAlCU andpH in 10'3 MNaCl with kaolinite volume fraction of 0.02 for wt % AlCl3 levels of(9) 0, (O) 2.5xlO'3, (a) 2.5xlO'2, (A) 0.25, (v) 2.5 [28].

Figure 7 - Zeta potential of ripidollite in Iff2 MNaCl versus pH - (a) natural sample, (M) - sample milled for 6 min. [47]. Electroacustic zp measurements in nondilute kaolinite suspensions against pH and Al3+ concentrations revealed remarkable features for different faces [28].

68

M.S. Çelik

Increasing Al3+ concentration shifted the iep values to higher pH and the zp values became more positive (Fig. 6) At low Al3+ levels, the zp values were represented by the silica-like kaolinite face, pH dependent face, and edge interactions. Sondi and Pravdic [4] have demonstrated a major change in the zp-pH profile of ripidolite and beidellite minerals upon grinding. While no iep was found for both minerals in the absence of grinding, an iep of ripidolite at pH 6 (Figure 7) and that of beidellite at pH 3 [47,52]. These findings were attributed to the increase of fraction of edge surfaces upon mechanical disintegration of clay particles and the corresponding increase in both surface areas and cation exchange capacities and in turn their favorable effect on electrokinetic properties. Indeed, exposure of reactive hydroxyl sites upon milling of ripidolite by a planetary ball mill for 6 min was shown to induce a 12.3-fold increase in specific surface areas and 3-fold increase in cation exchange capacities [47]. Kaolinites exhibit surface acidity due to terminated bonds and structural coordination across the edge faces. The character of acid sites are usually ascribed to Bronsted acidity arising from broken -O-Si-O-Si networks or to Lewis acidity from strained gibbsite layer [43]. Bronsted acid generation may derive either from silanol protonation or aluminol protonation both of which will equally contribute to the generation of acid sites and also to electrokinetic charge. The formation of various aluminum species particularly those of A1(OH)2+ and colloidal A1(OH)3 and their subsequent adsorption onto the solid surface are important in the charging of kaolinite. The effect of a series of metal chlorides of different valency on electrophoretic mobility of kaolinite as a function of their concentration is presented in Figure 8. It is evident that while the zp profiles in NaCl and MgCl2 suspensions exhibit negative charges in the entire concentration range, lanthanum and particularly aluminum ions are capable of reversing the sign of zp. Although both La and Al have the same trivalent charge, Al appears to reverse the charge at much lower concentrations than lanthanum. The presence of highly charged polyhydroxy aluminum species such as A14(OH)2O4+ particularly at pH 6 is plausible based on the species diagram of Aluminum and also the work of Matijevic et al [53]. A change in the valency of anion for potassium salts showed a much smaller effect compared to that of cations, with the effect becoming relatively small as the size of the anion increases from chloride to iodide [54]. In another work of Buchanan and Oppenheim [55]. Comparison of raw and leached kaolinite at pH 2.25 and 6 have yielded different zp-pH profiles, which were attributed to preferential removal of aluminum leaving a dealuminated surface layer. The solution behavior of multivalent metal cations (Mn+) has been extensively covered by various researchers [56,57,7]. The hydrolyzed species will undergo the following modifications in aqueous solution [7]: Mn+ + x O H " » M(OH)x(n-x)+

(10)

M(0H)x(n"x)+ + O H " o M(OH)x+1(nx+1)+

(11)

M(OH)X + +H+ 0 and therefore form stable suspensions with water (hydrophilic character). Some silicates such as talc and pyrophylite, on the other hand, show -ve values of AG. The hydrophobic character of talc is also indicated by high contact angles (75°) and low yAB values (4mJ m"2), both y+ and y" values being low.

Surface Thermodynamics of Clays

97

OTable 1 - Thermodynamic parameters for Wyoming montmorillonite saturated with a specific metal cation and organic cations with different number of carbon atoms and tetraalkyl ammonium ions. Reprinted by kind permission of [Colloid and Surface Properties of Clays and Related Minerals (231-243,105, 2002)]. Swy I cec = 68 meq/lOOg

AG

AG

AG

LW

K

na

NH4

Cs

li

Mg

Ca

Ba

Sr

Nat

-6.6

-7.,1

-6.4

-6.:5

-6.7

-7.1

-6.9

-5.6

-6.2

-6.1

7.6

15 .4

16.2

2.7

7.2

25.1

21.5

-8.1

7.7

11.0

1.02

8.:50

9.80

-4. 14

0.48

18.00

14.65

-13.74

1.43

4.83

iWi

AB

iWi

IF

iWi

« 0 9.9

AG

6 -38 .5

7 -40.3

NUMBER OF CARBON ATOMS" 8 11 12 9 10 -47.4 -52.1 -42.0 -44.0 -25.1

13 -71.3

14 -53.8!

15 -89.6

IF

iWi

0 and therefore form stable suspensions with water (hydrophilic character). Some silicates such as talc and pyrophylite, on the other hand, show -ve values of AG. The hydrophobic character of talc is also indicated by high contact angles (75°) and low y*6 values (4mJ m"2), both y* and y' values being low. Interaction of clays with organic substances, both by adsorption on the surface (e.g., amine) and by ion- exchange (e.g., quaternary ammonium cation) renders the clay mineral hydrophobic (AGIF = -ve). This takes place by the neutralization of Lewis basic sites (O atoms on the surface) by the organic cation. Consequently, there will be a reduction of the term YL+YC in the equations given above causing a reversal of sign on the AG jfwc term resulting in the organo clay becoming hydrophobic (Table 1). 1.5 - Hydrophobic and hydrophilic nature of clays - relation to interparticle free energy. The behaviour of clay minerals to form stable aqueous suspension is also attributed to the layer charge. In the case of silicate minerals the charge at the edge sites

98

B.S. Jai Prakash

and that arising due to isomorphous replacement necessitates the addition of another term AGEL in the equation of total interaction energy between the clay particles immersed in water. AG o - cresol > m - nitro phenol. The Langmuir, dual mode sorption and Redlich - Peterson models were tested to fit the sorption isotherms of single solute systems, whereas the Langmuir competitive model was used to describe bisolute sorption equilibria. Thermodynamic parameters (AH0 and AS0) and the mean free energy (E) for the sorption were determined from the temperature dependence of the distribution constant and the DKR equation, respectively. The value of sorption free energy obtained was indicative of ion exchange. The AH0 values for the sorption of phenol were in the range -3.6 to - 6.7 kj mol"1 and AG values were in the range - 9.7 to -12.8 kJ mol"1. The adsorption showed a gain in entropy. Removal of Cu2+ and Zn2+ from aqueous solutions by sorption on the montmorillonite modified with sodium dodecyl sulphate (SOS) was investigated by Lin and Juang [61]. The DKR parameters were determined. The sorption energy (E) values were -12.6 kj mol"1 for Zn2+ sorption and - 13.8 kJ mol"1 for Cu2+ sorption on the modified clay. This lies in the range of 8 - 1 6 kJ mol"1, indicative of ion-exchange

114

B.S. Jai Prakash

reaction. The sorption capacity in the DKR equation is found to be 31.9 mmol kg"1 for Zn2+ and 83.0 mmol kg*1 for Cu2+ for the clay having a cec of 37.2 mmol / lOOg. The values of AHC and AS0 are 7.39 kJ mol"1 and 6.39 J mol"1 K"1 respectively for Zn2+ and 7.05 kJ mol"1 and 9.09 J mol"1 K"1 respectively for Cu2+. A slightly positive entropy change was attributed by the authors to the fixation of ions on the exchangeable sites of the randomly distributed surfactant species. Doula et al [62] studied the thermodynamics of copper adsorption - desorption by Ca - kaolinite. Both the Freundlich and Gouy - Chapman models were found to describe the adsorption successfully. The adsorption enthalpy ranged from -2.71 to 4.78 kcal mol"1 indicating a physical process. The Cu2+ desorption was endothermic, since an increase in desorption was observed with increasing temperature. It was, however, observed by the authors that the calculated AH0 values include the enthalpies of hydration, mixing and exchange. Because of uncertainties about the energy of hydration of the adsorbed -desorbed ions, most of these terms cannot be calculated with a high degree of accuracy. Similar observations have been made on K -Ca and Mg - Ca exchange on the surface of kaolinite soil clay by Udo [63] and in calcium-bentonite clay byDoulaetal[64]. A study has been made recently to adsorb lead ions on to surfactant immobilized interlayered species bonded to clays (SUS-Clay) by Mahadevaiah et al [67]. Chromate immobilized by HDTMA surfactant was found to easily adsorb lead ions. The magnitude of DKR sorption energy (E) of lead ions on SHCr- montmorillonite was found to be 51.5 kJ mol"1. This value is greater than the order expected of an ion exchange mechanism [59] showing that the adsorption may be due to precipitation. The adsorption capacity Cm at the DKR region is calculated to be 119 mmol, kg"1 for the adsorption of lead ions which is less than the adsorption observed at the Langmuir region [67] 3 - Conclusions Probing the surface of a solid by measuring contact angle at the solid-liquid-air interface using the famous Young-Dupre equation which relates the surface tension to thermodynamic parameters has been extended to the study of surface of clays and related minerals. The surface tension components theory of van Oss - Good -Chaudhury has been successful in interpreting the contact angles and distinguishing the Lifshitz-van der Waals ) and the acid-base forces (y+ and y") on the surface. The average value of LW y , y" and y" for clay surfaces in mJ m"2 are, respectively 43, 1.6 and 36.9. The higher value of y indicates the predominance of the oxygen of the surface hydroxyl group in donating a pair of electrons. The magnitude of y+ is also indicative of the high tendency of the surface to hold through van der Waals forces. The sum total of the non covalent interactions between surfaces of particles immersed in water are designated as interfacial (IF) interactions. The interfacial free energy between clay particles immersed in water (AG c w c = AGLW + AGAB) are mostly positive with an average value of +10.6 mJ m"2 indicating hydrophilic character of clays. The value changes to negative when the clay surfaces are modified with organic molecules. Negative values of interfacial free energy is a quantitative measure of the degree of hydrophobicity of the clay. Thus organo clays are hydrophobic indicating their tendency to flocculate. A value of y" = 27.9 mJ m"2 is fixed as the boundary between hydrophobic (0.3. 1 0 - Summary At the microscopic level, three main interactions can be considered. Firstly; the dispersive interaction, which can be attributed to a "network", i.e. to the clay considered as a whole. The complexity of the sandwiching enhances the attraction force. The lamellarity also has an influence here, due to an increase of dispersive force with increasing of the dimensions of the sheet. Secondly, the donor-acceptor interaction, which occurs between polar groups and water. The active surface polar groups of clays are mainly silica rings. Associated with this siloxane surface is a roughly hexagonal cavity, formed by the bases (triads of oxygen ions) of six corner-sharing silica tetrahedra. The reactivity of the siloxane surface depends on the structure where it is involved and on the value of the charge in the clay mineral layer. If cation substitutions are absent from the underlying layer structure, a siloxane surface will function as a weak electron donor. If substitution of Al3+ by Mg2+ occurs in the clay layer, the resulting excess of negative charge distributed over several surface oxygens increases the electron donor effect. If isomorphic substitution of Si4+ by Al 3+ occurs in the tetrahedral sheets, the excess negative charge is greater and the electron donor effect is stronger. If the hydrogen of water is pointed towards the clay surface, the interaction is easy between the water LUMO and the clay HOMO localized on oxygens of siloxanes. Therefore, it is probably the favored

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situation. Concerning electron acceptor effect, it is dubious to state that they will be localized upon some surface groups. Something looking like an hydrogen bond can probably be formed between the oxygen of the incoming water and the LUMO of the clay surface. When cations are present, the electrostatic forces are predominant. This explains why in theoretical simulations, water molecules are grouped around interlayer cations. Energies of the different surface hydroxides have been widely investigated [128-132] in terms of acid-base strengths or hydrophilic character. However, it appears that the hydrophilicity of clays cannot be related to the number of hydroxyls located on the surface. The poor hydrophilic character of talcs presenting siloxane surfaces and where the silanol groups are absent casts a shadow on this reasoning. However, the reactivity of the siloxane is greatly influenced by its environment. Therefore, the main reason for the poor hydrophilic character of talc is more its hardness than the absence of hydroxyl groups. The third major type of interaction is due to cations. Water adsorption onto cations located inside sites creates two competitive charge transfers. The first is between the electrons of an oxygen atom from the solid, and a hydrogen atom from a water molecule. The second is between the cation and the oxygen atoms of water molecule. These two effects compete with the initial transfer between cation and oxygen atoms of the solids, and facilitate the extraction of the ion. The low level of extraction energies measured by TSDC shows this. Adsorption at high partial pressure is due to direct interactions between water vapor molecules and adsorbed molecules. The energies are the order of liquefaction energies and the limiting effect is the structure of the first layer, which is imposed by the heterogeneity of the solid surface. Some samples with "middle" energy surfaces can adsorb high quantities of water probably because they present on a same plane electron donor sites at appropriate distances compared to the liquid water structure. Approximate but realistic energies of surface and of adsorption can be computed from theoretical models using crystallographic neutron data. The main theoretical problem remains the estimation of the energy of the individual surface groups on real clay samples. 11 -References [1] R.C. Mackenzie, De Natura Lutorum, in Proceedings of the Eleventh National Conference on Clays and Clay Minerals, Pergamon Press, pp. 11-28, 1963. [2] S. Guggenheim and M. T. Martin, Clays and Clay Miner., 43 (1995) 255. [3] P.A. Schroeder, Web site: http://www.gly.uga.edu/schroeder/geol6550 /reservebook.html. [4] S.W. Bailey, Summary of Recommendations of AIPEA Nomenclature Committee, Clays and Clay Miner., 28 (1980) 73 [5] S.W. Bailey Ed., Reviews in Mineralogy, vol. 13: Micas, Series Editor P. H. Ribbes, Mineralogical Society of America Publications, 1984. [6] W.A. Deer, R.A. Howie and J. Zussman, Rock Forming Minerals, 2nd ed., Longman, 1986. [7] S.W. Bailey Ed., Reviews in Mineralogy: vol. 19: Hydrous Phyllosilicates (exclusive of Micas), Series Ed. Paul H. Ribbes, Mineralogical Society of America Publications, 1988. [8] M.J. Singer and D.N. Munns, Soils an introduction, MacMillan Publishing Company, 1987. [9] L. Yan, C.B. Roth, and P.F. Low, Langmuir 12 (1996) 4421.

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[10] Web site: http://mineral.galleries.com/minerals/silicate/class.htm. [11] S.A. Nelson, Web site: http://www.tulane.edu/~sanelson/geol211 /clayminerals.pdf. [12] Web site: http://www.man.ac.uk/Geology/MineralWeb/MineralWeb.html. [13] G.W. Brindley, G. Brown, Crystal structures of Clay Minerals and Their X-Ray Identification, Mineralogical Society, London, 1980. [14] A.W. Adamson and A.P. Gast, Physical Chemistry of Surfaces, Wiley, New York, 6th ed., 1997. [15] W. Rudzinski. and D.H. Everett, Adsorption of Gases on Heterogeneous Surfaces, Academic Press, London, 1992. [16] R.K. Her, The Chemistry of Silica, John Wiley and Sons, New York, 1979. [17] Z. Zhou and W.D. Gunter, Clays and Clay Miner., 40 (1992) 365. [18] M.J. Sparnaay, The Electrical Double Layer, Pergamon Press, Oxford, 1972. [19] J.A. Greathouse, S.E. Feller, and D.A. McQuarrie, Langmuir, 10 (1994) 2125. [20] P. Hobza, J. Sauer, C. Morgeneyer, J. Hurych and R.J. Zahradnic, J. Phys. Chem., 85(1981)4061. [21] W.F. Bleam, Clays and Clay Miner. 38 (1990) 522. [22] J. Lyklema, Fundamentals of Interface and Colloid Science, Academic Press, New York, 1995. [23] C.S. Johnson, Jr. and D.A. Gabriel, Laser Light Scattering, Dover, New York, 1981. [24] S.J. Gregg and K.S.W. Sing, Adsorption, Surface Area and Porosity, Academic Press, London, 1982. [25] J. Rouquerol, D. Avnir, C.W. Fairbridge, D.H. Everett, J.M. Haynes, N. Peraicone, J.D. Ramsay, K.S.W. Sing and K.K. Hunger, Pure Appl. Chem., 66 (1994) 1739. [26] D.H. Everett, Basic Principles of Colloid Science, Royal Society of Chemistry, 1988. [27] R. Defay, I. Prigogine, A. Bellemans and D.H. Everett, Surface tension and adsorption, Longmans, London, 1966. [28] R.G. Linford, Chem. Rev., 78 (1978) 81. [29] V. Medout-Marere, H. Belarbi, P. Thomas, F. Morato, J.C. Giuntini and J.M. Douillard, J. Colloid Interface Sci., 202 (1998) 139. [30] C.H. Bridgeman, A.D. Buckingham, N.T. Skipper and M.C. Payne, Mol. Phys., 89 (1996) 879. [31] B.J. Teppen, K. Rasmussen, P.M. Bertsch, D.M. Miller and L. Shafer, J. Phys. Chem. B, 101,(1997)1579. [32] A. Delville and M. Letellier, Langmuir, 11 (1995) 1361. [33] J.W. Gibbs, The Collected Works of Josiah Williard Gibbs, Green and Co, London, 1928. [34] J.D.V. Van der Waals und P. Kohnstamm, Lehrbuch der Thermostatik, Leipzig, 1927. [35] J.S. Rowlinson and B. Widom, Molecular Theory of Capillarity, Clarendon Press, Oxford, 1982. [36] J.W. Cahn and J.E. Hilliard, J. Chem. Phys., 28 (1958) 258 [37] E.A. Guggenheim, Thermodynamics, 3 ed., North-Holland Publishing Company, Amsterdam, 1957. [38] J.C. Ericksson, Arkiv. fur Kemi, 25 (1965) 331; 25 (1965) 342; 26 (1966) 49. [39] A.I. Rusanov, Progress in Surface and Membrane Science, Academic Press, New York, 1971.

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[40] E. Tronel-Peyroz, J.M. Douillard, L. Tenebre, R. Bennes and M. Privat, Langmuir, 3 (1987) 1027. [41] M. Privat, R. Bennes, E. Tronel-Peyroz and J.M. Douillard, J. Colloid Interface Sci., 121 (1988) 198. [42] E. Tronel-Peyroz, J.M. Douillard, R. Bennes and M. Privat, Langmuir, 5 (1989) 54. [43] J. Fraissard and C.W. Conner, Eds., Physical Adsorption: Experiment, Theory and Applications, Nato ASI Series, Kluwer AC, Dordrecht, 1997. [44] T.L. Hill, J. Chem. Phys. 17 (1949) 520, J.A.C.S. 74 (1952) 1598. [45] J.M. Douillard, Immersion Phenomena, Interfacial Dynamics, (Surfactant Science Series/88), Ed. N. Kallay, Marcel Dekker, New York, 1999. [46] J.M. Douillard, J. Colloid Interface Sci., 188 (1997) 511. [47] R.G. Pearson, Chemical Hardness, Wiley-VCH, New York, 1997. [48] C.S. Brooks, J. Phys. Chem., 64 (1960) 532. [49] W.A. Zisman, Adv. Chem. Ser., vol.43: Contact Angle, Wettability, and Adhesion, Ed. R.F. Gould, p. 1, A.C.S., Washington DC, 1964. [50] J.M. Douillard and V. Medout-Marere, J. Colloid Interface Sci., 223 (2000) 255. [51] D.Y. Kwok, D. Li, and A.W. Neumann, Langmuir, 10 (1994) 1323. [52] J.M. Douillard, J. Zajac, H. Malandrini and F. Clauss, J. Colloid Interface Sci., 255 (2002)341. [53] D. Aspnes, Optical Properties Of Solids - New developments, Chap. 15: Spectroscopic Ellipsometry of Solids, Ed. B. Seraphin, North-Holland, 1976. [54] V. Laperche, J.F. Lambert, R. Prost and J.J. Fripiat, J. Phys. Chem., 94 (1990) 8821. [55] M.B. Me Bride, T.J. Pinnavaia and M.M. Mortland, J. Phys. Chem., 79 (1975) 2430. [56] N.T. Skipper, M.V. Smalley and G.D. Williams, A.K. Soper, C.H. Thompson, J. Phys. Chem., 99 (1995) 14201. [57] J. Rouquerol, K.S. Sing, F. Rouquerol, Adsorption by Powders and Porous Solids, Elsevier, 1999. [58] B. Fubini, Thermochimica Acta 135 (1988) 19 [59] A. Saada, E. Papirer, H. Balard and B. Siffert, J. Colloid Interface Sci., 175 (1995) 212. [60] H. Balard, O. Aouadj, E. Papirer, Langmuir, 13 (1997) 1251. [61] H. Balard, Langmuir, 13 (1997) 1260. [62] J.T. Yates Jr. and T.E Madey, Eds., Vibrational Spectroscopy of Molecules on Surfaces, Plenum Press, New York, 1987. [63] A. Auroux and A. Gervasini, J. Phys. Chem., 94 (1990) 6371. [64] J. Vanderschueren and J. Gasiot, Thermally Stimulated Relaxation in Solids, Chap.: Field-Induced Thermally Stimulated Currents, Ed. P. Braunlich, pp. 135-223, Springer-Verlag, New York, 1979. [65] S. Devautour, J.C. Giuntini, F. Henn, J.M. Douillard, J.V. Zanchetta and J. Vandeschueren, J. Phys. Chem. B, 103 (1999) 3275. [66] V. Medout-Marere, H. Belarbi, A. Haouzi, J.C. Giuntini, J.M. Douillard, J.V. Zanchetta and J. Vanderschueren, J. Colloid Interface Sci., 223, (2000) 61. [67] P. Comba, T.W. Hambley, Molecular Modeling of Inorganic Compounds, WilerVCH, Weinheim,2001. [68] J.B. Foresman, A. Frisch, Exploring Chemistry with Electronic Structures Methods: A Guide to Using Gaussian, Gaussian Inc. Pub., 1993. [69] G.H. Grant and W. Graham Richards, Computational Chemistry, Oxford Science Publications, Oxford, 1995.

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[70] F. Frank Jensen, Introduction to Computational Chemistry, John Wiley & Sons, 1999. [71] A.R. Leach, Molecular Modelling: Principles and Applications, 2nd Edition, Prentice Hall, 2001. [72] R.T. Cygan and J.D. Kubicki, Eds., Reviews in Mineralogy and Geochemistry, Vol. 42: Molecular Modeling Theory: Applications in the Geosciences, Mineralogical Soc. America Ed., 2001. [73] R.G. Parr, W. Yang, International Series of Monographs on Chemistry, n° 16: Density-Functional Theory of Atoms and Molecules, Oxford University Press, N.Y., 1994. [74] S. Hwang, M. Blanco, E. Demiralp, T. Cagin, and W.A. Goddard III, J. Phys. Chem. B, 105(2001)4122. [75] B.J Teppen, D.M. Miller, S.Q. Newton, L. Schafer, J. Phys. Chem., 98 (1994) 12545. [76] N.T. Skipper, F.R Chou Chang, and G. Sposito, Clays and Clay Miner., 43 (1995) 285. [77] S.H. Park and G. Sposito, J. Phys. Chem. B, 104 (2000) 4642. [78] J.A. Greathouse, K. Refson, and G. Sposito, J.A.C.S., 122 (2000) 11459. [79] R. Sutton, G. Sposito, J. Colloid Interface Sci. 237 (2001) 174. [80] S.H. Park and G. Sposito, Phys. Rev. Lett., 89 (2002) 85501. [81] M.L. Huggins and J.E. Mayer, J. Chem. Phys., 1 (1933) 643. [82] R. Car, M. Parinello, Phys. Rev. Lett., 55 (1985) 2471. [83] CM. Bertoni, A.I. Shkrebtii, R. Di Felice and F. Finocchi, Progr. Surf. Sci., 42 (1993)319. [84] J.E. Huheey, E.A. Keiter, R.L. Keiter, Inorganic Chemistry: Principles of Structure and Reactivity, Harper Collins, 1993. [85] J.M. Douillard and M. Henry, J. Colloid Interface Sci., 263 (2003) 554. [86] M. Henry, Solid State Sci., 5 (2003) 1201. [87] F. Zigan, R. Rothbauer, Neues Jahrbuch fuer Mineralogie, 4 (1967) 137. [88] H. Saalfeld, M. Wedde, Zeitschrift fuer Kristallographie, 139 (1974) 129. [89] M.T. le Bihan, A. Kalt, R. Wey, Bull. Soc. Franc. Min. Crist., 94 (1971) 15. [90] B. Perdikatsis, H. Burzlaff, Zeitschrift fuer Kristallographie, 156 (1981) 177. [91] M. Mellini and P. F. Zanazzi, Am. Min., 72 (1987) 943. [92] R.A. Young, A.W. Hewat, Clays and Clay Miner., 36 (1988) 225. [93] S.I. Tsipursky and V.A. Drits, Clay Miner. 19 (1984) 177. [94] R. Calvet, Thesis, Hydratation de la Montmorillonite et Diffusion des Cations Compensateurs, Paris 6 University, 1972. [95] D.O. Nelson, S. Guggenheim, Am. Min., 78 (1993) 1197. [96] J.M. Douillard, V. Medout-Marere, Acid-Base Interactions: Relevance to Adhesion Science and Technology, vol. 2, Surface Energy and Acid-Base Properties of Solids studied by Immersion Calorimetry, pp. 317-347, Ed. K. Mittal, VSP, The Netherlands, 2000. [97] H. Chermette, J. Comp. Chem., 20 (1999) 129. [98] M. Henry, in "Modelling of Minerals and Silicated Materials", Eds. B. Silvi and P. d'Arco, pp. 273-334, Kluwer Ac. Pub., The Netherlands, 1997; Chem. Phys. Chem., 3 (2002) 561; Chem. Phys. Chem., 3 (2002) 607. [99] L.C. Allen, J.A.C.S., 111 (1989) 9003. [100] J.B. Mann, T.L. Meek, and L.C. Allen, J.A.C.S., 122 (2000) 2780. [101] J.B. Mann, T.L. Meek, E.T. Knight, J.F. Capitani, and L.C. Allen, J.A.C.S., 122 (2000)5132.

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[102] W. Hertl and M.L. Hair, J. Phys. Chem., 72 (1968) 4666. [103] T. Morimoto, M. Nagao and J. Imai, Bull. Chem. Soc. Jpn., 44 (1971) 1282. [104] C. Contescu, J. Jagiello and J.A. Schwarz, Langmuir, 9 (1993) 1754. [105] R. Chi, Z. Xu, T. Difeo, J.A. Finch, J.L. Yordan, J. Pulp Pap. Sci., 27 (2001) 152. [106] R. Keren and I. Shainberg, Clays and Clay Miner., 28 (1980) 204. [107] I. Berend, J. M. Cases, M. Francois, J.P. Uriot, L. Michot, A. Masion and F. Thomas, Clays and Clay Miner., 43 (1995) 324. [108] B. Janczuk, E. Chibowski, M. Hajnos, T. Bialopiotrowicz and J. Stawinski, Clays and Clay Miner., 37 (1989) 269. [109] M. E. Schrader and S. Yariv, J. Colloid Interface Sci., 136 (1990) 85. [110] J. Burgess, Ions in Solutions, Ellis Horwood, 1988. [111] H. Malandrini, Ph. D. Thesis, Montpellier (France), University of Sciences, 1995. [112] J. M. Douillard, H. Malandrini, T. Zoungrana, F. Clauss, S. Partyka, J. Therm. Anal., 41 (1994)1205. [113] J. Zajac, H. Malandrini, Pol. J. Chem., (1997) 686. [114] A. Delville, J. Phys. Chem., 99 (1995) 2033. [115] J. Sauer, P. Hobza and R. Zahradnik, J. Phys. Chem., 84 (1980) 3318. [116] N. T. Skipper, K. Refson and J. D. C. Me Connel, Clay Miner., 24 (1989) 411. [117] H. Malandrini, F. Clauss, S. Partyka and J.M. Douillard, J. Colloid Interface Sci., 194(1997)183. [118] H. Malandrini, R. Sarraf, B. Faucompre, S. Partyka and J.M. Douillard, Langmuir, 13 (1997) 1337. [119] V. Medout-Marere, A. el Ghzaoui, C. Charnay, J.M. Douillard, G. Chauveteau and S. Partyka, J Colloid Interface Sci., 223 (2000) 205. [120] V. Medout-Marere, S. Partyka, R. Dutartre, G. Chauveteau+ and J.M. Douillard, J. Colloid Interface Sci., 262 (2003) 309. [121] J.J. Fripiat, A. Jelli, G. Poncelet and J. AndrP(Ci)2+

(Eq. 7)

since this reaction differs from Eq.(5) it demands a different definition for the binding coefficient for the formation of a charged complex of a monovalent organic cations. Introducing into it the concentration of a neutral complex as calculated using (Eq.4), thus: Ki =

[P( C l ) 2 J [PCi][Ci(0n

[P(CQ2 ] Ki[P][Ci + fy(0) 2

(monovalent organic cations only) (Eq. 8)

Apparently, the same model should have been used for divalent organic cations, thus: PCi+Ci(0) + + » P ( C i ) 2 + +

(Eq. 7b)

However, Rytwo et al [22] had shown that charged complexes for adsorbed divalent organic cations as diquat and paraquat, arise from the binding of one divalent molecule to one monovalent site, as shown in Eq. (5) and Fig. (2e). It should be emphasized that in Eqs (3)-(8) the concentrations of the cations should be obtained near the surface of the clay platelet. This is not the concentration at the equilibrium solution. Since the distribution of the cation as a function of their distance from the surface, depends on the electric potential, we may use Boltzmann's equation:

[Ci(x)] = [Ci(oo)]e

kT

(Eq.9)

were e is the absolute magnitude of an electronic charge, q>0 is the potential at the surface of the sorbent, k is Boltzmann's factor, and 7 is the absolute temperature. For the sake of

Adsorption/Desorption of Ions on Clay Surfaces

163

simplicity we may define a variable named y(x), were x denotes the distance from the surface as:

y(x) = e

kT

(Eq. 10)

and Eq.(9) at the surface of the platelet (x=0) may became: [Ci(0)] = [Ci(a>)].y(0)Zl

(Eq. 11)

where y(0) denotes the value ofy at the surface of the sorbent. Thus, for each ion its concentration near the surface may be calculated upon knowing its concentration in the equilibrium solution, Ci(<x>), and the potential of the surface which yields a specific value of y(0). The value obtained for Ci(0) might be used afterwards to evaluate all complexes of the cation in case, using Eqs. (3)-(8). In summary, for each cation in the system the following details are needed: a. The valency of the cation, Zi b. Total concentration of the cation in the whole system, Cilol c. Binding coefficient of the cation for the formation of a neutral site, Kt d.

Binding coefficient of the cation for the formation of a charged site, Kt

In the worksheet version of the model, all those parameters are introduced in "ions input" worksheet. For each cation in the system there is a line in the worksheet named "output", that contains all the details about it: those that were introduced as input (name, valency, total concentration and binding coefficients), those that represent the different complexes formed (neutral and charged) and the surplus of each cation in the double layer, which is described in the next section. More specific versions of the model had considered also the formation of mixed complexes between two different monovalent organic cations, aggregation of organic cations in solution [18], adsorption to neutral sites [20] and even adsorption on two different type of charged sites, with different surface potential, PT and A [41, 23]. Each different complex might be added to the model, upon considering its influence on the mass balance of the cations in case, and the changes in the surface potential. 2.3 - Cations in the diffuse layer In the equilibrium solution there is equality between the amount of negative and positive charges. Near the negatively charged surface, we will usually find a surplus of cations neutralizing the surface charges. The assumptions in Gouy-Chapman theory lead to the conclusion that all ions with the same valency behave exactly the same in the double layer. This assumption is merely an approximation, as Cs+ will neutralize sites more effectively than Na+, due to it less tightly attached hydration shell. However, considering the basic assumption as correct, the proportion between all the Zi valent ions in the equilibrium solution will be exactly the same as in the double layer. For example: If Na+ is 50% of the monovalent cations in the bulk solution, it will still remain 50% of the monovalent cations in the diffuse layer, although the exact amount will obviously not be the same, due to the amplification caused by the negative potential of the charged surface. The same arguments can be used for anions, and the only difference is that their concentration near the surface will be lower than at infinity. If we define as Qz the surplus of the z-valent ions, then we might define Di, the excess

164

G. Rytwo

concentration of the Z valent ion / in the double layer region above the equilibrium concentration, were Z can be any integer number between (-2) and (+4) except 0, (thus can be either a mono or divalent anion or a mono-, di-, tri or tetra-valent cation) as: [Di] = Q z ^ C i ( c ° ) ]

(Eq-12)

were XCi(z>(°o) will be the sum the concentrations of all the Z valent ions in the equilibrium solution. The quantities Qz are a result of an integration of the excess of cations of valency Z over the double layer region. When the system includes only mono and divalent cations, and one type of anions, they can be calculated analytically [11]. For the general case, that will be described here, these values can be obtained numerically [42]. We define as Csj the excess of ions of type ;' in the double layer region above the equilibrium concentration. As said before, this excess would be the sum over the whole width of the double layer, denoted as d, of the concentration of ions / at the segment dx width, (Cj(x)) minus the concentration of those ions at the segment at infinite distance of the surface (Cj(oo)). Reducing dx to an infinitesimal value, would transform the summation into an integration. Introducing the relationship between the concentration as a function of the potential (Boltzmann distribution, Eq.(9)) and the definition of y(x)(Eq.(lO)) we may write: J

d

CS1 = J(C,(x)-C,(*))dx=C,(<x>) J(y(x)Zl -l)dx 0

(Eq- 13)

0

Thus, if we will be able to obtain a value for y(x) for any distance from the surface x, the integration in Eq.(13) can be made numerically. If a function y(x) is given, then evaluations of its value for any x can be made by developing the function to a Taylor series:

y(x+ h) = y(x) + hg + ^-§ + .... dx

(Eq.14)

2! dx

we may begin at the surface of the sorbant, and advance each time and use y(x) to calculate y(x+h), providing we know the value of y(0) and at least the first and second derivative of y(x). The full development of the first derivative is presented in Section 2.10. The first derivative ofy(x) is: ^tyCx^Cl^C.MiyW21-!)

(Eq. 15)

the sign + is essential due to the square root operation. Cl is a constant that depends on the units in use. When non-rationalized electrostatic units are used, C12=—— To make EkT the second derivation easier we will rewrite after assigning:


S y =£c,(»)(y(x) Zi -l)

165

(Eq. 16)

Eq. (15) then becomes: y" y

dx

2

(Eq. 15b)

and the second derivative ofy can be easily calculated from that, yielding:

2

dx

s " 2 ^ + y(x).S - - - I . ^ ^ l [

y

dx

yy

'

y

(Eq. 17)

2 dy dx\

and from the definition of Sj, (Eq. 16), we can calculate: ^=yZi-Ci(oo)-y(x)Zi-1 dy *—'

(Eq. 18)

and infroduce the value in (17). By knowing the first and the second derivative of y we can evaluate y(x) at any value of x using Eq.(14), if an initial value of y(0) is known. Although the calculations are only approximate, the use of the second term of the Taylor series, and a small length step (h), minimizes the error. The value ofy(x) at any x allows to perform the numerical integration in Eq. (13). The values of Qz for the different valencies can then be calculated, by summing the results of Eq.(13) for all ions with valence z. In the worksheet version of the model, double layer calculations are numerically performed in "double layer" worksheet. The length of the step h is limited to a maximum value of 0.2 nm, but close to the platelet it calculated as a function of the first derivative of y(x) in order to account for sharp changes when the slope is steep. The numerical integration is performed up to a distance of approximately 80 nm from the surface. Such distance is long enough even for very low ionic strengths that yield relatively high surface potentials. Closure of the system was achieved even at values of y(0)=300 (equivalent to cpo=-15O mV). In any case, width of the double layer may easily be increased by adding rows in the relevant worksheet, although that increases slightly the time of each calculation. After obtaining values of Qz for all valencies, the surplus in double layer of each ion can be calculated using Eq.(12), if the equilibrium concentrations of all ions are known. 2.4 - Mass balance of ion /'. Most ion adsorption models calculate the adsorbed amounts by taking the equilibrium concentration as known. However, for most applications we are interested in evaluating sorbed amounts as a function of the total amount of nutrient or pollutant spilled over the system.

166

G. Rytwo

Figure 3 - Illustrative representation of the different contributions to the total amount of a monovalent ion. Table 2 -Relevant parameters for each ion in the model Relevant equations (2.1-2.5) 3,4,9, 11, 13, 15, 16, 18, 19b 9, 11, 12, 13, 15, 16, 18, 19, 19b 3,4,5,6, 11,

Parameter

Symbol

Units

Remarks

Valence of the ion

Zi

absolute integer

external input

Concentration of the ion at equilibrium solution

Ci(«)

M

calculated

Ci(0)

M

calculated

Ci,o,

M

external input

19, 19b

Ki

M"1

external input

4,19b

Ki

M"1

external input

6, 8, 19b

Pci

M

PC i

M

P(Ci) 2 +

M

Di

M

calculated

12, 19

Qz

M

calculated

12, 19b

calculated

12, 19b

Concentration of the ion near surface total concentration of the ion binding coefficient for neutral complex binding coefficient for charged complex neutral complex Charged complex of di or trivalent Charged complex of monovalent surplus of ion at diffuse layer Proportion factor of Z valency ions surplus in diffuse layer sum of al Z valent ions in the equilibrium solution

calculated using Ci(0), K, P" and Zi calculated using Ci(0), K, P" calculated using Ci(0), K, PCi

3,4,6, 19 5, 19 7 8


167

Clearly, if the amount of total cation is constant, sorption processes will reduce the equilibrium concentration, thus- the latter is not known a-priori in most cases. The model presented here considers the system as closed- thus a mass conservation balance for each ion in the system is constantly calculated, and adsorption of a molecule causes a reduction of its concentration in the equilibrium solution, and vice-versa. The total amount of each cation ,Cilot, includes (see Fig. 3): a. b. c. d.

the sum of the ions in the equilibrium solution- Ci(ao), the amount bound as neutral complexes- PCi, (Eq.(4)) the amount bound as charged complexes- PCpi, (Eq.(6) or (8), depending in the valency of the cation) the surplus of the specific cation in the diffuse layer around the sorbent- Di (Eq. (12)).

If the total amount of a cation is known, the concentration at equilibrium can be calculated by evaluating the mass balance of the cation: [Ci tot HCi(»)]+[PCi]+[PC p i]+[Di]

(Eq. 19)

In previous versions of the model, Ci(ao)v/as specifically calculated. For example, for the case of a divalent cation, Eq.(19) would become: [Ci tot ] = [Ci(»)]+Ki^[Ci(»)]y 0 Z l +Ki[p-][Ci(«)]y o zi

+ Q z

=i^L(Eq.l9b)

As can be seen, if the amount of empty sites F, the potential at the surface (and y(0), that is related to it by Eq.(lO)), Qz and the concentration of all other ions is known the equilibrium concentration can be isolated and calculated from a quadratic equation based on Eq.(19b). In the worksheet version of the model, the calculation of Ci(<x>) is numerically performed, based on the "goal seek" function present in worksheet programs. This procedure is fast and accurate, and allows fast convergence of the model. Table 2 concentrates all the relevant parameters for each ion. Additional species (i.e., complexes between ions, dimers, aggregates, binding to another type of sites, etc.) might be introduced to the system, as long as they are also considered in the mass balance for each relevant ion. 2.5 - Evaluating the potential at the surface and y(0). As can be understood from previous sections, the potential at the surface is essential in order to evaluate all other parameters: without it, it would be impossible to calculate the concentration at the surface needed for the Stern layer, neither the distribution in the double layer needed for Di. Without all those, the mass balance would not be calculated. This specific model is based on the Gouy-Chapman equation. Details on the steps needed for the development of the equation can be obtained in Singh and Uehara [33]. In the worksheet model, the parameter evaluated is y(0) which is a function of physical constants and the potential at the surface, q>(0) (see Eq.(lO)). Although there are several studies and models based on the same equation, different notations might be found for the Gouy-Chapman equation. In order to clarify the apparent discrepancies, we will quote three different examples:

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G. Rytwo

g2=2sRTC,

7i

(oo) r . ^ ZiF(p(0) ^ V 2RT )

(

-ZiF(p(0)

basgd

md

Uehara

[33]

(Eq 20a)

N

RT _ j

e

on Singh

a2=—^ni(oo)(y0Zl-l)

based on Obi etal [43]

(Eq. 20b)

based on Nir etal [4]

(Eq. 20c)

In all the three versions, \ (Eq. 39)

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G. Rytwo

Since the concentration of empty sites will be in all the relevant cases considerably less than 1 M, thus [P"]«l, that means that KA>1. Evaluating the free energy of such reaction, presented in Eq.(36), and using AG = -RTlnK, will lead to the conclusion that the formation of multiple charged sites is thermodynamically favored, since AGc,si), the proton polarisation is transferred to the rare spin nucleus. After the removal of the BiCorSi field, the 13C (or 29Si) signal is detected while pursuing the 'H irradiation. The theoretical gain of sensitivity is proportional to the ratio of the gyromagnetic ratios TH/YCSI- The Hartman - Hahn condition is strictly valid for static spectra but remains a good approximation for spinning rates at the magic angle typically lower than ca. 5kHz. Fast MAS could be very useful in many cases and the above condition is no more fulfilled. Instead, polarisation transfer occurs at different frequencies, symmetric to the position of the Hartman - Hahn condition. As the relevant side bands are narrower than that in the static case, a careful calibration is required to get the maximum effect. Proton polarisation can also be transferred to quadnipolar nuclei. A recent paper deals with all these aspects [78]. During the cross polarisation sequence, the 13C magnetization M increases exponentially with the time constant T C H whereas the decrease of the proton magnetization with the relaxation time in the rotating frame (T]p(H)) leads to smaller cross polarisation reducing the 13C signal intensity. The variation of the carbon magnetization is ruled by the equation: M(t) = Mo [exp(-t/Tlp(H) - exp(-t/TCH)]/ [1 - TCH(Tlp(H))-1] (Eq. 10) where t is the contact time and Mo is the equilibrium magnetization. The cross polarisation is most efficient for static 13C (29Si) - 'H dipolar interaction and mobile carbon groups exhibit greater TCH values. This equation is valid assuming TCH/TI P (H) and TCH/Tlp(C) ratios are small. A full analysis of the plot of the magnetisation as a function of the contact time is required for quantitative measurements. Indeed, the intensity ratio changes drastically with the value of the contact time (figure 5). By contrast the equilibrium magnetization ratio from the two curve fittings is equal to 2.1, very close to the theoretical ratio. All these aspects are summarized in a recent review [79]. 13 C CP MAS NMR experiments are used to probe the molecular structure and dynamics of organic cations intercalated in smectites. One of the first study includes complexes of tetramethyl ammonium (TMA) and hexadecyltrimethylammonium (HDTA) cations with a montmorillonite and a vermiculite that have iron contents of 3.4 and 7.4%, respectively. Rapid motion is detected in these complexes, as seen from the averaging effect of the ! H - 13C heteronuclear dipolar interaction. The cation motion appears to be isotropic and anisotropic for intercalated TMA and HDTA, respectively [80]. The time constant TCH has been used to probe mobility of the dimethyldistearylammonium (DDSA) ion intercalated in montmorillonite. Here, the steric effects seem to be predominant since T C H values < 200 us indicate low amplitude and/or frequency of the CH2 motion. The adsorption of methanol vapor on the organoclay leads to larger values. A decrease of the dipolar interaction between the more distant CH2 groups of two neighbouring chains, brought by methanol insertion, enhances alkyl chain mobility [81].

233

Nuclear Magnetic Resonance Spectroscopy

The conformational heterogeneity and mobility of 1-octadecylamine (ODA) intercalated in montmorillonite has been described by NMR relaxation techniques and 2D WISE NMR (dispersion of proton lines along the 13C chemical shift scale).

Figure 5 - Plot of the signal intensity as a function of the contact time (DHEMHA: solid line: NCH3; dashed line: CH2OH) The proton line widths can directly reflect the mobility of the surfactant chain, supporting information from the TCH values. The main signal in the 35 - 30 ppm range is assigned to the inner CH2 groups of the long alkyl chain. The chemical shift of these groups depends on the conformation of the two y positions: trans - trans, trans - gauche, gauche - trans or gauche - gauche. The signals near 34 and 32 ppm are assigned to the trans and gauche conformations, respectively. The total content of gauche conformation has been calculated to be approximately 18%. The trans conformer is more rigid as indicated by shorter TCH values and broader line widths [82]. Table 3 - Most investigated surfactants Acronym BTA DDSA HEDMHA DHEMHA HDTA OA ODA PA TMA

Cation (CH3CH2)3N+CH2H5C6 (CH3)2N+((CH2)17CH3)2 (HOCH2CH2)N+(CH3)2(CH2),5CH3 (HOCH2CH2)2N+(CH3)(CH2)15CH3 (CH3)3N+(CH2)15CH3 H3N+(CH2)7CH3 H3N+(CH2)17CH3 H3N+(CH2)2H5C6 N+(CH3)4

Reference [88] [81] [87] [87] [80],[86] [90] [82],[85],[86] [88] [80],[91]

234

J. Grandjean

The proximity of organic cations to structural iron leads to a very effective 13C ('H) relaxation and an important line broadening. Indeed, the magnetic moment of the unpaired electron is 658 times larger than that of the proton, inducing a strong nuclear relaxation mechanism that results from direct or indirect electron — nucleus dipolar interaction. This paramagnetic effect in clay systems has been later studied in detail. Among different possible mechanisms, spin exchange between electrons on different ferric ions, leading to fluctuation of the dipolar interaction, is the dominant effect in the shortening of 'H and 13C relaxation times Tj and T2. Extensive line broadening (tiny T2, see figure 2) can give rise to a substantial signal loss. The paramagnetic effect results from statistical variation in local Fe concentration within a pair of clay layers rather than variations in the distance from the clay midplane [83,84]. Dynamics of cationic surfactants intercalated in montmorillonite and silylated montmorillonite have been studied. With the same iron content in the two clays, variation of the relaxation parameters means mobility difference. The larger relaxation times observed with the silylated materials imply a higher degree of surfactant mobility [85]. However, in any case, the paramagnetic effect generates line broadening with a partial loss of the signal intensity and resolution, preventing any analysis along the alkyl chain of the surfactant. Furthermore, the estimation of the paramagnetic effect on the nuclear relaxation process requires several assumptions. Therefore, NMR study of non-paramagnetic clay systems should provide more insight on the behaviour of the organic cations incorporated in clay galleries. Thus, the 13C CP MAS spectrum of the HDTA-Laponite exhibits wellresolved signals. The 'H relaxation times (in the laboratory frame and in the rotatory frame) do not show any significant variation along the long hydrocarbon chain, as spindiffusion evens out the differences in local magnetization during the measuring time. The spin-diffusion rate depends on the proton density which is not uniform in the case of partial exchange of the sodium cations. Thus, 29Si detected ! H relaxation times is described by a biexponential law, corresponding to silicon nuclei near exchanged (high proton density from the close organic cation) and unexchanged sites. In contrast to the *H relaxation time, the 13C relaxation time in the laboratory frame T^C) and the time constant TCH increase regularly from the head group to the terminal methyl group, according to a mobility increase along the hydrocarbon chain. Decrease of the Ri(C) values is characteristic of high mobility (figure 2) [86]. By contrast, the corresponding values obtained with HDTA-saponites are not significantly different whatever the carbon nucleus may be. More rigidity is indeed observed and Ri(C) values are near the maximum of the curve (figure 2). The 13C NMR spectra of HDTA- and ODA-Laponite exhibit much narrower lines than those obtained with the ODA-montmorillonite [82]. Paramagnetic effect (clay characteristics were not reported [82]), mobility reduction from the higher platelet size, higher charge density and/or charge originating from substitution in the tetrahedral layer may account for that observation. As indicated previously, the main signal in the 35 - 30 ppm range is assigned to the inner CH2 groups (C4 - C14) of HDTA. In the spectrum of the pure solid surfactant, the chemical shift at ca. 34 ppm is associated with the all-trans conformation of the alkyl chain (figure 6). Two partly resolved signals are observed in the spectra of HDTA- and ODALaponite, resulting from a mixture of trans (34.4 ppm) and gauche (32.7 ppm) conformations. In contrast to ODA-montmorillonite [82], the gauche conformer is dominant [86]. The same behavior has been obtained with HEDMHA- and DHEMHALaponite (figure 7).


235

Figure 6 - The all-trans conformation of the long alkyl chain of the DHEMHA iodide

Figure 7 - nC CP MASNMR spectrum of DHEMHA-Laponite The location of cation isomorphous substitution and the charge density influence the properties of the intercalated cation. Thus, the HDTA, HEDMHA and DHEMHA surfactants intercalated in the lowly charged saponites (0.3-0.4 charge per half-unit cell) show a bilayer structure with a high trans conformer content (ca. 70%) of the alkyl chains lying down on the clay surface . For higher charge clays, the mean surface area per cation decreases and the extended more rigid trans conformation of the surfactant cation is less easily formed. Accordingly, the population of the less densely packed gauche conformer increases (60-70%) as well as the basal spacing. The further shortening of the surface area per cation occurring with the highest charge saponites (0.75 and 0.80), changes the balance between repulsive and attractive interactions, leading to a different arrangement. The interchain Van der Waals interactions are enhanced and the trans conformation raises until ca. 90%. This is consistent with X ray diffraction data accounting for all-trans alkyl chains being tilted at 54.5° to the clay surface, the angle of the tilt that optimises the binding of the head group to the clay

236

J. Grandjean

surface [87]. 13C CP NMR has been used to identify a proton transfer and ion migration in a solid amine-clay mixture. The authors compare the spectra of phenethylammonium (PA) and benzyltriethylammonium (BTA) cations incorporated into hectorite or Laponite. The line width of the aromatic part is similar when PA or BTA is exchanged on hectorite and for a ground mixture of the clay and the chloride salt of PA. The ground mixture of the bromide salt of BTA is characterized by a much narrower line, similar to that of the pure salt. It has been suggested that proton transfer and subsequent ion migration occurring between clay protons and chloride ions are responsible for the broader line observed with PA [88]. During the synthesis of hectorite by hydrothermal crystallisation of a magnesium silicate, tetraethyl ammonium ions are used to aid crystallization and become incorporated as the exchange cations within the interlayer. This process has been followed by 13C MAS NMR to support a possible clay crystallization mechanism [89]. In clay suspensions, 2H NMR has revealed preferential orientation of molecules and ions near the mineral surface. Fast mobility strongly reduces the static quadrupolar splitting. Although slower motions are present in solids, the static 2H NMR spectrum remains sensitive to mobility variation. The ammonium protons of OA have been deuterated before intercalation in saponite. Variation of the deuteron splitting has been used to study the temperature influence on the head group dynamics. At 116K, a rather weak splitting value of 50 kHz has been determined, indicating internal rotation of the ND3+ group about its C3 axis is already operative. Cationic uniaxial rotation along the long axis of the surfactant cation gradually occurs in the 116-400K temperature range. The progressive reduction of the quadrupolar splitting is also indicative of a wide distribution of correlation times associated to this rotation. Large-amplitude motion of the whole ion in the interlayer space accounts for the quadrupolar splitting values obtained at higher temperatures [90]. ! H longitudinal relaxation times T! of TMAsaponite have been measured as a function of temperature. Under molecular isotropic rotational diffusion, the T! plot versus the inverse of temperature (proportional to the correlation time) shows one minimum (figure 2). A quite different plot is obtained for more complex motions. The relevant equation also predicts a co2 dependence of Tj when COTC> 1, whereas a much weaker dependence of « co1 ° below 140K is observed. This is indicative of a continuous distribution of the correlation times. The observed T! curve as a function of the inverse of temperature has been reproduced assuming two motional modes, isotropic cationic rotation as a whole and translational self-diffusion [91]. 4.3 - Intercalated (adsorbed) molecules As pointed previously, the presence of paramagnetic metal ions (mainly iron) in most natural clays induces line broadening that limits the suitability of NMR methods. For this reason, such studies have been restricted to synthetic samples or to natural smectites with very low iron content, especially hectorite. The NMR studies are concerned with characterization, structural and dynamic aspects of the clay-organic complexes. Sorption of triethylphosphate (TEP) on ion-exchanged smectites has been studied by 31P and 13C (CP) MAS NMR spectroscopy. TEP molecules progressively release coordinated water molecules from the solid interface. The 31P CP MAS spectrum of the adsorbed molecules shows two lines characterizing two motionrestricted phases attributed to monolayer and bilayer complexes. NMR (and IR) results suggest that the TEP molecules are directly coordinated to the interlayer cations via the


237

phosphoryl group P = O. The mobility of the sorbed species depends on the charge density of the clay [92,93]. Tributylphosphate (TBP) is one of the organic compounds which is used as ligating agent for nuclear fuel processing and is a widespread contaminant in ground water and soils surrounding processing facilities. The europium isotopes are among the release contaminants. Pure TBP exhibits a 31P NMR signal at 0.3 ppm, but in the Eu(NO3)3 - TBP complex, the signal is shifted to ca. -180 ppm. The pseudo-contact interaction between unpaired electrons of the (paramagnetic) cation and 31 P nuclei is responsible for it. Hectorite adsorbs this complex from solution, as shown by the signal near -180 ppm of the 31P MAS NMR spectrum. Another signal occurs at 5 ppm when TBP is in excess, corresponding to uncomplexed TBP that is not exchanged with the absorbed complex. When Eu-hectorite is put in contact with excess of TBP, no complex is formed. Based on these results, the authors conclude that actinides and lanthanides can enter a clay as a cation, presumably from a highly aqueous environment or as an organic complex formed prior to the sorption into the clay [94]. Small organic molecules such as urea, formamide or dimethyl sulfoxyde (DMSO) can also penetrate the kaolinite (phyllosilicate 1:1) galleries. NMR studies of these intercalates deals with the orientation, conformation and dynamics of the guest molecules. The 13C signals of the intercalated molecules are shifted downfield in response to increased hydrogen bonding after intercalation. The 13C spectrum of the kaolinite-DMSO intercalate shows two equally intense methyl signals which have been first assigned to two inequivalent methyl groups in the same DMSO molecule, one methyl group being keyed into the ditrigonal holes of the silicate layer and the other standing approximately parallel to the sheets [95,96]. 2H NMR has been used to study the molecular motion of the intercalated DMSO molecules. Another model has been proposed assuming two kinds of interlayer DMSO sites: one DMSO molecule has one methyl group keyed like in the previous model, and the other DMSO molecule, not keyed, adopts another orientation [97,98]. In both models the sulfonyl oxygen is hydrogen-bonded to the inner surface hydroxyls. More recently, a multinuclear NMR study has been performed in the temperature range 170-380 K [99-101]. The main conclusions of these papers can be summarized as: - Below 320 K, all interlayer molecules are equivalent with one methyl group keyed in the pseudohexagonal cavities of the silicate sheet. Above 320 K, some of the keyed methyl groups are released from the trapped holes giving rise to two coexisting DMSO sites in the interlayer space. Increasing temperature enhances this process and all the interlayer molecules are essentially free at 415 K. - The methyl groups of the intercalated DMSO molecules undergo free rotation around their C3 axis over the investigated temperature range. This symmetry axis is fixed at low temperature (ca. 160 K). A wobbling motion of the methyl groups is induced at higher temperature. After being released from the ditrigonal holes, the DMSO molecules undergo an anisotropic rotation of the whole molecules. Intercalation of formamide (FA), N-methylformamide (NMF), and Ndimethylformamide (DMF) has been studied similarly. In the kaolinite - FA system, outer surface and interacalated molecules are characterized by two carbonyl signals at 166.4 and 168.4 ppm, respectively. Molecules loosely adsorbed on the outer surface of the host have great mobility and produce negligible cross polarization efficiency. Accordingly, the outer surface signal at 166.4 ppm is not present in the 13C CP MAS NMR spectrum of the kaolinite - FA system. Among the three amide molecules,

238

J. Grandjean

intercalation of FA molecules induces the strongest downfield shift of the carbonyl signal. Weak Van der Waals interaction between the guest and the host results in a deshielding of the resonance, and the strength of the interaction is in the order FA > NMF » DMF. Therefore, mobility of the intercalated molecules, as deduced from the 13 C line widths, is in the reverse order FA < NMF < DMF. The *H chemical shifts confirm the existence of hydrogen-bonding between the amide protons of FA and the oxygens of the silicate sheets. No such hydrogen-bonding with NMF exists at room temperature. In contrast to intercalated DMSO molecules, the chemical shift value of the hydroxyl groups in the host kaolinite structure is not affected by intercalation. The structure of the three kaolinite intercalates is schematically shown in the figure 8 [102]. Table 4 - Studied molecules Acronym (name) Acetone Benzene DMF DMSO FA HEX MMA NMF PEG PEO PS Pyridine TBP Trichloroethylene TEP TNT

Molelecule (CH3)2C=O C6H6 (CH3)2NCOH (CH3)2S=O H2NCOH H2C=CH(CH2)3CH3 H 2 OC(CH 3 )COOCH 3 H3CNHCOH H(OCH2CH2)nOH (-CH2CH2O-)n (-CH2CH(C6H5)-)n C6H5N (CH3(CH2)3 O)3P=O CC12=CHC1 (CH3CH2O)3P=O (NO2)3H2C6CH3

Reference [107] [105,106] [102] [95-101] [102] [103] [117] [102] [110] [83],[111-116] [116] [107] [94] [107] [92],[93] [108]

Interaction of interlayer cations of Laponite and hex-1-ene (HEX) has been studied by 'H, 13C NMR, and 27A1 MAS NMR. All the expected I3C signal of HEX are detected with the Na-Laponite. By contrast, the peaks of the olefinic carbon atoms are not visible when the clay counterion is Al3+. This suggests some direct or indirect interaction of the double bond with aluminum ions leading to extensive line broadening whereas the saturated part of the hydrocarbon chain moves rather freely. Comparison between the 27A1 MAS NMR spectra of Al-Laponite and the clay-organo complex support this sorption mechanism [103]. Crown ethers and cryptands are known to complex cations. Their intercalation in montmorillonite and hectorite results in the formation of 1:1 and 2:1 ligand/cation interlayer complexes. In Na-hectorite, the apparent 23Na shift (second order effects have not been suppressed) varies with the nature of the macrocycle [104].


239

Figure 8 - Schematic representation of the structure of kaolinite intercalated compounds with (A) FA, (B) NMF, and (C) DMF. (Reprinted with permission from [102]. Copyright (1999) American Chemical Society). Macroscopic ordering of clay aggregates allows the determination of the arrangement of molecules intercalated between the clay platelets from the determination of the chemical shift anisotropies, because the principal axes of the chemical shift tensor are related to the molecular coordinate system. The 13C NMR spectra of benzene sorbed on Ag-exchanged hectorite show a single line whose position depends on the orientation of the clay platelets with respect to the external magnetic field. At room temperature, benzene molecules are mobile as inferred from the rather narrow signal. Molecular motion is restrained at lower temperature. Below 251 K, the anisotropy pattern is only detected. The equation (1) adapted to the chemical shift anisotropy is used to simulate the experimental spectrum. To reproduce the experimental patterns, it was necessary to tip the benzene C6 axis up out the ab plane of the clay platelet by about 15°. The proposed dynamic model includes rotation of the benzene molecules about the C6 axis and about the normal to the clay layer. The latter motion is quenched at 77 K [105]. More recently, local motion of benzene adsorbed on Ca-montmorillonite has been studied by 2H NMR. These results suggest that adsorbed benzene molecules first form n complexes with Ca2+ in the interlayer space of the clay. At or below 198 K, the quadrupolar splitting of the adsorbed molecules is close to half of the static-pattern splitting. From the equation (1), adapted to this system, local molecular motion can be deduced. Adsorbed benzene molecules undergo a small-angle wobbling of the C6 axis accompanied by discrete jumps about the hexad axis. At higher temperatures, benzene molecules start to perform large-angle wobbling of the C6 axis with extremely fast jumps around the axis, and eventually desorb from Ca2+ to tumble freely in the interlayer space of the clay. Fast motion cancels the quadrupolar interaction as shown by the presence of a central signal in the spectrum [106]. Using similar computer

240

J. Grandjean

simulations of 2H line shapes, this research group has also studied local motions of deuterium-labelled organic pollutants (trichlorethylene, acetone, pyridine, benzene or ethylene glycol) adsorbed on montmorillonite. Local mobility of the adsorbed trichloroethylene molecules remains partly restricted even above the 185 K melting point of the pollutant. This indicates a strong Cl2C=CDCl/Ca-montmorillonite interaction. Rapid and largely isotropic reorientation only occurs at room temperature when the spectrum exhibits a narrow single line. At similar low temperatures, the spectrum of adsorbed acetone molecules is narrowed by roughly a factor of 3 compared to the width of the adsorbed trichloroethylene spectrum. This is consistent with fast rotation of the CD3 group about the C-C axis but a composite motion consisting of three-site hop of the CD3 group and two-site hop about the C=O axis also accounts for the observed spectra. In the case of pyridine adsorbed on a substrate with acidic sites, one may expect at least some contributions from hydrogen-bonded complexes (N — HA). Motion about the Cpara — N —H (hydrogen bond) axis can reproduce the experimental spectrum at low temperature [107]. Dynamics of adsorbed trinitrotoluene has been studied by 2H MAS NMR. Magic angle spinning rate slower than the quadrupolar interaction gives rise to a narrow central band and spinning site bands spaced at intervals equal to the MAS rate [12]. Analysis of the spinning side band pattern leads to the mode of motion of the CD3 group. Intercalation of TNT in the acidtreated K10 montmorillonite provides a 2H NMR spectrum simulated by assuming C3 ring jumps (55%) and free rotation around the axis perpendicular to the aromatic ( and mineral siloxane) plane. The adsorbed TNT binding is weaker with Ca-montmorillonite, and free ring rotation dominates [108]. Intercalation of polymer molecules has attracted considerable attention during the last few years. NMR spectroscopy has been used for characterization and dynamics of the intercalated organic material. Thus NMF in NMF-kaolinite has been replaced by acrylamide which was then polymerised in situ. The process was followed by the change of the signal intensity of the ethylenic carbon atoms of the 13C CP MAS NMR. One-hour treatment is required to complete the polymerisation [109]. Polymerization of ethylene glycol intercalated in kaolinite was not observed. However, poly(ethylene glycol)-kaolinite intercalates have been prepared by releasing intercalated DMSO. 13C CP MAS NMR spectra, in combination with IR and X-ray diffraction, indicate that the intercalate polymer (PEG) is more constrained in the interlamellar space of the clay than it is in the bulk. The intercalated polymers are arranged in flattened monolayers where the ethyleneoxy groups show their oxygen atoms facing towards the hydroxyl surface of kaolinite [110]. Poly(ethylene oxide) (PEO) complexes are interesting materials due to their anisotropic ionic conductivity. The 13C CPMAS NMR spectra of Na-, K-, and Bahectorite/PEO complexes consist of a unique signal at « 70 ppm that has been assigned to a gauche conformation of the methylene groups, suggesting that the helicoidal conformation of the polymer PEO is maintained after intercalation. The 23Na NMR spectrum of the relevant complex shows also a unique peak, indicating an homogeneous environment inside the polymer helix and sodium ions are directly coordinated to the ethylenoxy units of the polymer [111]. Later, 2H solid-state NMR has been used to probe dynamics of the polymer in deuterated PEO/Li-fluorohectorite intercalates. The temperature dependence of the quadrupolar NMR powder patterns indicates that PEO, even at 220 K, possesses some small amplitude dynamics. At higher temperatures, even above the bulk polymer melting point, the spectra still retain residual powder patterns in


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addition to an intense central line, indicating restricted polymer motion between silicate layers. In addition, the authors indicate that there is a competition between the PEO oxygens and the surface oxygens of the silicate layers for interaction with Li+ cations [112]. PEO intercalated in montmorillonite and hectorite has also been studied by solidstate NMR to probe cation dynamics in these nanocomposites. In montmorillonite, the paramagnetic iron content produces an extra broadening of 15 kHz, compared to the 7Li NMR spectrum of the hectorite nanocomposite at 220 K. As the temperature increases, the 7Li linewidths observed in the montmorillonite systems undergo narrowing beyond that associated to the paramagnetic effect. Ion dynamics, responsible for this additional narrowing, is approximately two orders of magnitude less than polymer orientation. Cation surroundings in the vicinity of Li+ differ from that found in the nanocomposites with larger alkali ions, as an effect of the greater cation - Fe3+ distance [113]. The theoretical background of the paramagnetic effect on the NMR parameters of these systems are described in the literature [83]. The OC - CO torsion angle of PEO with 13% 13C - 13C labelled units can be estimated from the simulation of the 13C twodimensional double quantum (2D DQ) NMR spectra. The gauche content is estimated to 90 + 5%, which provides valuable constraints on the possible conformation in the intercalation gap [114]. Numerous solid-state NMR techniques are currently used to characterize the structure and dynamics of solids. The PEO-hectorite intercalate has been considered in order to optimise the conditions of several NMR experiments dedicated for studying organic materials near the silicate surfaces [115]. The intercalation of poly(styrene - ethylene oxide) block polymers (PS-fe-PEO) into hectorite has been studied by multinuclear solid-state NMR. Polymer intercalation is assessed by 2D 'H-29Si heteronuclear correlation spectra. Two copolymers with similar PEO block lengths (7 and 8.4 kDa) but different PS block lengths (3.6 versus 30 kDa) are compared. While the PS block is found not to be intercalated in either copolymer, definite proof of PEO intercalation in the sample with the shorter PS block is provided by a 'H- 1 3 C heteronuclear correlation experiment. In the PS-rich sample, the amount of intercalated PEO is much smaller, and a significant fraction of PEO is not intercalated. A schematic model is shown in Figure 9 [116]. Intercalation of copolymer of methyl methacrylate (MMA) (Table 4) and MDEA (OCH3 of MMA replaced by O(CH2)2N+(CH2CH3)2CH3r) into hectorite has been performed in two ways: either the formed copolymer has been directly intercalated in the clay or copolymerisation has been realized in situ on MDEA-exchanged hectorite. Different 13C relaxation times in the laboratory and the rotating frames are measured for the solid copolymer (MMA/MDEA ratio of 8) and the copolymer directly intercalated in hectorite, indicating the intercalation process affects copolymer dynamics. By contrast, the 13C relaxation times in the laboratory and the rotating frames are similar for the solid copolymer (MMA/MDEA ratio of 6) and the copolymer-hectorite complex prepared in situ. This could be due to a small percentage of MMA copolymerisation between the clay layers, most of MMA unit sequences are formed mainly out of the layers [117]. Polyaniline can exist in several oxidation states ranging from the completely reduced base state to the completely oxidized state. The most studied form of the polymer is emeraldine, in which the number of reduced and oxidized units is equal. This insulator can be converted to a conducting form by proton doping. 2H MAS NMR studies of the polymer indicate that polarons play an important role in the conductivity mechanism. After intercalation into montmorillonite, the polymer spectrum is similar to

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that of the non conductive emeraldine base. Failure to observe a (metallic) Knight shift in the nanocomposite, is consistent with the importance of 3D mechanisms of charge transfer for bulk conductivity in polyaniline [118].

Figure 9 - Schematic structural models of PS-b-PEO intercalated in hectorite (PEO blocks: dashed lines); PS blocks: solid lines); a and b are the side and top views for the copolymer with short chains of PS; c and d the corresponding views for the copolymer with long chains of PS. (Reprinted with permission from [116]. Copyright (2003) American Chemical Society). Interaction of polypeptides with clay minerals is at the basis of both natural biochemical processes and technological applications. Direct observation of polylysine side-chain interaction with montmorillonite interlayer surfaces is shown through 2D 'H27 A1 heteronuclear correlation NMR spectroscopy. Indeed, a peak correlates the NH3+ protons of the polylysine side chains and octahedral Al(III) of the mineral. Fast rotational motion around the C3 axis of the NH3+ group allows resolution of this small signal [119]. Adsorption of polylysine and polyglutamic acid on montmorillonite has been studied by 13C CP MAS NMR. The chemical shift of the backbone a- and carbonyl carbon nuclei have shown that both polypeptides, which exhibit a mixture of ct-helical and random conformation in the bulk, tend to unfold and adopt a more extended random coil structure on adsorption. Furthermore, analysis of the resonance line widths indicates an increase ordering of the positively-charged side chain of polylysine but not in the case of the negatively-charged side chain of polyglutamic acid [120]. 5 - Conclusions The diversity of the NMR experiments allows to study clay systems, ranging from water-rich suspensions to dried solids. The determination of the chemical shifts, quadrupolar splittings, relaxation rates or self-diffusion coefficients provides a detailed picture on the structure and dynamics of molecules and ions near a clay surface. These studies have also benefited from the more recent developments of NMR technology


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such as two-dimensional or multiple quantum coherence experiments. The survey of literature indicates that NMR study of clay systems is a growing and promising area. Acknowledgments I am grateful to F.N.R.S. (Brussels) for grants to purchase a solid-state NMR spectrometer and for support in our studies on clay materials. 6 - References [I] T.R. Jones, Clay Miner. 18 (1983) 399. [2] D.P. Siantas, B.A. Feinberg and J.J. Fripiat, Clays Clay Miner., 42 (1994) 187. [3] J.T. Smith and R.N.J. Comans, Geochim. Cosmochim. Acta, 60 (1996) 995. [4] Y.O. Aochi and W.F. Farmer, J. Colloids Interface Sci., 161 (1993) 106. [5] L. Bailey, M. Keall, A. Audibert and J. Lecourtier, Langmuir, 10 (1994) 1544. [6] E.S. Boek, P.V. Coveney and N.T. Skipper, J. Amer. Chem. Soc, 117 (1995) 12608. [7] M. Ogawa and K. Huroda, Bull. Chem. Soc. Jpn., 70 (1997) 2593. [8] M. Zanetti, S. Lomakin and G. Camino, Macromol. Mater. Engn., 279 (2000) 1. [9] M. Alexandre and P. Dubois, Mater. Sci. Engn., 98 (2000) 1. [10] D. Canet, Nuclear Magnetic Resonance: Concepts and Methods, J. Wiley & Sons, Chichester, 1996. [II] M. H. Levitt, Spin Dynamics: Basis of Nuclear Magnetic Resonance, J. Wiley & Sons, Chichester, 2001. [12] D.D. Laws, H.-M.L. Bitter and A. Jerschow, Angew. Chem. Int. Ed., 41 (2002) 3096. [13] J. McConnell, The Theory of Nuclear Magnetic Relaxation in Liquids, Cambridge, University Press, Cambridge, 1987. [14] J.-J. Delpuech Ed., Dynamics of Solutions and Fluid Mixtures by NMR, J. Wiley & Sons, Chichester, 1995. [15] O. Sodermann and P. Stilbs, Progr. NMR Spectr., 26 (1994) 445. [16] W.S. Price, Annu. Rep. NMR Spectrosc, 32 (1996) 53. [17] J. Grandjean, Annu. Rep. NMR Spectrosc, 35 (1998) 217. [18] J. Grandjean, in Encyclopedia of Surface and Colloid Science, A. Hubbard Ed., M. Dekker, (2002) 3700. [19] D.E. Woessner and B.S. Snowden Jr., J. Chem. Phys., 50 (1968) 1516. [20] B. Halle and H. Wennerstram, J. Chem. Phys., 75 (1981) 1928. [21] J. Grandjean and P. Laszlo, J. Magn. Reson., 83 (1989) 128. [22] D.T. Edmons and A.L. Mackay, J. Magn. Reson., 20 (1975) 515. [23] D.T Edmons and A. Zussman, Phys. Lett., 41A (1972) 167. [24] J. Grandjean and P. Laszlo, ACS Symp. Ser., 415 (1990) 396. [25] J. Grandjean and P. Laszlo, Clays Clay Miner., 37 (1989) 403. [26] D. Petit, J.-P. Korb and A. Delville, J. Grandjean, P.Laszlo, J. Magn. Reson., 96 (1992)252. [27] J. Grandjean and P. Laszlo, Clays Clay Miner., 42 (1994) 652. [28] C.A. Weiss and W.V. Gerasimowicz, Geochim. Cosmochim. Acta, 60 (1996) 265. [29] J. Grandjean, J. Colloid Interface Sci., 185 (1997) 554. [30] A. Delville, J. Grandjean and P. Laszlo, J. Phys. Chem., 95 (1991) 1383. [31] J.A. Ripmeester, L.S. Kotlyar and B.D. Sparks, Colloids Surf. A, 78 (1993) 57. [32] J. Grandjean and P. Laszlo, J. Amer. Chem. Soc, 116 (1994) 3980.

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PESTICIDE-CLAY INTERACTIONS AND FORMULATIONS JUAN CORNEJO*, RAFAEL CELIS, LUCIA COX and M. CARMEN HERMOSIN Instituto de Recursos Naturales y Agrobiologia de Sevilla, CSIC. P.O. Box 1052. 41080 Sevilla - SPAIN. * E-mail: [email protected]


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1 - Introduction During the last few years it has been reported that pesticides have been found in several compartments like soil, ground water, surface water, sediments, air, foods and even animal tissues. This situation is viewed with great concern due to the environmental problems arising from the use of enormous amount of pesticides and specially for those persistent and mobile molecules affecting soil-water ecosystems entering later in the trophic chain. Production and uses of pesticides are still increasing. However, there are also several significant developments that will have long term impact on pesticide usage and residues in water. There has been a steady decrease in the amount of herbicide needed to control weeds since 1940's (Figure 1), because the specificity of the new molecules seems to be more effective using lower dose than earlier ones [1]. The soil is the main recipient of organic agrochemicals used in plant protection to control or destroy weeds, insect, fungi, and other pests in a deliberated way. However, sometimes the soil receives those chemicals in accidental ways like spillages from broken containers, industrial wastes disposal, etc. Thus the soil plays a central role in the environmental fate of these chemicals and in the protection of ground and surface waters. The soil behaves as a filter, where the organics are chemically or/and biologically degraded but it can also be able to retain most of these chemicals avoiding or limiting their leaching to deeper soil horizons and ground waters. For these reasons the soil can not be regarded as a sink for chemicals or as a compartment with unlimited contaminant loading capacity or unlimited natural attenuation power.

Figure 1 - Historical trends in recommended herbicide application rates, by chemical class. After Nash and Leslie [I]. Once the pesticide is applied to targeted objectives (weeds, plants pest, soil, etc), most of the chemical finally reaches the soil either directly or from leaves or air deposition. After entering the soil, pesticides undergo different processes in the soil environment mainly of adsorption-desorption, transport and physico-chemical and biological degradation [2].

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The above-mentioned processes are of course dependent on three big groups of factors that are associated with the soil properties (structure, texture, pH, % Organic Matter (OM), microbiological activity, etc.) and soil management (traditional or conservation tillage, irrigation method, etc.) to the pesticide properties (water solubility, stability-pH-UV, formulation, etc.) and to the climate conditions (rainfall, temperature, UV, etc.). The organic fraction of the solid phase of the soil is often assumed to dominate the interactions of pesticides in the soil environment. This assumption is taken by modelers and specially in studies dealing with rich organic matter soils obscuring the important role played by the mineral surfaces on the fate of pesticides in soil. However, mineral surfaces may dominate the fate of organic chemicals arriving in the soil. It is very well known that in Mediterranean, semiarid and arid zones the soils are very poor in OM content being the mineral fraction the responsible colloidal particles for surface interactions. Even in normal soils and subsoils it is expected an important contribution of the mineral fraction to the interaction of polar or weakly polar organic molecules like some pesticides, due to the higher percentages of the mineral fraction over the organic one. It is also necessary to consider that both soil fractions are not generally isolated but forming organo-mineral associations with surface properties very different from those they own separately contributing to sorption process [3-5]. In summary, clay-pesticide interactions are here considered under two complementary points of view: pesticides interaction with the clay minerals of the soils and the design of clay-based formulations for an improvement of the agronomical and environmental use of pesticides. 2 - Pesticides Agricultural pesticides are often detected in natural waters, and therefore, they are an important group of organic pollutants which production and uses must be controlled to minimize the health and environmental problems. There is a strong relationship between the amount of pesticide applied and the amount detected in soil and water. It has been pointed out about the decrease of the amount of new pesticides needed for control some pests. However, it is also true that the total amount of pesticide marketed in the world increased because of the change in farming practices, beginning in the 90's. No-tillage or conservation tillage is being widely used in developed countries. This type of soil management needs higher amounts of herbicides with the corresponding environmental impact but reducing the soil erosion and increasing the water infiltration because seeds are drilled directly into the soil containing plants residues from the previous crop instead of plough the fields before planting. Before drilling the seed, all weeds are destroyed with the appropriated herbicide. Pesticide is a generic name for compounds used for pest control most of them used in agricultural practices. On a weight basis, agriculture is the largest user of pesticides (77%) the rest being used for industrial and commercial activities (16%) and for home and garden sectors (6%). The three main groups of pesticides are insecticides controlling insects, herbicides for weeds, and fungicides for plant diseases. There are some other small groups of chemicals with specific objectives like rodenticides, nematicides, fumigants, molluscicides and plant growth regulators. There is a large number of pesticides currently in use, with a wide range of physicochemical properties which determine its behavior in the environment. Molecular size, ionizability, water

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solubility, lipophilicity, polarizability and volatility are all important properties. Pesticides can be classified in many different ways. Figure 2 is a classification scheme for selected pesticides on the basis of their significant chemical properties and reported behavior in soils and water [6].

Figure 2 - Classification of pesticides by Gevao et al [6]. Clays and clay minerals are very special natural minerals with so many specific properties that they have originated a whole research world. In previous sections most of these properties have been shown but in any case most of these properties were known by the soil science studies. The interaction of clays and pesticides has been studied for many years. Cruz et al [7], Weber [8], Bailey and White [9], Mortland [10], White [11] and many others were the pioneer scientists who created the scientific basis of the actual knowledge of the new wonderful world of the applied clay science. 3 - The pesticide sorption process The natural process of sorption by soil solids, mainly those constituting the soil colloidal fraction, determines the amount of pesticide that can reach the target organism and the amounts available for other processes such as volatilization, degradation and leaching [12-14]. The sorption of pesticides by soil colloidal particles is also of interest in the transport of these compounds in runoff and surface waters, and even in ground waters, because this paniculate matter can act as a carrier of organic contaminants from point source [2,12,15-17]. As it was mentioned above, organic matter is recognized as the primary factor related to sorption of non-polar organic pesticides in soil or sediment/water systems [18,19], but for polar organic pesticides the behavior is not expected to be the same [20,21], specially for soils and sediments with low organic carbon content [22-29].


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Sorption of pesticides by diverse materials is also a method used for their elimination from water or their immobilization in contaminated soils [30,31] and a basis for controlled release formulations, which can decrease their contamination potential [32] and can be more economical than decomposing non-bioactive amounts of pesticide in the environment [33]. Recently, there has been an increasing interest in the use of natural and modified clays as supports or carriers to reduce the leaching of soil-applied herbicides or insecticides [34-40]. In this chapter we will first discuss pesticide-clay interactions, and second how these interactions can be used in pesticide formulations. 4 - Pesticide-clay interactions Soil clay fraction as a whole (mineral and organic components) has been shown to be the responsible for the sorption of many polar pesticides [26,28,41-45]. According to Mingelgrin and Gerstl [20], the mineral clay fraction of the soil increases its importance in pesticide soil sorption when organic matter content of the soils is low and when pesticides are ionic, ionizable or polar. For these pesticides, Hermosin et al [46] observed that variation coefficients decreased when sorption was considered in the clay fraction basis, indicating a more homogeneous behavior of the clay fraction in pesticide sorption as compared with the whole soil. For all pesticides considered in this study (Figure 3), variation coefficients were higher in the case of sorption coefficient Kj (Kj= pesticide sorbed/pesticide in solution) than in the case of sorption coefficient Kclay (Kclay= IQ/Vo clay * 100). For every pesticide considered, clay was the soil characteristic better or equal than organic carbon or organic matter correlated to pesticide soil sorption.

Figure 3 - Kj and K^ variation coefficients for selected pesticides (Ac= acephate, At= atrazine, Br~ bromacil, Ch= chlordimeform, Dq= diquat, Du~ diuron, Epeihylparaoxon, Ef= Ethofumesate, Fl=fluoridone, lm= imazethapyr).

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On the contrary, the organic matter associated to the colloidal fraction of soils seems to be less reactive in sorption of polar compounds [44,47]. This is clearly shown in Figure 4 for the triazinone herbicide metamitron: its affinity for Fe-saturated SWy montmorillonite is much higher than for commercial (Fluka) humic acid or soil humic acid [48]. Clay minerals play an important role in adsorption of polar organic pesticides mainly due to the high surface areas associated to their small particles size, and smectites are among all clay minerals the most important regarding sorption of organic compounds such as pesticides since they contribute most of the inorganic surface area in soils [25]. Smectites have swelling or expandable structure that makes the interlamellar surface accessible to interchange the cations and to water and polar organic molecules such as pesticides [9,17,43,49-54]. Clay minerals, due to isomorphous substitution in their structural framework, carry a permanent negative charge which, in their natural state, is compensated by inorganic cations. Hence, cationic pesticides can be adsorbed to clay minerals by ion exchange processes and sorption is directed by charge pattern interactions between cations and the surface charges of the clay mineral [33,55-57]. Anionic pesticides can also interact with clay minerals through positive edge charges (variable charge) of the silicate layers, through hydrogen bonds or attached to multivalent metal ions at exchange sites through cation bridges [33]. However, in general, anionic pesticides are weakly retained by most of soil or sediment components due to repulsion between clay minerals surface negative charges and organic anions [48,58,59]. Sorption capacity of clay minerals for hydrophobic pesticides is often considered considerably reduced, due to the highly hydrophilic environment provided by hydration water of exchangeable cations [10,60]. However, clay minerals also have "hydrophobic microsites" where pesticide molecules also sorb. In fact, even very highly hydrophobic organic pollutants, such as phenanthrene [61], have been shown to sorb on smectites. This was also proposed by Laird [62], who suggested that the herbicide atrazine initially sorbs on smectites as molecular species on these hydrophobic microsites on the clay siloxane surface. The same explanation was given by Celis et al [4] studying sorption of simazine and atrazine on model soil colloidal components. Moreover, van Oss and Giesse [63] used the surface thermodynamic theory to demonstrate the hydrophobicity of clay minerals. Figure 5 summarizes the different main sorption sites on a model smectite for model cationic (C), anionic (A), polar (P) or hydrophobic (H) pesticide molecules. The nature of the exchangeable cations of the clay mineral greatly influences sorption. The type of exchangeable cation adsorbed to the siloxane surface of kaolinite determined the adsorption of nitroaromatic compounds on kaolinite: significant adsorption was observed in the presence of weakly hydrated cations (i.e., Cs+, Rb+, K+, or NH4+) while strongly hydrated cations (H+, Na+, Ca2+, Mg2+ o Al3+) prevented any specific interaction [64]. Oxygen ligands at the external siloxane surface of kaolinite are accessible only in the presence of weakly hydrated cations. The same observation was made by these authors in sorption studies with other clay minerals such as illite and montmorillonite [65]. On another hand, sorption of polar molecules in the interlamellar spaces of montmorillonite by substitution of water molecules hydrating the exchangeable cation have been shown to be facilitated by small ionic potential of these cations [43,49,53,66,67], since the opening of the silicate layers is much easier and


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thereby the substitution of the hydration water molecules of the exchangeable cations by pesticide molecules is facilitated.

Figure 4 - Metamitron sorption isotherms on FeSWy montmorillonite and soil and commercial (Fluka) humic acids.

Figure 5 - Main sorption sites on a model smectite for model cationic (C), anionic (A), polar (P), and hydrophobic (H) pesticide molecules. An exception to this is the case of transition metals such as Fe3+ and Al3+, which has been shown to increase adsorption capacity of montmorillonite through the formation of coordination bonds between the electron donor groups of organic molecules and the metal ions and through protonation of groups such as NH. The last

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was observed for the triazine herbicide simazine [4] or the polar insecticide imidacloprid [54]. The high sorption of these pesticides on Fe saturated montmorillonite has been attributed to the acidic media provided by the saturating cation, which allows protonation of metamitron, simazine and imidacloprid molecules close to the montmorillonite surfaces and, thus, protonated molecules can easily be adsorbed on negatively charged surfaces on the interlayer space of the montmorillonite by a cation exchange mechanism. Figure 6 show this process in the case of simazine. The high polarizing power of the saturating cation allows protonation of simazine to H+-simazine which sorbs to a higher extent. Surface acidity also leads to further hydrolysis of sorbed simazine, since nucleophilic attack by water molecules is facilitated [4]. Iron oxides and humic acid coatings of the montmorillonite seem to favor this process, since they can act as proton donors [4].

Figure 6 - Protonation, sorption, and hydrolysis to hydroxysimazine of simazine molecule at the clay surface. Surface properties of the clay mineral determine their sorption capacity for pesticide molecules. Low CEC and layer charge of the clay mineral facilitates the opening of the silicate layer and the adsorption of polar organic molecules in the interlamellar space of these clay minerals [43,49,50,53]. Laird et al [68] found that atrazine sorption on 14 reference soil smectites ranged from 0 to 100 % depending on surface properties of the smectites and found a high negative correlation between sorption and surface charge density. This is clearly shown in Table 1 for three different organic compounds: two polar pesticides, the carbamate insecticide methomyl and urea herbicide thiazafluron, an a organotin biocide monobutyltin. Sorption of methomyl and thiazafluron is negatively correlated with layer charge and CEC (SAz montmorillonite > SWy montmorillonite> SH hectorite), whereas higher sorption of monobutyltin is observed in low layer charge montmorilloite SWy than in high layer charge smectite SAz. This table also shows the lack of sorption on kaolinite KGa measured in the case of the polar compounds and the lower sorption in the case of the MBT cation.

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Conversely to smectite, kaolinite has a rigid nonexpandable structure (Figure 7) which gives rise to a lower affinity for organic compounds including organic cations such as monobutyltin. Table 1 - Date obtained on different smectites calculated from Freundlich adjustment of isotherms. Smectite SSAf SHCa-1 SWy-Ca SAz-Ca Kga

(mV) 63 31 97 10

LC{ (molc unit cell"1) 0.31 0.68 1.13 0

CEC (mmolc kg"1) 439 764 1200 40

MET 24.66 10.21 6.96 0

Kf THIA 134.2 79.8 2.5 0

MBT nd§ 2332 1681 38

SSA = Specific surface area; LC = layer charge; CEC = cation exchange capacit. Sorption coefficients for: MET = carbamate insecticide methomyl [53], THIA = urea herbicide thiazafluron [43] and MBT = organotin cation monobutyltin [16]. f Van Olphen and Fripiat [69];} Jaynes and Boyd [70]; § not determined

Figure 7 - Structures of smectite and kaolinite. Many studies dealing with interactions of pesticides with pure minerals have been carried out in order to elucidate the possible binding mechanisms in soils [42,49,50,53,71], but very little information is available on adsorption of herbicides by the clays separated from soils, whose surface properties may be different than those for the pure minerals [25,42-44,47]. Although model adsorbents facilitate the study of pesticide adsorption, the actual mineral surfaces in soils can be interassociated with other soil components such as iron oxides or humic substances which might block adsorption sites on the clay minerals surfaces, giving rise to different adsorption capacities than that of pure minerals [43,48,72,73]. Association of iron oxide (ferrihydrite) to Ca-SWy montmorillonite increased sorption capacity of the clay mineral for the triazine herbicides simazine and atrazine [5]. This increase in sorption has been attributed to partially dissociated H2O molecules surrounding hydroxy Fe polymers in the interlayer of the montmorillonite promoting protonation of the herbicide molecules. The association of this montmorillonite with humic acid also increased sorption of these herbicides on the clay mineral and even the sorption capacity of the humic acid, which has been attributed to changes in

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conformation of the humic polymers increasing sorption on humic coatings [5]. Different results were obtained in the case of the urea herbicide thiazafluron of very high water solubility [74]. The association of iron oxide to clay mineral reduced sorption of thiazafluron by blocking the access to the interlamellar spaces of the smectite and by coating the external surface of montmorillonite. The same result was observed in the case of clay mineral association with humic acid. The high affinity of the highly polar thiazafluron for sorption sites in montmorillonite is reduced when montmorillonite surface is blocked by iron oxides and/or humic acid. In the case of the anionic herbicide 2,4-D [75], no sorption was measured on montmorillonite clay mineral due to repulsion between negatively charged clay surface and 2,-D anions, whereas ferrihydrite and humic acid coatings on the smectite surface provided sorption sites. The different behavior observed with these three different pesticides (Figure 8) reveals that the surface properties of soil clays are even more important than the relative amount present in soil, since these surfaces may not be accessible to organic compounds, due to interactions with other soil components such as humic substances, which may compete with pesticides for sorption sites [25,48,71,74,75].

Figure 8 - Molecular structures of the herbicides atrazine, thiazafluron, and2,4-D. 5 - Pesticide interactions with modified clays Because of the hydrophilic, negative character of their surfaces, clay minerals, in particular 2:1 phyllosilicates, have been shown to be very good adsorbents for cationic and highly polar pesticides, but their adsorption capacity for poorly soluble, nonionic pesticides is usually low [10,51,60,76]. The strong hydration of natural inorganic exchange cations produces a hydrophilic environment at the clay mineral surface that considerably reduces the sorptive capacity of clays for hydrophobic compounds. Replacement of natural metal-exchange cations with organic cations through ion exchange reactions has been shown to change the nature of the surface from hydrophilic to hydrophobic, and hence, this simple modification has been proposed for the improvement of the sorptive properties of clay minerals for hydrophobic organic compounds, including hydrophobic pesticides [77-79] (Figure 9). The organic cations most commonly used for clay mineral modification are quaternary ammonium ions of the general form [(CH3)3NR]+ or [(CH3)2NR2]+, where R is an aromatic or aliphatic hydrocarbon. Incorporation of large alkylammonium cations, such as octadecylammonium (ODA), dioctadecylammonium (DODA) and hexadecyltrimethylammonium (HDTMA), in the interlayers of smectitic clays has resulted in organoclays with enhanced affinity for neutral [38,80,81] and even acidic pesticides [51,77,82-85]. Celis et al [38] found that adsorption of the uncharged


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fungicide triadimefon by Arizona montmorillonite increased from 0 to > 90% after modification with HDTMA cations. Increases in adsorption after modification of montmorillonite with large alkylammonium cations have also been reported for acidic pesticides, such as 2,4-D [51,76], imazamox [85], bentazone [82], dicamba [83,84], and picloram [86] (Table 2), especially at low pH levels where the protonated form of the pesticide predominates [85]. It appears that the interlayer phase formed from large alkylammonium alkyl groups functions as a partition medium for nonionic organic compounds and effectively removes such compounds from water [60,77,87].

Figure 9 - Preparation of organoclays through ion exchange reactions. The adsorptive characteristics of organoclays formed using small quaternary ammonium cations, such as tetramethylammonium, are much different, since small organic cations exist as discrete species on the clay mineral surface and do not form an organic partition phase [78,87-89]. In these organoclays, the organic cations act as nonhydrated pillars that prop open the clay layers exposing the abundant siloxane surface area [89]. Low-charge montmorillonite modified with small alkylammonium cations were found to be particularly effective in removing alachlor, norflurazon, and hexazinone from aqueous suspensions [90,91]. Besides the size of the hydrocarbon chains, the surface charge of the clay mineral and the amount of organic cation in the interlayer have been shown to be major factors influencing the adsorptive properties of alkylammonium-exchanged clays (Table 2), because these parameters determine the arrangement of the organic cation in the clay mineral interlayer, and in turn the presence of space available to host pesticide molecules [38,60,85,87]. A recent research line on organoclays as adsorbents of pesticides is the selective modification of clay minerals with organic cations containing appropriate functional groups to maximize the affinity of the adsorbent for a given pesticide. Although this concept has been applied to develop organoclays with increased affinity and selectivity for heavy metal ions [92-94], little information exists on how organic cations with different functionalities influence pesticide adsorption by organoclays. Very recently, Nir et al [90] pointed out the importance of the structural compatibility between the pesticide molecule and the alkylammonium cation preadsorbed on the clay mineral in determining the performance of organoclays as adsorbents of pesticides. Alachlor and metolachlor, both containing a phenyl ring in their structure, displayed greater affinity for montmorillonite exchanged with organic cations with phenyl rings, such as benzyltrimethylammonium or phenyltrimethylammonium, than for alkylammonium-exchanged montmorillonites.

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Table 2 - Freundlich coefficients (Kf) for the adsorption of some acidic pesticides by unaltered and modified montmorillonite samples (data from references [83], [85] and [86])f. OctS (%)

Piclora m

Dicamb Kf Imazamox a

SWy-2 SAz-1 SWy-2 SWy-2 SWy-2

Main Interlayer cation Na+ Ca+ ODA ODA HDTMA

60 90 56

0 0

0 0

SWy-2

HDTMA

83

37

1

36

SAz-1

ODA

67

175

1

ODA HDTMA HDTMA

98 54 85

504 40 240

2 1 1

163 20 115 37 92

Sample

Montmorillonite

SW AS ODA-SW! ODA-SW2 HDTMA-

13 3 62 1 1

0 0 100 117

4

1 6 2

55

5

44

0 3

SWi

HDTMASW2 ODA-SAi

ODA-SA2 SAz-1 HDTMA-SA, SAz-1 HDTMA-SA2 SAz-1

2 1 2

5

167

2

352 77 272

1 0 24

SWy-2: Wyoming montmorillonite (CEC= 76 meq/WOg), SAz-1: Arizona montmorillonite (CEC= 120 meq/lOOg), ODA: octadecyl-ammonium, HDTMA: hexadecyltrimethyl-ammonium, OCtS: percentage of the CEC of the clay mineral compensated with organic cations.

Figure 10 - Simazine adsorption isotherms by untreated montmorillonite (SWy-2) and montmorillonite modified with different natural organic cations (data from reference 95).


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Based on this concept, Cruz-Guzman et al [95] hypothesized that natural organic cations with appropriate polar functional groups could allow one to selectively modify clay mineral surfaces to maximize their affinity for selected pesticides. Using Lcarnitine, L-cystine dimethyl ester and thiamine cations as modifying agents, the authors found that the chemical nature of the interlayer organic cation greatly influenced simazine adsorption by exchanged clays (Figure 10), probably through a combination of functionality and steric effects, and suggested the possibility to selectively modify clay mineral surfaces with organic cations containing appropriate functional groups to create an interlayer microenvironment designed to improve the affinity of the clay mineral for a given pesticide. The suitability of natural organic cations for this purpose was considered to be particularly interesting to minimize the environmental impact of the adsorbent when incorporated into natural ecosystems for practical applications. 6 - Pesticide-clay formulations The environmental problems associated with pesticide use, particularly the use of highly mobile pesticides, have become a current concern because of the increasing presence of these agrochemicals in ground and surface waters. To compensate for transport and degradation losses and to ensure adequate pest control for a suitable period, pesticides are applied at concentrations greatly exceeding those required for control of the target organisms, thus increasing the likelihood of runoff and leaching and hence the risk of surface and ground water contamination [96,97]. This problem is exacerbated in the case of highly soluble pesticides, because the risk of offsite movement from the intended target area increases as the pesticide is quickly dissolved in the soil solution [98]. Most pesticide formulations in current use deliver the bulk, if not all, of the active ingredient in an immediately available form that is readily released to the environment [99]. For highly soluble pesticides, these formulations may result in great pesticide losses shortly after application, before molecules have time to diffuse into soil aggregates and reach adsorption sites at the soil surfaces [39,100]. Recently, increased attention has been directed to reduce pesticide transport losses by the development of less hazardous formulations, such as slow-release formulations, in which only a part of the active ingredient is in an immediately available form; the bulk of the herbicide is trapped or sorbed in an inert support and is gradually released over time [101]. Beneficial effects related to the use of slow-release formulations include reduction in the amount of chemical required for pest control, decrease in the risk of environmental pollution as a result of pesticide transport losses (i.e., leaching or volatilization) (Figure 11), savings in manpower and energy by reducing the number of applications required in comparison to conventional formulations, increased safety for the pesticide applicator, and a general decrease in nontarget effects [97]. Among the various materials proposed as carriers in pesticide formulations [102], recently there has been a renewed interest in the use of natural soil constituents, like clays, iron oxides or humic acids [34,35, 96,98,99,103-105]. Clays have unique properties, such as their high specific surface areas associated with their small particle size, low cost, and ubiquitous occurrence in most soil and sediment environments. Margulies et al [34] used montmorillonite and sepiolite as carriers for the volatile herbicide S-ethyldipropylcarbamothioate (EPTC) and showed that volatilization losses of EPTC were greatly reduced by the use of the clay minerals as pesticide carriers. At 30°C, the half-life time (T1/2) of EPTC in its free form was 10 h, whereas when adsorbed

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to montmorillonite the r M was more than 5 days. Similarly, when EPTC was incorporated into soil, the TI/2 values were 4 and 9 days for the free and adsorbed forms, respectively. Control release of atrazine and alachlor was achieved by Johnson and Pepperman [99] by addition of selected minerals, including calcium bentonite, finegrind bentonite, montmorillonite K10, kaolinite, and iron oxide. Cox et al [39] used Fe(III)-treated Wyoming montmorillonite to enhance the affinity of the clay mineral for simazine and 2,4-D, and prepared clay-pesticide complexes that displayed reduced leaching of the pesticides in hand-packed soil columns compared to the free form of the pesticides. Similar results were reported by Celis et al [86,91] for the herbicides picloram and hexazinone preadsorbed on Fe(HI)-montmorillonite.

Figure 11 - Pesticide transport processes as affected by the formulation. Application as a clay complex enhances the soil sorption process, reducing pesticide losses by volatilization and leaching. In addition to the use of clay minerals as such, the possibility exists to selectively modify clay mineral surfaces, for instance through the incorporation of organic cations in the interlayers, to improve their adsorption capacity for selected pesticides and to control the desorption rate once added to the environment. Different types of pesticide formulations based on organoclays, particularly alkylammoniumexchanged clays, have been proposed [33,38,40,82,83,105,106]. However, while much attention has been given to describe the diversity of organoclay-pesticide interactions [51,75,77,81-84,86,90,107], information on the factors controlling the release rate and extent of pesticides from organoclay formulations is less abundant. Particularly limited are studies that validate the behavior of organoclay formulations of pesticides under real, field conditions, although the comparable or even improved weed control efficacy reported by several authors for organoclay-based formulations of herbicides compared to standard formulations seems promising for practical application of these formulations [37,108-110]. One major obstacle of many clay-pesticide formulations assayed has been observed to be the high amount of pesticide not released from the formulation, which would result in soil contamination and would require higher application rates for the same amount of active ingredient to be released [91,110]. Formulations of hexazinone


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supported on Fe(III)-montmorillonite released from 75% to only 20% of the herbicide associated with the clay mineral [91]. Gerstl et al [96] also observed that a large portion (35-70%) of the active ingredient remained adsorbed in controlled release formulations of alachlor at the end of release experiments. Similarly, Johnson and Pepperman [99] found that from 5 to 27% of alachlor and atrazine formulated with bentonite, montmorillonite, kaolinite and iron (III) oxide was not released, with the greatest retention by bentonite formulations. To circumvent the above-mentioned limitation, factors influencing the release rate and extent of pesticides from clays and organoclays need to be considered. The characteristics of the clay mineral, the amount and nature of exchangeable cations, the clay-pesticide ratio, and the procedure followed to prepare the formulation all affect the interaction of the pesticide with the sorbent and in turn the release rate and extent from the resulting formulation [37,40,96,91,105-107,110]. Thus, El-Nahhal et al [37] found that organoclay complexes with 0.5 mmol benzyltrimethylammomum/g of montmorillonite gave larger adsorbed amounts and better formulations of alachlor as compared to benzyltrimethylammonium preadsorbed up to the cation exchange capacity of the clay (0.8 mmol/g). Hermosin et al [105] showed that the release of fenuron from organoclays was inversely proportional to the adsorbent power of the organoclay and the fenuron-organoclay mixing time. Similarly, loosely-bound preparations of hexazinone with hexadecyltrimethyl-ammonium-montmorillonite (i.e., simple mechanical mixtures) have been shown to lead to greater amounts of herbicide released than formulations where an intimate association was promoted by the use of organic solvent [91].

Figure 12 - Hexazinone breakthrough curves in hand-packed soil columns after application as commercial formulation and HDTMA-montmorillonite formulations containing 20%, 10%, and4% active ingredient (a.i.).

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In a recent study, Celis et al [110] found hexazinone formulations based on hexadecyltrimethylammonium-montmorillonite displayed slow release properties in water, retarded herbicide leaching through soil columns and maintained a herbicidal efficacy similar to that of the free form of the herbicide, whereas formulations based on phenyltrimethylammonium-montmorillonite released the herbicide instantaneously and did not display slow release properties. High organoclay-herbicide ratios made the interaction of the herbicide with the organoclay more intimate and reduced the release rate of hexazinone as well as its leaching through soil columns (Figure 12). Therefore, the possibility exists to select all these variables to optimize the performance of the formulation for practical applications. 7 - Summary and conclusions Clay-pesticide interactions are considered under two complementary points of view: pesticide interactions with the soil clay minerals and the design of clay-based formulations for an improvement of the agronomical and environmental use of pesticides. Clay minerals increase in importance on pesticide soil sorption when organic matter content of the soils is low and when pesticides are ionic, ionizable or polar. Smectites are among all clay minerals the most important regarding sorption because they have swelling or expandable structure that makes the interlamellar surface accessible to interchange the cations, water and pesticide molecules. Surface properties of the clay minerals determine their sorption capacity for pesticide molecules. Although model sorbents facilitate the study of pesticide adsorption, the actual mineral surfaces in soils can be interassociated with other soil components such as iron oxides or humic substances which might block adsorption sites on the clay minerals surfaces, giving rise to different adsorption capacities than that of pure minerals. The surface properties of soil clays are even more important than the relative amount present in soil, since these surfaces may not be accessible to organic compounds, due to interactions with other soil components which may compete with pesticides for sorption sites. Replacement of natural metal-exchange cations with organic cations through ion exchange reactions has been shown to change the nature of the surface from hydrophilic to hydrophobic increasing clay mineral affinity for hydrophobic organic compounds, including hydrophobic pesticides. A recent research line on organoclays as adsorbents of pesticides is the selective modification of clay minerals with organic cations containing appropriate functional groups to maximize the affinity of the adsorbent for a given pesticide. These organoclays can be used in clay-pesticide formulations. Factors influencing the release rate and extent of pesticides from clays and organoclays are still issues which need to be addressed. The characteristics of the clay mineral, the amount and nature of exchangeable cations, the clay-pesticide ratio, and the procedure followed to prepare the formulation all affect the interaction of the pesticide with the sorbent and in turn its release from the formulation. 8 - References [1] R.G. Nash and A.R. Leslie, Eds., Groundwater Residues Sampling Design, American Chemical Society, Washington D.C., 1991. [2] J. Cornejo, P. Jamet and F. Lobnik . Pesticide/soil interactions: Some current research methods: Introduction, Eds. J. Cornejo and P. Jamet, INRA, Paris, 2000.


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[3] J. Comejo and M.C. Hermosin, Humic Substances in Terrestrial Ecosystem: Interaction of humic substances and soil clays, Ed. A. Piccolo, Elsevier, Amsterdam, 1996. [4] R. Celis, J. Cornejo, M.C. Hermosin and W.C. Koskinen, Soil Sci. Soc. Am. J., 61 (1997) 436. [5] R. Celis, J. Cornejo, M.C. Hermosin and W.C. Koskinen, Soil Sci. Soc. Am. J., 62 (1998) 165. [6] B. Gevao, K.T. Semple and K.C. Jones, Environ. Pollut, 108 (2000) 3. [7] M. Cruz, J.L. White and J.D. Russell, Israel J. Chem., 6 (1968) 315. [8] J.B. Weber, Res. Rev., 32 (1970) 93. [9] G.W. Bailey and J.L. White, Res. Rev., 32 (1970) 29. [10] M.M. Mortland, Adv. Agron., 23 (1970) 75. [11] J.L. White, Bound and Conjugated Pesticide Residues: Clay-particle interactions, Eds. Kaufman et al, ACS Symposium Series. Washington, 1970. [12] J.F. Mcarthy and J.M. Zachara, Environ. Sci. Technol., 23 (1989) 497. [13] A.J. Beck, A.E.J. Johnston and K.C. Jones, Crit. Rev. Env. Sci. Technol., 23 (1993) 219. [14] J. Cornejo and P. Jamet, Eds., Pesticides/soil interactions: Some current research methods, INRA, Paris, 2000 [15] M.C. Hermosin, J. Cornejo, J.L. White and F.D. Hess, J. Agric. Food Chem., 30 (1982) 728. [16] M.C. Hermosin, P. Martin, and J. Cornejo, Environ. Sci. Technol., 27 (1993) 2606. [17] L. Cox , M.C. Hermosin, W.C. Koskinen and J. Cornejo, Clay Miner., 36 (2001) 267. [18] S.W. Karickhoff, D.S. Brown and T.A. Scott, Water Res., 13 (1979) 241. [19] C.T. Chiou, SSSA Special Publication 22: Reactions and Movement of Organic Chemicals in Soils, Eds. B.L. Sawney et al, ASA and SSSA, Madison, 1989. [20] U. Mingelgrin and Z. Gerst, J. Environ. Qual., 12 (1983) 1. [21] U.A.T. Brinkman, Environ. Sci. Tecnol, 29 (1995) 79. [22] K.S. Reddy and R.P. Gambrell, Agric. Ecosyst. Environ., 18 (1987) 231. [23] R. Calvet, Environ. Health Persp., 88 (1989) 147. [24] W.R. Roy and I.G. Krapac, J. Environ. Qual., 23 (1994) 549. [25] D.A. Laird, P.Y. Yen, W.C. Koskinen, T.R. Steinheimer and R.H. Dowdy, Environ. Sci. Technol., 26 (1994) 1054. [26] I. Roldan, M.C. Hermosin and J. Cornejo, Sci. Total Environ., 132 (1993) 217. [27] L. Cox, M.C. Hermosin and J. Cornejo, Int. J. Environ. Anal. Chem., 58 (1995) 305. [28] L. Cox, M.C. Hermosin and J. Cornejo, Chemosphere, 32 (1996) 1381. [29] M.C. Fernandes, L. Cox, M.C. Hermosin and J. Cornejo, Pest Manag. Sci., 59 (2003) 545. [30] F.H. Yelverton, J.B. Weber, G. Peedin and W.D. Smith, Pub. AG-442 Agricultural Extension Service North Carolina State University, Raleigh, NC, 1990. [31] J. Wagner, H. Chen, B.J. Brownawell and J.C. Westall, Environ. Sci. Technol.,. 28 (1994)231. [32] D.J. Park, W.R. Jackson, I.R. McKinnon and M. Marzhall, Controlled-Release Delivery Systems for Pesticides, Ed., H.B. Scher, Marcel Dekker Inc, New York, 1999. [33] G. Lagaly, Appl. Clay Sci., 18 (2001) 205.

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[90] S. Nir, T. Undabeytia, D. Yaron-Marcovich, Y. El-Nahhal, T. Polubesova, C. Serban, G. Rytwo, G. Lagaly and B. Rubin, Environ. Sci. Technol, 34 (2000) 1269. [91] R. Celis, M.C. Hermosin, M.J. Carrizosa and J. Comejo, J. Agric. Food Chem., 50 (2002) 2324. [92] L. Merrier and C. Detellier, Environ. Sci. Technol., 29 (1995) 1318. [93] G. Sheng, S. Xu and S.A. Boyd, Soil Sci. Soc. Am. J., 63 (1999) 73. [94] R. Celis, M.C. Hermosin and J. Cornejo, Environ. Sci. Technol., 34 (2000) 4593. [95] M. Cruz-Guzman, R. Celis, M.C. Hermosin and J. Cornejo, Environ. Sci. Technol., (in press). [96] Z. Gerstl, A. Nasser and U. Mingelgrin, J. Agric. Food Chem., 46 (1998) 3797. [97] Z. Gerstl, A. Nasser and U. Mingelgrin, J. Agric. Food Chem., 46 (1998) 3803. [98] M. Fernandez-Perez, E. Gonzalez-Pradas, M. Villafranca-Sanchez, F. FloresCespedes and M.D. Urena-Amate, J. Environ. Qual., 29 (2000) 304. [99] R.M. Johnson and A.B. Pepperman, Pestic. Sci., 53 (1998) 233. [100] T.J. Gish, M.J. Schoppet, C.S. Helling, A. Schirmohammadi, M.M. Schnecher and R.E. Wing, Trans. ASAE, 34 (1991) 1738. [101] M. Fernandez-Perez, E. Gonzalez-Pradas, M.D. Urefta-Amate, R.M. Wilins and I. Lindrup, J. Agric. Food Chem., 46 (1998) 3828. [102] H.B. Scher, Ed., Controlled-Release Delivery Systems for Pesticides, Marcel Dekker Inc., New York, 1999. [103] M.E. Carr, R.E. Wing and W.M. Doane, Starch/Staerke, 46 (1994) 9. [104] Y. El-Nahhal, S. Nir, C. Serban, O. Rabinovitch and B. Rubin, J. Agric. Food Chem., 48 (2000) 4791. [105] M.C. Hermosin, M.J. Calderon, J.P. Aguer and J. Cornejo, Pest. Manag. Sci., 57 (2001) 803. [106] T. Undabeytia, S. Nir and B. Rubin, J. Agric. Food Chem., 48 (2000) 4767. [107] M.J. Carrizosa, M.C. Hermosin, W.C. Koskinen and J. Cornejo, Soil Sci. Soc. Am. J., 67(2003)511. [108] Y. El-Nahhal, S. Nir, T. Polubesova, L. Margulies and B. Rubin, Pestic. Sci., 55 (1999)857. [109]Y. El-Nahhal, Environ. Pollut, 124 (2003) 33. [110] R. Celis, G. Facenda, M.C. Hermosin and J. Cornejo, Pest Manag. Sci., (2004) (submitted).

PHARMACEUTICAL AND COSMETIC APPLICATIONS OF CLAYS ALBERTO LOPEZ-GALINDO "' and CESAR VISERAS 2 1

Instituto Andaluz de Ciencias de la Tierra (CSIC-UGR). Facultad de Ciencias, Campus Fuentenueva. 18071 - Granada - SPAIN. 2 Departamento de Farmacia y Tecnologia Farmaceutica, Facultad de Farmacia. Universidad de Granada. 18071, Granada - SPAIN. E-mail: [email protected] * E-mail: [email protected]


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1 - Introduction Clays are substances found throughout the earth's surface, as they are the main component of soils and pelitic sedimentary rocks. Because of their frequency of occurrence and their particular properties, they have been used by man since prehistoric times for therapeutic purposes, such as to cure wounds, relieve irritations or treat gastrointestinal disorders. In Europe, Asia, Africa and America, most ancient civilizations used some form of clays in this manner, the best known examples being those of Mesopotamia, Egypt, Greece and Rome as they were mentioned by numerous classical authors. The medicinal "earths" were normally named according to their place of origin, and were thus known as Egyptian, Nubian, Lemnian, Samian, Cimolian earths, Armenian bole, etc. Lemnian earth, from the Greek island of Lemnos, can be considered the first medicine recorded in history [1] and was in use until the beginning of the last century. Its importance is reflected in its being mentioned, among others, by Homer, Theofrastes, Pliny the Elder, and Galenus, who twice travelled to Lemnos in the Aegean Sea to study its preparation. In the Middle Ages the Arabs added new varieties to those familiar to the Greco-Roman world, with significant contributions by Avicena and Averroes. Later, both the Spanish king Alfonso X the Wise, in the collection of his texts and previous translations known as the Lapidario, and Agricola, in his De Re Metallica, dedicated extensive chapters to the properties and applications of medicinal earths. During the Renaissance, when the first Pharmacopoeia appeared, the use of these clays was regulated to a certain extent. In modern times, with the change of mentality brought about by scientific and technological progress, their use has become considerably more restricted, although they continue to be used as natural remedies for the prevention, relief or cure of certain pathologies of the skin, inflammations, dislocations, contusions and the treatment of wounds. Those interested can find in literature numerous examples of such applications in historic times for both health and beauty [2-6]. The development of some branches of sciences such as Mineralogy, Chemistry and Pharmacy in the 18th and 19th centuries was decisive for understanding the nature of these materials. But it was not until the beginning of the 20th century, with the improvement in instrumental techniques, in particular the discovery of X-ray diffraction, when the causes of the singularly useful properties of clay began to be understood. This are related to directly to their small particle size and their crystalline structure, which makes them suitable for application as absorbent, sterilising, antiinflammatory and detergent substances. At present, environmental awareness and interest in the use of natural products has led to an increase in the use of clayey geomaterials in medical and thermal treatment. In Europe, where the greatest number of spas and therapeutic centres using clays is found, Italy is probably the country with the longest tradition and most frequent use of these materials. For this reason courses and scientific meetings have recently been organised there on the subject [3,4] and where the protocols and norms qualifying the different materials used in fangotherapy are drawn up [7,8]. We should here point out that there is some confusion in the literature regarding the terms "clay mineral" and "clay". The former is a mineralogical term referring to part of a family (the phyllosilicates) consisting of hydrated aluminosilicates containing considerable amounts of Mg, K, Ca, Na and Fe and, occasionally, less

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common ions such as Ti, Mn, or Li. Despite their varied chemical composition, they can be classified in just a few major groups - smectites, micas, kaolin, talcum, chlorites, vermiculites, fibrous and interstratified. The word "clay", on the other hand, refers to natural materials composed of very fine-grained minerals, with some plasticity when mixed with water and which harden on drying. It is, therefore, applicable to all smallsized particles, normally < 2 (im, found in soils or sediments, including, apart from the phyllosilicates mentioned above, other minerals and/or organic products such as quartz, feldspars, carbonates, sulphates, Fe and/or Al oxides, humus, etc. The expression "healing clays" applies mainly to the second term and refers, therefore, to natural clays that, after appropriate treatment to bring out a particular property, are used for pelotheraphy in spa centres. 2 - Structure and texture Clay minerals are among the most widely used materials in pharmaceutical formulation, because of their properties as excipients and/or their biological activities [2,6,9-14). These features depend on both their colloidal dimensions and high surface areas (basic properties), resulting in optimal rheological characteristics and/or excellent sorption capacities. For these reasons, clays have been used for many years in the formulation of solid (tablets, capsules, and powders), liquid (suspensions, emulsions) and semisolid (ointments, creams) dosage forms, either for oral or topical administration. Only some clay minerals are used in pharmacy, including kaolin, talc, smectites (montmorillonite and saponite), and fibrous clays (palygorskite and sepiolite). The kaolin group is a family including kaolinite, halloysite, dickite and nacrite, of which kaolinite is the most common mineral, so that kaolin and kaolinite frequently become synonymous [13]. The smectite group includes, among others, montmorillonite, beidellite, nontronite and saponite, although rocks containing montmorillonite as main mineral are also referred to as bentonites [15]. Finally the palygorskite-sepiolite group includes two minerals - palygorskite (often known as attapulgite) and sepiolite. The particular use of a clay mineral for any specific pharmaceutical application depends firstly on its structure. The structural unit of clay minerals consists of a combination of Al or Mg octahedra and Si tetrahedra, resulting in layered structures that may be organised as consecutive strata of octahedral and tetrahedral sheets (T:O or 1:1 clays), or structures with one octahedral sheet "sandwiched" between two tetrahedral ones (T:O:T or 2:1 clays), allowing for an initial classification. The main difference in the behaviour of these two classes is their performance when dispersed in polar solvents. 1:1 clays do not swell, whereas 2:1 ones do, creating highly structured systems with interesting rheological properties. Further distinction can be made on the basis of chemical differences. In some of these minerals, isomorphic substitution in the octahedral or tetrahedral layers creates negative charges compensated by exchangeable ions in the interlayer space. The swelling properties of clay minerals are strongly affected by the type and hydration grade of the predominant exchangeable ion [16]. Finally, textural differences between structurally and chemically identical minerals affect their adsorptive and rheological properties [17]. As a result of their structural and chemical characteristics, both kaolinite (1:1 layered silicate of Al) and talc (1:1 layered silicate of Mg) show minimal layer charges, presenting low cation-exchange capacities (< 15-20 mEq/lOOg). On the other hand,

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smectites are 2:1 layered silicates, characterised by octahedral and tetrahedral substitutions and high ion-exchange capacities (100-200 mEq/lOOg). Differences in the number of cations in octahedral sites lead to the division of smectites into di- and trioctahedral groups, montmorillonite falling into the first group and saponite into the second. Finally, sepiolite and palygorskite are 2:1 phyllosilicates, but, unlike other clay minerals, they have a fibrous morphology resulting from the 180° inversion occurring every six (sepiolite) or four (palygorskite) silicon tetrahedra, causing a structure of chains aligned parallel to the "a" axis, each of which has a 2:1 structure. This threedimensional ordering also causes open channels measuring 3.7 x 6.4 A (palygorskite) and 3.7 x 10.6 A (sepiolite) and containing zeolitic and crystallization water. Sepiolite has a BET surface area of approximately 300 m2/g and palygorskite 120-180 m2/g. These values can increase as the adsorbed and zeolitic water evaporates when the mineral is heated. Figure 1 shows the most common morphology of these minerals when observed under scanning electron microscope (SEM).

Figure 1 - Usual clay morphology observed by SEM. A) Smectite; B) Talc; C) Kaolinite; D) Palygorskite Although all the particles are small-size (always < 5 microns) the differences between the different types of phyllosilicates are clear. Smectites (A) are usually present


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as scarcely differentiated planar particles with quite irregular edges and a size of less than 2 microns, while the particles of talcum (B) are clearly individualised, with clear edges and a quite larger size. The particles of kaolinite (C) are better crystallised, with abundant pseudo hexagonal shapes, while palygorskite samples (D) are normally made up of large aggregates containing randomly arranged microfibres, although this mineral is sometimes found as small fibres covering other crystals. As a result of these basic characteristics, clay minerals are used in numerous industrial applications involving ceramics, plastics, paper, paint, catalysis, cosmetics, etc., as reviewed in the literature [18-26]. 3 - Use in pharmaceutical formulations 3.1 - Pharmaceutical denominations Both the European Pharmacopoeia (EP) and the United States Pharmacopoeia (USP) contain monographs regarding clay mineral materials. In the EP 4th [27], official monographs of "Aluminium Magnesium Silicate", mixture of montmorillonite and saponite (a), "Bentonite" (montmorillonite, b), "Kaolin" (c), "Magnesium Trisilicate" (sepiolite, d)) and "Talc" (e) are included. Besides these monographs the USP 25 [28] adds the following: "Activated Attapulgite" (f), "Alumina and Magnesium Trisilicate Oral Suspensions" (g), "Alumina and Magnesium Trisilicate Tablets" (h), "Bentonite Magma" (i), "Colloidal Activated Attapulgite" (j), "Magnesium Trisilicate Tablets" (k) and "Purified Bentonite" (1). Table I summarises the clay minerals included in EP 4th and USP 25, with their chemical and commercial correspondences. Some ambiguities between mineralogical, chemical and pharmaceutical names can be observed. The term "bentonite" is too generic, as it can be used both for a rock consisting mainly of smectites (mineralogy) or a material mainly containing montmorillonite (pharmacy). On the other hand, "Aluminium Magnesium silicate" (EP 4th) and "Magnesium Aluminum silicate" (USP 25) are not univocal names, creating some confusion. Both mainly refer to blends of montmorillonite and saponite, according to their specific Al/Mg ratio (between 95 and 105% w/w of that stated on the label). Moreover, palygorskite samples are frequently commercialised under these denominations, as well as attapulgite. Finally, sepiolite seems to correspond to the so-called "Magnesium Trisilicate", described as a blend of Si and Mg oxides prepared to meet the pharmacopoeia requirements. USP 25 requires not less than 20% w/w of MgO and not less than 45% w/w of SiO2, where EP 4* indicates not less than 29% w/w of MgO and not less than 65% w/w of SiO23.2 - Pharmaceutical specifications As intended for use in the preparation of medicines, clay mineral materials must fulfil certain requirements concerning their chemistry (stability and high chemical inertia), physical characteristics (texture, water content, dimensions) and toxicological nature (chemical safety, microbiological purity). Some of these properties, such as those regarding safety and stability, are vital. It must be remarked that minerals intended for use as pharmaceutical materials may contain crystalline silica (both quartz and cristobalite), that should be controlled and avoided as far as possible, as it is classified by the International Agency for Research on Cancer (IARC) as a product with sufficient evidence of carcinogenicity in laboratory animals and limited evidence in humans (group 1, IARC Monographs) [29]. On the other hand, amorphous silica, found in nature as biogenic silica (e.g.

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diatomaceous earth) and as silica glass (volcanic genesis), is not classifiable as carcinogenic to humans (group 3) [29]. Regarding kaolin, USP 25 specifies that it must be "powdered and freed from gritty particles by elutriation". Impurities such as quartz, mica, hematite or pyrite are mainly contained in the coarse fraction of the rock, and should be eliminated. Concomitant administration of drugs, such as some antibiotics (amoxicillin, ampicillin, clindamycin), cimetidine, atropine, phenytoin, digoxin and quinidine could reduce drug absorption as a result of drug-kaolin interaction, and should be avoided [30-36]. Talc, presented as a white or almost white, impalpable and unctuous powder, may contain variable amounts of other minerals, such as hydrated Al silicate, magnesite, calcite and dolomite that can remain when used as excipient. Talc containing asbestos is, however, not suitable for pharmaceutical use because of its carcinogenic activity in humans. According to the IARC monograph [37,38], talc not containing asbestiform fibres is not classifiable as to its carcinogenicity to humans (group 3), while there is sufficient evidence for the carcinogenicity to humans of talc containing asbestiform fibres (group 1). In fact, the EP 2002 monograph on talc includes specific tests (infrared, X-ray diffractometry and optical microscopy analysis) to detect asbestos and to determine asbestos character in talc. Because of their cation-exchange capacity smectites can interact with certain drugs affecting their bioavailability. Nevertheless, this interaction could be advantageous in the formulation of controlled release systems, which is one of the most attention-grabbing fields of clay applications at present, as discussed below. Table 1 - Pharmaceutical, mineral, chemical and commercial correspondences among clays used in Pharmacy Clay Mineral

Rock

Pharmacopoeia] name

Kaolinite

Kaolin

Kaolin, Heavy (EP 4th)

Talc

Talc

Kaolin (USP 25) Talc (EP 4th and USP 25)

Montmorillomte

Beniorute

Bentonite (EP 4th and USP 25)

Group: Smectites Subgroup: diocthaedral Saponite

Purified Bentonite (USP 25)

Chemical name and CAS registry number Hydrated aluminium silicate (1332-58-7)

Empirical formula

Usual names

Al,Si,O s (OH)«

Talc (14807-96-6)

Mg 3 Si 4 O l() (OH)i

Aluminium magnesium silicate (1302-78-9) Aluminium magnesium silicate (12511-31-8)

(Na.Ca.KWAl.Mgfe Si 3 Oi 0 (OH)-.nH,O

China Clay, bolus alba. porcelain clay, weisserton, white hole Magsil osmanthus, Magsil star. powdered talc, purified french chalk, purtalc, soapstone. steatite Mineral soap, clay soap, taylorite, wtlkinite, Veegum HS, Albagel, mineral colloid

Bentonite

Aluminium magnesium silicate (EP 4th) Magnesium aluminium silicate (USP 25)

Aluminium magnesium silicate (12511-31-8) Magnesium aluminium silicate (1327-43-1)

(Ca,Na,K) W3 (Mg,Fe) 3 (Si, Al) a Oi 0 (OH) : .nH ; O

Veegum R-K-HV-T-F, Carrisorb, Gelsorb, Magnabites Colloidal, Colloidal complex

Suberoup: triocthaedral Palygorskite

Palygorskite

Attapugite (USP 25)

(Mg,Al.Fe) 5 (Si,Al) 8 0 : o (OH), (OH2), (H : O) a

Attapulgite, Attasorb, Pharmasorb

Sepiolite

Sepiolite

Magnesium trisilicate (EP 4th) and (USP 25)

Aluminium magnesium silicate (12511-31-8) Magnesium aluminium silicate (1327-43-1) Hydrated magnesium trisilicate (39365-87-2) Magnesium aluminium silicaie (1327-43-1) Anhydrous magnesium trisilicate, magnesium metasilicate, magnesium ortnsilicare

Mg 3 Si i : 0 3 0 (OHMOH : )4 (H2O)B

Silicic acid, hydrated magnesium salt, meerschaum, parasepiolite, sea foam, talcum plasticum.

Group: Smectites

Finally, regarding fibrous clays, although this is not a specific requirement of any Pharmacopoeia, the particle size of fibrous minerals must be carefully controlled because of its possible biological effect. In a preformulation study, Viseras et al [39] showed that samples corresponding to different clay minerals used in pharmacy


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presented particle sizes lower than the value generally accepted for defining a particle as a fibre (> 5 urn in length and a length/diameter ratio > 3:1) [40]. Moreover studies carried out on humans exposed to some sepiolite samples confirmed that exposure to these minerals involves no risks [41-43]. The I ARC clearly distinguishes between palygorskite and sepiolite [44,45]. Palygorskite samples are classified as long (>5um) clay fibres and short(5um) fibres. 3.3 - Use as excipients In the preparation of pharmaceutical products particular importance is attached to the selection of suitable excipients, i.e., auxiliary substances contained in the formulation with the purpose of providing the product with an adequate presentation. Excipients must facilitate the administration of the active ingredients, improve their efficiency and ensure stability until the expiry date for usage by the patients. The fundamental property for a product to be used as excipient is it being innocuous, while attention should also be paid to other attributes affecting the organoleptic characteristics of the end product, such as taste, smell and colour. Clays are regarded as essentially non-toxic and non-irritant materials at the levels used in pharmaceutical excipients and are included in the Inactive Ingredients Guide [46] published by the Food and Drug Administration (FDA). This guide contains all inactive ingredients present in approved (or conditionally approved) drug products marketed for human use. Table 2 shows a summary of the applications of clay minerals as pharmaceutical excipients in drug products as provided by this guide. 3.3; 1 - Solid dosage excipients In Table 3, clay minerals are classified according to their functionality as excipient in solid dosage forms. Kaolinite is mainly used as a diluent because of its white to greyish-white colour. Its suitability as pharmaceutical excipient greatly depends on the geological nature (sedimentary, residual, and hydrothermal) and mineral composition of the deposits, which have an important effect on texture and particle size distribution, and consequently, on the rheological properties (flow) of the powder mass [26,47,48]. Talc is mainly used as diluent, glidant and lubricant in tablet and capsule formulations. In addition, talc is used as an additive to promote film coating of tablets and particles [13,49,50]. Several studies have shown that the chemical composition and physical properties of talc depend on the source and the method of preparation [47,48,51-55]. Smectites, such as bentonite and Mg Al silicate, are used in solid dosage forms as tablet and capsule disintegrants, tablet binders and adsorbents. The use of bentonite in the formulation of tablets has been studied in the past by several authors [56-58]. Feinstein and Bartilucci [59] investigated the efficiency of disintegration of bentonite, concluding that its effectiveness is comparable to other typical disintegrants, such as cellulose derivatives. Wai et al [60] indicated that laminar clays are not good disintegrants when used as intragranular agents. In contrast, Fielden [61] proposed that a suitable technological procedure results in good disintegrant characteristics, even when

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used as an intra-granular agent. Table 2 - Pharmaceutical applications of clays as excipients in drug products for human use. Pharmacopeial name

Administrati ion

Dosage form

Potency range (*)

Immediate release (IR) Kaolin or Heavy kaolin

Oral

and Modified release (MR)

— N o n specified

(delayed or sustained) tablets Film coated tablets

Talc

0.189-204mg

Oral

MR (Sustained or Repeat actiion) 0.2 - 3 mg (sustained) and 73.93 mg (repeat) tablets

Sublingual

Tablet

5 mg

Lotion Topical

Ointment Powder

Non specified

Capsules Oral Bentonite or Purified Bentonite

Tablets Suspensions

0.45 % w/w

Topical

Suspensions

2.1 %w/w

Transdermal

Film

Vaginal

Suppository

Non specified

Drops Granule Oral Mg Al or Al Mg silicate

Reconstitution granules

— N o n specified

Syrup Suspensions

0.15-2 %Wv

Tablets

8mg

Rectal

Suspension

Vaginal

Ointment

Non specified

Emulsion (creams) Topical

Lotion

1.5% w/w

IR Tablets Magnesium Trisilicate

Oral

Coated Tablets

Non specified

Sustained Release Tablets (*) POTENCY RANGE: Minimum and maximum amounts of inactive ingredient for each route/dosage form

The use of fibrous clays in the formulation of tablets is based on their properties as glidants and binders. The suitability of some laminar and fibrous phyllosilicates as additives in solid dosage forms was recently investigated [12,62]. Fibrous clays can also be used as disintegrants. Viseras et al [63] showed that, in comparison to other silicates, sepiolite could be used as direct compression disintegrant even at low concentration. Regarding binding properties, Angulo et al [64] showed that sepiolite considerably improved the durability and quality of pellets. Moreover, their


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high surface area allows fibrous clays to be used in solid formulations as adsorbents of liquid drugs. Finally, unlike palygorskite, sepiolite may be used as a pharmaceutical excipient for drugs subject to oxidative degradation, such as hydrocortisone. Sepiolite avoids degradation because of its lower ferric iron content in comparison with palygorskite [65,66]. Table 3 - Uses of clay minerals as excipients in solid dosage forms Excipient

Dosage forms

Functional category

Kaolin and Heavy Kaolin

Tablets and capsules

Diluent and adsorbent

Talc

Tablets, capsules and powders

Coating aid, lubricant, diluent and glidant

Tablets, capsules and granules

Adsorbent, 1Dinder and disintegrant

Tablets and capsules

Adsorbent, |jlidant, binder and disintegrant

Bentonite Magnesiurr i Aluminium Silicate Magnesiurr 1 Trisilicate

3.3.2 - Liquid and semisolid dosage excipients Pharmaceutical dispersions are shaken several times during their "life", leading to changes in the system structure, and when administered orally they encounter a special pH environment that may severely affect their properties. Both suspending and anticaking agents are used to prevent drastic changes in dispersion properties. Some types of laminar and fibrous clays are particularly useful as stabilisers because of their positive thixotropic nature [39,67-70]. Table 4 summarises the main uses of clays in liquid and semisolid formulations. Kaolinite and talc are employed in liquid formulations as suspending and anticaking agents [13]. Lagaly [71] pointed out the importance of particle morphology and surface charge in the rheological behaviour of kaolin suspensions. Yuan and Murray [72] compared the rheological characteristics of kaolin dispersions prepared with different crystal morphologies (planar kaolinite and tubular or spherical halloysite), concluding that particle morphologies strongly affected the dispersion viscosities. Bentonite and Magnesium Aluminum Silicate are commonly used as suspending and stabilising agents in the formulation of suspensions, gels, ointments and creams for oral or topical administration. USP 25 describes four types of Magnesium Aluminum Silicate (IA, IB, IC, IIA) with different viscosity and Al/Mg ratio contents. When laminar clays are dispersed in a polar medium, face-edge and face-face interactions are the two major mechanisms implied in the formation of a rigid network [73-78]. Recently, the colloidal and rheological properties of bentonite suspensions were reviewed by Luckam and Rossi [79], who emphasise that laminar silicate gels are sensitive to the addition of electrolytes. In addition, Ma and Pierre [80] considered the influence of Fe3+ ions on the colloidal behaviour of montmorillonite suspensions, concluding that both Fe3+ and its hydrolytic products acted as counter ions to neutralise the electric double layer around clay particles. By means of absorption on clay particles,

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the hydrolysis products could also modify the surface charge of the clay thus improving suspension coagulation. The effect of ion type and ionic strength on the sol-gel transition of sodium montmorillonite dispersions was studied by Abend and Lagaly [81], who obtained phase diagrams of different states (sol, repulsive gel, attractive gel, sediment) of the dispersions, showing that the borderline between gel and sediment depends on the type of counter-ion and co-ion. Table 4 - Uses of clays as excipients in liquid and semisolid dosage forms Excipient

Dosage forms

Functional category

Creams and pastes

Emulsifying agent

Suspensions

Suspending and anticaking agent

Kaolin and Talc

Bentonite and Magnesium Aluminium Silicate

Magnesium Trisilicate

Ointments, Creams and Gels Emulsifying agent Suspensions


Suspensions


Fibrous clays dispersed in water form a three-dimensional structure composed of interconnecting fibres [82]. Fibrous clay gels retain their stability in the presence of high concentrations of electrolytes, thus making them ideal for such an application [8385]. Some investigations have focused on the effects of hydrodynamic factors, such as size and shape of the particles, on the final product properties. Viseras et al [39] assessed the effects of shear history on the rheology of laminar and fibrous clay dispersions, concluding that the degree of dispersion and the structural changes resulting from differences in particle shape significantly affect the rheological properties of the systems. A linear relation was found between mixing energy and apparent viscosity in the laminar systems, while apparent viscosity was related to mixing power for the fibrous ones. A subsequent study examined the filtration behaviour of some Spanish clay-water dispersions, the results of which were compatible with the rheological properties of the systems [70]. Some authors have evaluated the use of clay minerals in combination with other agents. Ciullo [86] showed a synergic effect of Veegum® and natural gums as stabilisers in the formulation of emulsions. Recently, Lagaly et al [87,88] studied the use of smectites in combination with non-ionic surfactants as stabilisers in the formulation of oil in water emulsions. The main mechanism of stabilisation was the formation of a mechanical barrier around the oil phase droplets, preventing their coalescence. The rheological behaviour of the emulsions was also investigated and a strong influence of the clay mineral and surfactant was found.


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3.4 - Use of clay minerals as active substances Clay minerals are also used in pharmacy because of their biological activity, both in the treatment of gastro-intestinal and topical diseases. Moreover, they are used in the treatment of some much more specific illnesses. Marketed preparations containing clays as active substances are summarised in Table 5. Table 5 - Uses of clays as active principles in marketed products Active

Therapeutic use

Antidiarrhoeal & gastrointestinal protectors

Kaolinite Antacid

Brand names EU: Dystomin-E, Entrocalm, Collis Browne's, Kaoprompt-H, Kaopectate, Kaopectate-N, Enterosan, Kaodene, Kalogeais, Pectipar, Carbonaphtine Pectinee, Kao-Pront USA: Kao-Spen, Kapectolin, K-P Generic, Kaopectate Other: Bipectinol, Donnagel-MB, Kaomagma, Kaomagma with Pectin, Chloropect EU: Neutroses Vichy, Neutroses, Kaobrol Simple, Kaomuth, Anti-H, Gastropax Other: De Witt's Antacid

Anti-inflammatory

EU: Cicafissan, Antiphlogistine USA: Mexsana

Homeopathic Product

Other: Alumina Silicata

Anti-rubbing

EU: Ictiomen, Aloplastine, Lanofene 5, Poudre T.K.C.

Anti-haemorrhoids

EU: Titanoreine

Talc Pleurodesis

Palygorskite (attapulgite)

Antidiarrhoeal

Formulated and prepared in hospitals just before their use USA: Diar-Aid, Diarrest, Diasorb, Diatrol, Donnagel, Kaopectate, Kaopectate Advanced Formula, Kaopectate Maximum Strength, Kaopek, K-Pek, Parepectolin, Rheaban and Rheaban Maximum strength, Quintess EU: Streptomagma, Actapulgite, Gastropulgite, Mucipulgite, Norgagil, Diasorb Others: Fowler's and Kaopectate

Magnesium Trisilicate

Smectite

Antacid

USA: Streptomagma, Kaopectate

Antacid

EU: Neutroses Vichy, Neutrose S. Pellegrino, Instatina, Masbosil, Silimag, De Witt's antacid, Anti-acide-GNR, Gastric Expanpharm, Gastropax, Magnesie compose Lehning, Triglysal, Contracide, Gelusil Other: Trisil,. De Witt's Antacid, Gasulsol Tab

Antacid Antipruritic and local anaesthetic Antidiarrhoeal & gastrointestinal protectors

EU: Smecta Others: Calamine Lotion

EU: Diosmectite

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3.4.1 - Antidiarrhoeal uses Antidiarrhoeals are usually categorized in four groups; antiperistaltics, adsorbents, antisecretory and digestive enzymes. Palygorskite and kaolinite are included in the second group of adsorbent agents [89,90]. Kaolinite, used as oral adjunct in the symptomatic treatment of diarrhoea because of its adsorbent properties, is administered orally in doses of about 2-6 g every four hours [13]. It may be formulated alone or in combination with other actives, such as pectin, loperamide, aluminium and magnesium salts, belladonna extract and morphine. As regards palygorskite, it has been described as even more effective than kaolinite in the symptomatic treatment of diarrhoea because of its capacity to adsorb and retain water, bacterium and some toxins [91]. Cerezo et a! [92] evaluated the possibilities of fibrous clays as non specific anti-diarrhoeic agents, concluding that both palygorskite and sepiolite comply with the pharmacopoeial specifications and may be taken into account. The daily dose of palygorskite can be up to 9 g in the form of oral suspension, conventional and chewable tablets. For oral suspension and tablets, the usual dose is 1200 to 1500 milligrams (mg) taken after each loose bowel movement, with no more than 9000 mg being taken in twenty-four hours. For chewable tablets, the dose is slightly less, and no more than 8400 mg should be taken in twenty-four hours. However, no conclusive evidence is available to show that palygorskite use may reduce the duration of diarrhoea, stool frequency, or stool fluid losses [93]. Smectite is equally effective in the treatment of infectious diarrhoea as it reduces the duration and frequency of liquid stool by mechanisms including absorption of water and electrolytes in the intestine, decrease of mucolysis caused by bacteria and protection of the luminal surface against pathogenic bacteria [94,95]. Moreover, some authors have described the use of smectite in the treatment of acute diarrhoea, although this clay is not currently recommended for this purpose [96-98]. On the contrary, Carretero [6] recently illustrated the use of Na+ smectite as osmotic laxative, although no experimental evidence supports this statement. 3.4.2 - Gastrointestinal protector Mucus forms a 200 micra thick layer of gel on the gastro-duodenal mucous membrane [99], which acts as a physical barrier preventing direct contact between the gastric enzymes and the cells of the mucous membrane, thus avoiding digestion of the latter [100] and mechanical erosion. In patients suffering from peptic ulcer, the thickness of the mucus layer decreases, while the mucolytic activity of the gastric juices and the enzyme levels increase [101,102). Clays provide multiple gastro-intestinal protection mechanisms associated with the different etiologies of deterioration mechanical erosion, enzyme attack, bacterial toxins, drugs, alimentary allergies, genetic factors, environmental factors such as tobacco, alcohol, etc. Several adsorbent agents (bentonite, kaolinite, active carbon) present anti-endotoxemic activity both in vivo and in vitro that reduces the alteration of the mucous membrane to the levels of healthy individuals [103,104]. The protective effect of clays on intestinal barriers is related to their influence on the rheological properties of the mucus. As discussed previously, clay particles dispersed in a water solution medium greatly increase its viscosity and, consequently, its stability. On the other hand, the mucoadhesivity of clays, i.e., their positive interaction with and binding to glycoproteins present in the mucus, is probably an important protective mechanism. Deterioration of glycoproteins by reactive agents, such as free


279

radicals, ethanol or some drugs, is reduced when the polymer is complexed to the clay [105-108]. 3.4.3 - Antacid uses Clay minerals such as palygorskite and magnesium trisilicate (sepiolite) may be used as symptomatic antacid agents, because of their capacity to neutralise acidity in the gastric secretions. They are used in combination with aluminium hydroxide and kaolinite in suspensions and chewable tablets. Magnesium trisilicate is given in doses from 1 to 4 g, reacting with hydrochloric acid to form magnesium chloride and silicon dioxide, with an H+ neutralising capacity of around 15 meq/g. It is indicated in the treatment of gastric and duodenal ulcers. Magnesium chloride resulting from the neutralizing action may induce diarrhoea in some cases. Kaolinite in combination with sodium bicarbonate and magnesium trisilicate is commercialized, having an H+ neutralising capacity of around 56 meq /g. 3.4.4 - Anti-inflammatory and antiseptic purposes Purified talc is used in dusting powders to calm irritation and prevent roughness, while kaolinite is used for sore throat symptoms, including tonsillitis, pharyngitis and stomatitis, and is responsible for the adsorption of waste products. Mixtures of kaolin, bentonite and palygorskite have been proposed for use as dressing for the treatment of skin injuries, especially burns [109]. Kaolinite is applied topically as kaolin poultice to reduce inflammation [110]. Finally, pastes of kaolin and salicylic acid are applied as percutaneous anti-inflammatory in the treatment of muscular pain and tendonitis. Fibrous clays are also used in the treatment of aqueous inflammations, adsorbing the aqueous fraction and probably also retaining the proteic fraction of the inflammation [111-113]. They are probably able to effectively retain toxins and bacteria as happens in the gastrointestinal tract. Bentonite is included in antipruritic and local anaesthetic preparations for topical use. 3.4.5 - Topical applications The use of clay minerals as actives in topical dosage forms (creams, milks and powders) has been proposed on the basis of their capacity to efficiently adsorb a variety of undesired substances, including greases, skin exudates and external agents such as bacterial toxins [114]. However, clay minerals in such formulations are normally employed as excipients, i.e., as auxiliary substances intended to maintain the dose of the active principle in the area to be treated by increasing the viscosity of the system, promoting skin-adhesivity and keeping a high concentration of drug in the proximity of the treated skin-area. 3.4.6 - Other uses Kaolin is included in human homeopathic preparations, when it is known as alumina silicate, in the form of drops, globules and oral granules. Bentonite may be used as adsorbent in paraquat poisoning [110]. Talc is concomitantly used with carrageenates in suppositories administered as mucoprotectors and lubricants of rectal mucosa in the treatment of haemorrhoids. Finally, this mineral is also indicated as the preferred treatment for pleural effusion, a complication in patients with malignant neoplasms caused by disturbance of the normal

280


reabsorption of fluid in the pleural space [115]. Talc pleurodesis for the treatment of malignant pleural effusion is an effective method, preventing recurrent effusion in 8090% of cases and being less painful than tetracycline [116]. Talc can be insufflated in a dry state or instilled as slurry. The dose should be restricted to 5 grams [117]. 4 - New uses in modified drug delivery systems Most of the clays used in pharmacy can interact with other components of the formulation and, in the specific case of drug-clay interaction, this can affect the bioavailability of the drug itself [118-120], among others. The best known cases are those of montmorillonite and saponite, which are fairly common, well studied smectites [121,122]. Later studies have evaluated the effect of several factors such as ionic strength, the dielectric constant of the medium and the addition of polymers, confirming that ionic exchange is the main mechanism involved in absorption [123-127]. More recently, Tolls [128] examined the influence of the molecule's lipophilia in the absorption by clays of various veterinary drugs. On the other hand, the oral administration of fibrous clays could also affect the bioavailability of some drugs, such as mebeverine, folic acid, contraceptive steroids, promazine, atropine, glycosides (digoxin, digitoxin), erythromycin, paracetamol, chloroquine, quinidine, propranolol and tetracycline. Moreover antimicrobial preservatives, such as parabens, could be inactivated [129-133]. In recent years, there has been discussion on how to take advantage of these interactions for aims that are biopharmaceutical (modification of drug release or solubility), pharmacological (prevention or reduction of side-effects) and chemical (increased stability) [134]. Ideally, a pharmaceutical form should be designed to fulfil the therapeutic requirements, while avoiding or minimising the side effects. Conventional pharmaceutical forms are designed to release the dose immediately and achieve rapid, complete absorption of the drug. However, immediate release forms require repeated administration to maintain efficient concentrations of the drug. To avoid this, modified release pharmaceutical forms attempt to fulfil the therapeutic requirements by optimising the time, rate and location of drug release [28] and are known as "sustained release" (reducing frequency of administration to at least half that of a conventional form), "delayed release" (releasing the drug over a predetermined period) and "sitespecific release" (releasing the drug at or near the place of physiological activity). These release objectives can be achieved by using products of drug-clay interaction. Delgado et al [135] examined the use of kaolinite samples with different degrees of crystallinity as vehicles for the controlled release of drugs and found a linear relation between the crystallinity of the mineral and the release of amilobarbitone. Halloysite, a tubular polymorph of kaolinite, has recently been proposed for pharmaceutical use and, specifically, the tubules of this mineral could act as natural vehicles for microencapsulation and controlled release of both hydrophilic and lipophilic agents [136-138]. Moreover, the alternative absorption of macromolecules of opposite charge, including proteins, clays and poly-ions, has been proposed for the preparation of immobilisation vehicles, characterised by their capacity to guarantee the biological activity of the enzyme [139]. The halloysite microtubes filled with NAD coenzyme were assembled with ADH (alcohol-dehydrogenase) coenzyme and used as sustained release vehicle of the cofactor of the immobilised enzyme [140]. Regarding the use of swelling clays, Cameroni et al [141] studied the effect of


281

different factors on drug release from compounds of papaverine and Veegum® (a commercial smectite) and found that the amount released depended on the pH, the ionic strength of the dissolution fluid and the elimination rate of the drug from the medium. Moreover, optimisation of the formulation, obtained by surface deposit of the papaverine on the compound, gave in vitro absorption profiles with zero kinetics, together with rapid achievement of constant drug concentrations in the gastro-intestinal tract [142]. Finally, Forni et al [143] showed that montmorillonite affects release in matrices of polyvinyl alcohol by interaction with the drug. More recently, Oya et al [144] proposed the use of Ag / montmorillonite compounds instead of a conventional organic agent, as a thermostable inorganic agent with high antimicrobial and antifungic activity for the treatment of muco-cutaneous conditions. Similar results were found using chelates of Ag and Tiabendazol in montmorillonite [145]. Fouche [146] examined the use of antibiotic-clay compounds in the treatment of gastric ulcer determined by Helicobacter pylori, with the conclusion that the clay aided penetration of the drug through the gastro-intestinal barrier. Absorption of 5-fluorouracil by montmorillonite has been considered for the development of new therapeutic systems for oral administration in the treatment of colo-rectal cancer [147]. hi recent years, five Spanish clays, including smectites and fibrous minerals, have been evaluated with regard to enzyme immobilisation, with the conclusion that at least the fibrous minerals could be used as vehicles for biotransformations [148]. These same clays can be used to obtain compounds with different types of drugs (timolol, tetracyclines, imidazolic antifungics) in which the release profiles have suitable kinetics for use as modified release systems [134,149-151). Preparation of the compounds is carried out by interaction of the solid (clay) with solutions of the drug in different media. However, Rives-Arnau et al [152] proposed a new dry process for the preparation of drug-clay compounds, as an alternative to the more common wet process, consisting in complexing by grinding the clay and the drug together. A third formation mechanism of these compounds would involve contact between the drug and the clay at the melting temperature of the active agent (Viseras et al, unpublished). 5 - Use in cosmetics The pharmaceutical (treatment) and cosmetic (care and beauty) uses of clay minerals are normally mentioned together, even though their aims are very different. It is therefore advisable to specify the intended use of a clay, as this will determine not only the technical aspects of its treatment, but also legal questions or matters of code of practice. A "cosmetic product" is any substance or preparation intended to be placed in contact with the various external parts of the human body (epidermis, hair system, nails, lips and external genital organs) or with the teeth and the mucous membranes of the oral cavity with a view exclusively or mainly to cleaning, perfuming, changing their appearance and/or correcting body odours and/or protecting or keeping in good condition (Council Directive 76/768/EEC on the approximation of the laws of the Member States relating to cosmetic products). On the other hand, Council Directive 2001/83/EC on the Community Code relating to medicinal products for human use defines "a medicinal product" as any substance or combination of substances administered to human beings for treating or preventing disease, making a medical diagnosis or restoring, correcting or modifying physiological functions.

282


A detailed study of all the possibilities of clays in the field of cosmetics falls outside the scope of this review, and so we shall concentrate on some examples that show the close relationship between their cosmetic applications and the properties resulting from their high specific surface and small size, which have been extensively discussed above. In most cases, cosmetic preparations make use of clays' rheological properties with the aim of the physical stabilising of the end product, just as they are used as excipients in medicinal preparations. Similarly, the use of clay minerals as active in cosmetics is closely related to their adsorbent capacity. They are used in deodorant powders and creams as they eliminate the gases responsible for the bad smell [153], in bath powders and baby powders, where they absorb sweat and humidity, keeping the folds of skin lubricated and thus avoiding friction; in facial powders to reduce the shine of talcum and increase the adherence of the preparation; and, finally, in face packs to clean the skin of grease. Other cosmetic uses are related to their emulgent capacity, whose mechanism has already been discussed, examples being the use of palygorskite and smectite in dry shampoos, which are widely used in North African countries. They can also be used as protection against external agents, in particular solar radiation, as proposed by Del Hoyo et al [154], who determined the capacity of phenylsalicylate complexes in sepiolite to prevent sunburn. In cosmetic preparations the clays act as a physical barrier against UV radiation, considerably increasing the protection factor of the compound. This is a question of much interest at present, given the appreciable increase in skin pathologies caused by radiation. 6 - Topical use: clays in spas Applications of clays to the human body for therapeutic purposes (geotherapy, fangotherapy and pelotheraphy) are very ancient techniques which have become increasingly popular in recent times. The beneficial effects for particular rheumaticarthritic pathologies and sporting injuries, as well as in dermatological and cosmetic applications, are based on the rheological properties, the high capacity for cation exchange and absorption, and the slow cooling rate of clays when properly prepared using different types of water. The term "peloid" refers to the product resulting from the mixture of a liquid phase (salt, saline or mineral-medicinal water), a solid inorganic phase (clay minerals and other minerals such as quartz, calcite, feldspars, etc.) and a third organic phase (bacteria, algae, diatomeas, protozoa, arthropods, etc.), which is applied topically as a therapeutic agent in the form of poultices or baths [6]. The preparation of thermal muds and peloids (medical and not mineralogical or geological terms) from clayey materials rich in smectites and other clays requires a process known as "maturation" affecting the clays when they are brought into contact with thermal and/or mineral water [155-160]. Traditionally, sulphurous water is used when the aim is to produce dermatological masks and bromo-iodic water for thermal treatment of bone and muscular injuries [161]. The maturing process lasts from 3 to 20 months and causes important changes technical properties of the clays, whose plasticity, absorption capacity, cooling index and grain-size alter as a result of the profound interaction between the different phases involved and the biological activity of the organisms themselves and their metabolic products. The nature of both the mineral and organic components involved is decisive for the final properties of the therapeutic mud [7,162-166], which varies from spa to spa according to the type of clayey material used


283

and the composition of the thermal-medicinal water. The peloid obtained after maturation is applied to the whole body or on selected parts of the patients for 10 to 15 days at a temperature of 40-45°C in 1 to 2 cm layers for 20-30 minute sessions. The application produces relaxing, anti-inflammatory and analgesic effects in the treated area due to vasodilation, perspiration and stimulation of the cardio-circulatory and respiratory systems. It is particularly beneficial in the treatment of degenerative arthropathies and the associated painful syndromes, bone and joint injuries, rheumatism and arthritis in different parts of the body, spondylosis, myalgia, neuralgia, chronic phlebopathy, certain skin ailments, etc. [167-169]. Although there is no specific protocol for qualification of any one "peloid" thermal mud, in recent years considerable progress has been made in this direction, particularly due to the various proposals of the Italian Group of the AIPEA [3,4]. In many spas, after the local reserves of clays are exhausted, artificial mixtures of clayey materials are used whose nature is not always clearly determined. The choice of a suitable material should be made with clear ideas as to factors such as mineral composition, chemistry, pH, grain-size, specific surface, cation exchange capacity (total and for the main cations Na+, K+, Ca2+, Mg2+), consistency parameters (liquid and plastic limits, plastic index), rheology (activity, adhesivity, viscosity, water retention), thermal behaviour (heat capacity and conductivity, cooling kinetics), and organic matter and micro-organisms content. The most suitable materials are those with a high content in swelling clay minerals, fine granulometry and a low amount of "abrasive" materials (quartz, feldspar) for a pleasant application of the "peloid" mud, good thermal, rheological and adhesive properties and a low content in hazardous trace elements and minerals (such as free silica and asbestiform minerals). In this sense, we should point out the importance of control of the contents in certain, potentially toxic trace elements and their mobility during the maturing process (such as As, Sc, Tl, Pb, Cd, Cu, Zn, Hg, Se and Sb) in order to avoid possible intoxication during treatment [164,170,171]. 7 - Concluding remarks The development of certain instrumental techniques during the second half of the 20th century led to the discovery of the enormous compositional and textural variability of clays, thus improving understanding of the different mechanisms involved in their physico-chemical properties, in particular, those related to their surface characteristics (adsorbent capacity and Theological properties). These theoretical advances, which helped understanding of the processes behind the traditional uses of clays since antiquity as natural products with therapeutic and cosmetic aims, also resulted in the development of new applications. Of all the applied sciences using these "new" materials, those concerning health seem to be where most future investigation will take place on clays, to determine their possibilities in the treatment of illnesses and in the care and protection of the human body. What is at present known, as briefly described in this survey, informs of the variety and number of applications in use and allows us to foresee important advances in the coming years, particularly in the development of new drug delivery systems. The global increase in standard of living also suggests that body care in specialized centres will become increasingly popular, involving a reconsideration of the geomaterials used in such centres, which will inevitably require a correct qualification of the materials used and the exchange mechanisms involved.

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REMOVAL OF METALS BY NATURAL AND MODIFIED CLAYS CRISTINA VOLZONE Centro de Tecnologia de Recursos Mmerales y Ceramica - CETMIC (CIC-CONICET-UNLP) - CC 49, Cno. Centenario y 506, (1897) M.B. Gonnet Provincia de Buenos Aires - ARGENTINA E-mail: [email protected] / [email protected]

Clay Surfaces: Fundamentals and Applications F. Wypych andK.G. Satyanarayana (editors) © 2004 Elsevier Ltd. All rights reserved.

Removal of Metals by Natural and Modified Clays

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1 — Introduction

It is well known that metals occur in nature and that from the early days man has been using some of them for different purposes. Copper, bronze and iron have been the most important ones in human history because they have marked an advance in civilisation as man started using tools made of them (approx. in 8,000 BC; 3,000 BC, and 1,200 BC, respectively). So important were these metals that historians took the name of some of them to describe periods, such as' Bronze Age' and 'Iron Age'. Metallic elements are classified on the basis of their physicochemical properties and these occupy three-quarters of the periodic table of elements. Growth in population and advances in science and technology have brought comfort, but a number of natural systems were altered and health and the environment have been affected as well. In our daily life, we are surrounded by metal ores and metallic elements that we have put to different uses. However, during the process of obtaining certain metallic elements from ores (metallurgical industry), a lot of waste is produced that is harmful to us. Acid mine water may show huge amounts of such metals, for example cadmium, copper, zinc, lead, mercury, etc. This water may occasionally contaminate the underground and natural water thus leading into a serious threat to living organisms. Therefore, the amount of metal in wastewater has to be reduced to prevent its accumulation in the biosphere. Heavy metals refer to high density metallic elements such as mercury, cobalt, copper, chromium (III), iron, etc. However, some of these metals including manganese, molybdenum, vanadium, strontium, zinc, etc, are essential to human, animal and plant life. On the contrary, excessive levels of some of them may be toxic. There are others, which are harmful to health even in low concentration such as mercury, lead, cadmium, chromium (VI), arsenic, and antimony. Treatment and location options for such heavy metals are to be taken into account to purify soils and waters. Toxic heavy metals such as cadmium and mercury may be included in the dietary habits of animals through environmental exposure, thus contaminating food products derived from those animals. There is an increasing interest around the world in cleaning up polluted rivers and lakes, and implementing systems that regulate the disposal of waste that contains metals. Among others, the International Environmental Protection Agency (EPA) has determined various levels in the concentration of metals beyond which organisms may be altered, even die, and produce corrosion on different solids. Most often, cadmium occurs in small quantities associated with zinc ores but also with copper and lead ores. Whenever cadmium compounds bind to sediments in rivers, they may be bio-accumulated or redissolved easily, and some cadmium compounds may leak through the soil and reach underground water. Environmental protection agencies have determined that the maximum level of cadmium in drinking water should not exceed 5 parts per billion (ppb) to overcome any health hazard. Copper is a metal found in natural deposits as ores containing other elements. It is widely used in household plumbing materials, and the maximum contamination level has been set at 1.3 parts per million (ppm). Copper is rarely found in underground water, but copper mining and smelting operations may be a source of contamination. The greatest percentage of total zinc in polluted soil and sediment is associated with iron and manganese oxides. Chromium is a metal found in natural deposits as ores containing other elements. It is mainly used in metal alloys such as stainless steel, protective coatings on metal, tannery industry, magnetic tapes, and pigments for paint products, cement, paper, rubber, floor covering, and other materials. The maximum level for chromium has been set at 0.1

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ppm. Mercury is a liquid metal found in natural deposits as ores containing other elements. Electrical products such as dry-cell batteries, fluorescent light bulbs, switches, and other control equipment account for half of the mercury in use. No more than 2 ppb of mercury is allowed in drinking water. Mercury is usually removed from wastewater by co-precipitation, coagulation, adsorption, ionic change, solvent extraction, electrooxidation, and flotation [1]. Lead is a metal found in natural deposits as ores containing other elements. It is generally the most widespread and concentrated contaminant at a lead battery-recycling site. Lead must be removed from drinking water, if it is above 15 ppb. Thus, it is imperative that processes for heavy metal decontamination have to be developed. These may be carried out through different techniques: chemical precipitation, solvent extraction, ultrafiltration, ionic exchange, adsorption, biosorption, photocatalysis, reverse osmosis, evaporation, non-conventional flotation, etc. [2,3]. Metal immobilization through precipitation and adsorption is considered a common mechanism to reduce metal in contaminated soils [4]. Different natural substances, like zeolites, carbon, and clays may be used as adsorbents for metal retention [5], although of late the preparation of modified clays and design of new adsorbent materials have been studied as a way to improving environmental conditions. This Chapter gives an overview of removal of such metallic impurities using both natural and modified clays. 2 — Retention of metals by clays The components of waste disposal are usually natural clay deposits or compacted bentonites liner owing to their high sorption capacity for cations and their low hydraulic conductivity. Aluminosilicates and oxides minerals are capable of removing many metals over a wide pH range and to much lower dissolved levels than the precipitation method [6]. Clays are the most important elements of the mineral kingdom and their use, mainly in ceramic products, dates from around 8,000 BC. Clay minerals are part of the soils and are essentially hydrous aluminosilicates, commonly known as phyllosilicates. The phyllosilicates contain bi-dimensional tetrahedral and octahedral sheets. The tetrahedral cations are normally Si4+, which may be replaced by Al3+ and/or Fe3+, Figure 1. The octahedral cations normally are Al3+, Mg2+, Fe2+/3+. The assemblage of one octahedral sheet between two tetrahedral sheets is known as the 2:1 layer (or T-O-T), whereas; the assemblage of one octahedral and one tetrahedral sheets form the 1:1 layer (or T-O).

Figure I - Schematic clay structure (2:1 layer).


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Kaolinite and smectite are examples of 2:1 and 1:1 layers, respectively. Charge imbalance due to isomorphic substitutions in the structure layers is compensated by cations such as Na+, K+, Mg2+, Ca2+, etc., placed in interlayer position, Figure 1.These cations are easily exchanged [7,8] and this property can be used for metal retention on clays. Nevertheless, the retention of metals by clays can also be controlled by properties such as surface area, surface charge, pH, ionic strength, etc. [6,9]. The different cation exchange capacity of natural clays plays an important role in the retention mechanism [10] and there is over-exchange when initial metal concentration exceeds the concentration corresponding to the cation exchange capacity of clay [11]. The amount of cation uptake by clays could be increased after different physical and/or chemical treatments. These treatments can modify structural (chemical composition, changes in interlayer distance, new species in interlayer position, etc.), textural (surface area, pore distribution, porosity, etc.) and/or acidic properties (Lewis and/or Bronsted sites) (Figure 2). The modifications on clays can be carried out by different methods: intercalation of inorganic and/or organic substances; intercalation of hydrolysed inorganic OH-cation species; pillaring clays; ligands intercalation, acid and alkaline treatments, etc. [12-16]. Pillared clays, also known us pillared interlayer clays (PILCs), are obtained by cation exchange of polynuclear hydroxy-cation species between the aluminosilicate layers followed by calcinations [17]. The cation of the polynuclear hydroxy-cation may be: Al, Cr, Fe, Zr, Ti, Ni ions, etc. There are many publications dealing with preparation, characterisation and uses of intercalated OH-cation onto clays, and pillaring clays [1933] in which textural, structural and acidic properties have also been analysed as well as different applications, such as in catalysis and as adsorbents. Acid modifications on clays are prepared by treatment of clays with different concentrations of acid (generally HC1, H2SO4) solutions at boiling temperature [34-39]. The acid treatment increases the surface area, porosity, pore, volume, and acid sites of the clay, and it can be used in catalysis, as bleaching, as adsorbents, etc.

Figure 2 - Schematic changes in clays after structural and textural modifications (M). To evaluate environmental impact, many researchers are studying in detail the interaction between clays and different metals from solutions. Studies about retention of metal ions by clays are important since they give further information on procedures in soil chemistry, hydrometallurgy, treatment of

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wastewater, etc. The main factors that affect the adsorption of metals are pH, temperature, ionic concentration, ionic strength, metal amount, etc. [40]. Soil clays and oxides have shown the ability to select different metal cations as Christensen [41], Zhu and Alva [42], Carey et al [43] mentioned. The retention of cadmium by clay has been studied under different conditions and this is depicted by the isotherms, which show the initial slope of the adsorption isotherm and the amount adsorbed by illite increased with pH [40]. The adsorption of cadmium by some soils is found to be pH-dependent and increases when increasing Cd concentrations [40,44]. Maximum adsorption values at unadjusted pH for soils containing different clays content ranged from 7.88mmol kg"1 to 64.8mmol kg"1. Clay content in soils is important for higher retention; however, Cd adsorption is related significantly to contents of organic carbon and low crystalline Fe. Adsorption of Cd onto phyllosilicate clays may occur both by specific and non-specific adsorption [45]. Cd complexation to the edge sites was studied by Zachara and Smith [46]. Cadmium sorbed on soil is strongly influenced by soil pH, cation exchange capacity (CEC), and organic content [47]. A nuclear magnetic resonance (NMR) study of Cd adsorption on montmorillonte indicates that Cd ion may be localised in interlayer and on the external surface. CdCl+ can also be adsorbed in the interlayer [48]. Similar results are found for kaolinite [49] where treatment with diluted Cd solution produces adsorption of Cd in interlayer, whereas in a treatment with a concentrate Cd solution, the ions can be situated in interlayer and on surface sites. And there is also adsorption of CdCl+ in the interlayer. The adsorption of cadmium on montmorillonite is low in highly concentrated chloride solutions ( > 1M), [50]. Lead tends to accumulate in the soil surfaces. In calcareous soils, Pb precipitates as Pb-carbonates, and all components of soils are responsible for adsorption of Pb [51,52]. Pure kaolinite retains Pb and Ca ions at low initial metal concentrations; and the adsorption of PbCl+ and CdCl+ becomes important with a higher metal concentration, where the most important retention mechanism is cation exchange [53]. The adsorption of Pb appears to progress the most slowly initially, but after equilibrium is reached the adsorption of Pb is the highest observed [54]. Modified clays, as phosphatic clays may be effective for immobilising heavy metals such as Pb, Cd, and Zn ions from aqueous solutions [4], The amount of metal desorbed onto phosphatic clays decreased in the following order: Pb>Cd>Zn. The significant differences between the amounts of metals sorbed from phosphatic clay suggest differences in their adsorption mechanisms. Results demonstrate that waste phosphatic clay could be a potential agent to treat Pb-contaminated soils [4]. Thiamont is one smectite covalent grafted with a chelating sulfhydryl functionality. It is found to be an effective adsorbent for Pb and Hg (70 and 65 mg metal /g adsorbent, respectively), but as a less effective adsorbent for Cd and Zn ions [55]. The adsorption of Cd (pH=6.9), Cu (pH=4.9), Pb (pH=4.9), and Zn (pH=4.9) by Al- and Zr-hydroxy intercalated bentonite is dominated by cation exchange. On the other hand, Al and, to a lesser degree, Zr-hydroxy intercalated and pillared bentonites exhibit high affinity for Zn ions, which is independent from the ionic strength of the solvent at neutral pH [9]. It was also demonstrated that from pH=4 to pH=6, selectivity for kaolinite appears to be Pb > Cd ions [53,54]. The metal adsorption in kaolinite is usually accompanied by the release of the hydrogen (H+) ion from the edge sites of the mineral [54]. Mercier and Pinnavaia [14] developed a porous clay heterostructure with uniform intragallery mesoporosity with important capacity for Hg retention. The Hg


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adsorption-desorption study by Yin et al [56] indicated that the contamination of any soils with a high concentration of Hg (II) could result in groundwater problems because a large fraction of Hg could eventually be leached out, however, clay content and organic matter play an important role in soil remediation. The adsorption of Hg (II) by kaolinite is initially influenced by pH, and the presence of Cl, Ni, and Pb reduce the Hg retention [57]. Ionic strength and the presence of SO4= and PO4= ions have relatively low impact on the adsorption of Hg (II). The silanol (SiOH) group is responsible for retaining the bulk of the adsorbed Hg (II), and both the silanol (SiOH) and aluminol (A10H) groups must be considered for adsorption of Hg (II) by kaolinite [57]. Zachara et al [58] and He et al [59] have also studied ion adsorption capacity on the clay surface by applying a triple layer model (TLM) as a composite of Si-OH and Al-OH sites. Copper, nickel cobalt and manganese retentions by natural kaolinite have been studied by Yavuz et al [60], which demonstrated the following adsorption affinity order for metal ions: Cu > Ni > Co > Mn. They concluded that kaolinite might be used to remove traces of heavy metals from an aqueous solution. Triantafyllou et al [11] demonstrated that bentonite retains an important amount of Ni and Co, but it presents higher affinity for Ni. Adsorption of heavy metals on Na-montmorillonite decreases when pH decreases and, at low pH values (2.5-3.5), the hydrogen ion competes with heavy metal [61]. Abollino et al [61] have found the following order for retention of metals by Na-montmorillonite: Cr > Ni > Mn > Zn > Cu > Co > Pb; and also have analyzed adsorption in the presence of different ligands (EDTA, tartaric acid, oxalic acid, citric acid, etc.) present in solution. Cu, Zn, Cd, Hg, Pb, Ca, and Na ion retentions by soils have been studied by Airoldi and Critter [44] who have suggested that the study of adsorption onto soil surfaces gives information about the exchange capacity of the matrix and that interactions occur by complex formation between the organic matter of the soil matrix and the cations dispersed in an aqueous solution. Adsorption of Cu and Zn by tropical peat soils indicate that Cu is retained more than Zn at the same pH value, and there is a relationship between proton release and Cu and Zn adsorption in the range of 1 to 2, suggesting that Cu and Zn replaced one or two protons from the sites [62]. Matthes et al [9], Volzone and Garrido [15,16] have studied the retention of heavy metals by pillared bentonites. The adsorptions of Cd, Cu, Pb, and Zn by bentonite were dominated by cation exchange. This tendency is similar for certain cations as a function of pH by using pillared bentonites or intercalated bentonites with hydrolysed cations [9]. The retention Zn at pH=6.9 suggests a complexation of Zn ions on surface hydroxyl groups of the intercalated polyhydroxy cations in bentonites and the pillared clays [9]. The Zn ions may also be adsorbed by altered tuffaceous material provided they contain smectite as a clay component [63]. Selective Zn adsorption by halloysite decrease when pH decreases, and all the Zn adsorbed was extracted with 0.1 M HC1 [64]. The adsorption capacity of the different types of clays for Zn ions follows the following order: sepiolite > bentonite > paligorskite > illite > kaolinite, where clays with the highest specific surface and cation exchange capacity show the strongest adsorption capacity for Zn [6]. Garcia Sanchez et al [6] have analysed the retentions of Cd, Cu, Zn, and Ni by different silicate minerals such as sepiolites, kaolinites, illites, bentonites, and palygorskite, containing different impurities, and have demonstrated that a factor like the

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reaction medium, such as pH and ionic strength, influence the adsorption process. Adsorptions of Pb, Zn, and Cd by kaolinite in different conditions of pH, metal concentration and exposure periods have been also analysed by Miranda-Trevino and Coles [54]. When different cations are present in equal amount in the solution, the cation with the smaller atomic radius and/or higher charge is preferentially adsorbed by the montmorillonite [9,65]. This behaviour is also observed even if the concentration of sodium is present twenty-two times or more in a higher molar concentration than chromium in a wastewater solution [65-67]. 3 - Special analysis: retention of chromium by clays There is not much scientific information about the interaction between chromium in solution and clays or soils solids. In 1976, Bartlett and Kimble [68,69] analysed the retention of chromium by soils. In 1977, Griffin et al [70] studied the adsorption of chromium by kaolinite and montmorillonite clays and in 1980 Kopperman et al[71], and Kopperman and Dillard [72] by using chlorite, illite and kaolinite clays. At the same time, Rengasamy and Oades, in 1977, [19] and Brindley and Yamanaka, in 1979, [20] started specific studies about the intercalation of polymeric chromium species onto montmorillonites. Later, from 1984 until today other researchers such as Pinnavaia et al [21], Tzou and Pinnavaia, [22], Carr, [23], Volzone et al [73], Volzone and Cesio [74-76], Volzone [77-79] and Yoong et al [49] continued with similar studies and also analysed the Cr-pillared clays. The most stable oxidation state of chromium is chromium (III). Chromium (II) compounds are reducing agents, and chromium (VI) compounds are strong oxidising agents. The trivalent chromium, Cr (III), and the tetravalent dichromate Cr (VI) in solution and in soils are the most important forms in the environment chemistry. The Cr (III) is low in toxicity and an essential trace ion for several biological activities whereas hexavalent chromium is highly toxic. Chromium is increasing as a pollutant, mainly due to industrial activities such as leather tanning, rubber, mineral mining, paint formulating, porcelain enamelling, electrical and electronic components, and non-ferrous metal manufacturing. According to EPA, chromium is one of the major threats to human health. Although each country has its own regulations on the maximum concentration of chromium in different media, in general, it is provided that the content of chromium (III) in solution should not be above 0.10 ppm [80].

Figure 3 - Flow diagram showing different steps in the analysis of retention of chromium by clays: (a) chromium solutions, (b) clay conditions, (c) combination of (a) and (b). Cr(s): Species chromium (III) solutions prepared in laboratory, Cr (w) chromium from -wastewater solutions.


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The evaluation of the removal of chromium by clays can be considered as a combined analysis of chromium in solution plus clays, which in schematic form is shown in Figure 3. Examples of chromium retention on clays or on modified clays by using chromium from an aqueous solution or from wastewater are shown in Figure 3 c. 3.1 - Chromium (III) solutions The chemistry of chromium in solution is complex and, whenever a researcher is evaluating chromium retention on solids, it is advisable to know which chromium species are present in solution. Polymeric chromium (III) species in solution have been meticulously studied by Ardon and Plane [81], Laswick and Plane [82]; Kolaczkowcki and Plane [83], Thompson and Connick [84], Finhol et al [85], Stunzi and Marty [86], Spiccia et al [87], Stunzi et al [88], Yoon et al [49]. Cr (III) in solution may be present in different hydrolysed forms as a function of pH, OH/Cr, hydrolysis time and temperature, etc.; it is possible then that the amount of chromium retention by clays may vary, even without interfering cation. Figure 4 shows the UV-visible spectra of three chromium (III) solutions prepared from chromium nitrate salt, Cr(NO3)3.9H2O, at different hydrolysed conditions [75,87]. The "M" solution is a fresh 0.05 M chromium nitrate salt. The "T" solution is prepared by mixing rapidly a 0.5 M chromium nitrate solution by the addition of 2 NaOH (OH/Cr=8) [73,75]. The operation has been carried out under certain conditions in order to obtain a chromite solution that has been then rapidly acidified with 2 N HC1O4 to produce the protoned oligomers in solution with total chromium [Cr3+] = 0.05M. The solution has been equilibrated at 25 °C during 30 hours before being added to the clay suspension. The P(l/60) solution is prepared from 0.1 M chromium nitrate solution by the addition of 0.2 M NaOH (OH/Cr=2) at 60°C, and hydrolysed for one day [73]. The total chromium in this solution is [Cr3+] = 0.05 M. The three solutions show two maximum absorption peaks at 408 nm and 575 nm for the "M" solution; at 421 nm and 581 nm for the "T" solution; and at 424 nm and 586 nm for the "P" solution. The bands of the "M" solution correspond to the presence of monomeric species, Cr(H2O)63+ [84]. The shifting of the peak to high wavelength is related to higher polymeric chromium species [84]. A high content of trimeric with low monomeric and dimeric species should be present in the "T" solution depending on the way of preparation [86,87]. Yoon et al [49] have been obtained trimeric chromium in organic media, such as the trimeric chromium oxyformate. The polymeric chromium solution, P(l/60), contains mainly trimeric-species, Cr3(OH)45+, followed by terra-, Cr4(OH)66+; mono-, Cr(H2O)3+, and dimer-Cr2(OH)24+ species [73]. The polyhydroxy chromium in solution may be obtained by adding NaOH to a nitrate chromium solution (OH/Cr=2) at different hydrolysed temperatures and time [73]. Figure 5 shows the pH of polymeric solutions vs. hydrolysis time as prepared at two different temperatures, 20°C and 60°C. The pH of 0.1 M chromium nitrate solution is 2.10, and it changes to pH=4.00 after a final addition of 0.2 M NaOH (OH/Cr=2). When hydrolysis time progresses, pH decreases close to constant values as shown in Figure 5, reaching pH values of 3.23 and 2.45 for hydrolysis temperatures of 20°C and 60cC, respectively. The absorption spectrophotometric characteristics for each solution are shown in Table 1, where Xmaxj and ^max2 are the maximum adsorption values, and Imax1/Imax2 are the ratio between the maximum intensities of the bands.

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Figure 4 - UVvisible spectra of chromium solutions: -'-'-' M" solution,— ' T " solution, "/"' solution [75].

Figure 5 - pH of polymeric solutions vs. hydrolysis time [73]. The absorption spectrophotometric characteristics for each solution are shown in Table 1, where Xmaxt and A.max2 are the maximum adsorption values, and Imax1/Imax2 are the ratio between the maximum intensities of the bands. In the same table, the M solution (a monomeric solution as mentioned in the previous paragraph) is included for comparison, where the ratio between the maximum absorption at 408 to 475 equals 1.17, which confirms the presence of monomeric Cr(H2O)63+ [33]. In the OH-Cr


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solution, hydrolysed at 20°C for one or 30 days, P(l/20) or P(30/20), trimeric and tetrameric species are abundant [75]. The P(l/60) solution contains mainly trimeric followed by tetra species, but if the hydrolysis time is longer, for example, 36 days, P(36/60) or more, e.g. 100 days, the P(100/60) spectrum characteristic (Tablel) changes and the monomeric species are the highest, although small amounts of tri- and dimer species were also found. Longer hydrolysis times at 60°C would then originate the depolymerisation of the species in solution. The hydrolysed chromium solution aged for 3 weeks at 25°C, prepared with chromium nitrate solution and Na2CO3, shows bands located at 420 and 580 nm, as reported by Tzou and Pinnavaia [22]. The same authors have analysed a similar solution but aged for 36 hours at 95°C. The spectrum of this sample has shown two bands situated at 420 and 586 nm. More polymerised chromium species may be present in the last solution. Table 1 - Maximum absorption values (Ajnax,, X,max2) and ratio between maximum intensity of the band (Imax,/Imax2) [73]. ^ma X l nm 408

A,max2 nm 575

Imaxi/Imax2

P(l/20) P(2/20) P(8/20) P(17/20) P(30/20)

424 424 424 423 423

583 583 583 584 584

1.34 1.36 1.38 1.39 1.45

P(l/60) P(6.5/60) P(36/60) P(67/60) P( 100/60)

424 423 422 420 419

586 586 586 586 586

1.39 1.33 1.36 1.32 1.32

Hydrolysed Cr solution P(day/°C) M:(NO 3 ) 3 Cr0.1 M

1.17

The basic chromium sulphate, [Cr(H2O)5(OH)SO4], is a widely used primary tanning agent. Figure 6 (a) shows the spectrum, in the UV-visible range, of the basic chromium sulphate solution prepared with 2,000 ppm Cr (mg L"1). It shows two bands at 422 and 586 nm with similar characteristics of solutions containing chromium species such as Cr2(OH)24+ [89]. This type of species is important for tanning processes. The spectrum of the tanning waste solution is shown in Figure 6 (b). This solution was kindly provided for this study by INESCOP (Technological Institute for Footwear and Leather, Spain). The tanning waste corresponds to the end of the tanning process in a tannery, and it contains 2,000 ppm of chromium, organic matter, solids in suspension, inorganic salts, and oil and fats [89]. The suspended solids have been separated by filtration before the spectrum. The maximum absorptions of the waste solution were at 419 and 576 nm, (Figure 6 b). The type of chromium species in waste solution is unknown. However, according to the characteristics of the spectrum, monomeric chromium species, Cr(H2O)63+, could be present [86]. The different types of chromium species among tanning salt and tanning waste is due to changes originated after the tanning process.

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Figure 6 - UV-visible spectra chromium solutions containing 2,000 ppm Cr. (a) Basic chromium sulphate salt solution, (b) tanning wastewater [89]. 3.2 — Retention of chromium from solutions by clays Different aspects are included in this section: influence of chromium (III) species, types of clays, and the effects of thermal treatment, all referred to retention of chromium on clays and modified clays (as shown in Figure 3 c, quadrants I and II). 3.2.1 - Influence of chromium (III) species on clays A montmorillonite clay after being treated with monomeric chromium, Cr(H2O)63+, from M solution; trimeric chromium species from T solution, and polymeric chromium (III) species, obtained at different hydrolysed conditions, shows similar hkl reflections except to 001 reflection, d(001) spacing, which is modified according to Cr cation or intercalated OH-Cr cation species intercalated, Table 2. The structure of the montmorillonite is preserved in spite of the different media used as indicated by the hkl reflection of the sample after treatment with blank solutions (a reactive without chromium) [75]. The original sample (M) and after treatment, monomeric chromium shows similar interlayer spacing values, M: 14.8 A and M-M: 15.0 A. Higher values (18.4 - 20.7 A) have been obtained in montmorillonite after treatment with trimeric, MT, and polymerised solutions at different time and temperature conditions (M-P(l/20), M-P(30/20) and M-P(l/60) solutions). The increase in the spacing is attributed to different polymerised OH-Cr-species retained in the montmorillonite. Montmorillonite treated with polymeric hydroxy-chromium solution prepared during a long hydrolysed time at 60°C, P(36/60) and P(100/60), shows two different interlayer spacings, 13.5 and 19.0 A for M-P(36/60), and 13.9 and 17.0 A for MP( 100/60) samples as shown in Table 2. Such solutions contain mainly monomeric Cr species with small amounts of dimer and trimer, as mentioned in 3.1. The higher interlamellar spacing of montmorillonite, d (001) spacing, is due to higher proportions of


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polymerised chromium species intercalated onto clay due to the different density per atom of chromium of the chromium species. Rengasamy and Oades [19], Brindley and Yamanaka [20], Pinnavaia et al[21], and Tzou and Pinnavaia [22] have obtained a maximum of d(001) spacing of 14.0, 16.8, 27.0, and 27.6 A, respectively, for OH-Cr-smectites by using OH-Cr solution with OH/Cr=2. Whereas, Tzou and Pinnavaia [22], using hydroxy-chromium solution from NaCC>3 with an hydrolysis temperature at 25 °C during 3 weeks, have obtained d(001) spacing of 17.7 A. On the other hand, Carr [23] has obtained a basal spacing of approximately 14.7 A using an OH/Cr =1 solution. These differences are attributed to the different procedures followed by the authors: i.e., preparation, hydroxy-chromium solution, hydroxy-chromium-smectite, and amount of Cr/sample. Table 2 - d(001) spacing, A, of montmorillonite after treatment with differently prepared OH-Cr solutions (M: monomeric, T: trimeric, and P: polimerics). Sample

M

d(001),A

14.8

M-M M-T 15.0

18.4

M-P (1/60) 20.7

M-P (36/60) 13.5, 19.0

M-P (100/60) 13.9, 17.0

M-P (1/20) 19.9

M-P (30/20) 19.9

Modified kaolins and bentonites have been used to retain chromium from a basic chromium sulphate salt solution. The spectrum of this salt in UV-visible range has been shown in Figure 6.a, and it contains a high proportion of dimeric species, Cr2(OH)24+ [89]. Two kaolins, A and E, with higher kaolinite content in sample A (further on this in 3.4) and two bentonites, Bl, and B2, with high Wyoming- and Chetotype montmorillonite contents, respectively, have been used. The clays A, E, and Bl have been modified with caustic potash solutions, and stabilised at pH 7 and temperature of 500 K (Ak, Ek, Blk). The B2 bentonite has been pillared with OH-A1 species and modified with hexamethaphosphate ligand (B2alh) [15]. Table 3 - Cr retention from basic chromium sulphate salt by clays.

Clay A Ak

Cr retention from salt, mg/g 1 day 8 days 4 5 18 26

E Ek

6 27

8 44

Bl Blk

8 30

8 45

B2 Balh

7 33

7.5 47

The retention of chromium by A, E, and B1 increases after treatment from 4, 6, and 8 mg Cr/g to 18, 27, and 28 mg Cr/g after a 1-day contact, respectively (Table 3).

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The different behaviour between Ak and Ek is due to the different mineralogical composition. The pH of the suspensions of modified clays and chromium salt solutions were close to 5.5. The B2alh adsorbent reached a pH suspension of 4 and it presents similar chromium retention than Blk. The original samples reach the maximum equilibrium of retention after one day of contact; however, in the modified clays retention increases after eight days, suggesting a different mechanism. i) Influence of the chromium added The interlayer spacing, d(001), of a montmorillonite treated with polymeric Cr species (from P(l/60) solution), increases from 14.8 to 20.7 A when the amount of Cr is increased in the range of 0.5-20 mmol Cr(III) per gram of clay (Table 4) [47]. The high spacings when adding 10 and 20 mmol Cr/g correspond to gallery heights of 10.9 11.lA. This is the difference between d(001) spacing and 2:1 layer (9.6 A). Two layers of trimer-(Cr3(OH)45+), tetramer-(Cr4(OH)66+), or dimer-(Cr2(OH)24+) species have perhaps been intercalated between the 2:1 layer of smectites because these species are 5, 6.5, and 4 A in height, respectively [77,87]. Figure 7 shows the characteristic spectra of the supernatants after montmorillonite treatments with different mmol Cr/g added (S-M-P0.5, S-M-P1.5, S-MP3.5, S-M-P5, S-M-P10, and S-M-P20) [77]. Table 4 - d(001) spacing after different Cr added. Sample d(001), A

M 14.8

M-P0.5 15.7

M-Pl .5 > 16i >

M-P3.5 18.6

M-P5 19.6

M-P10 20.5

M-P20 20.7

Figure 7 - Absorption in visible spectra and pH of the OH-Cr-solution P(l/60), and supernatant after treatment with different mmol Crper gram of sample [77].


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The starting solution, P(l/60), shows two bands at 424 and 586 nm. The supernatant after treatment with 0.5 mmol Cr/g is colourless and then does not show bands in the spectrum. As a consequence, the chromium added is retained by montmorillonite. The bands of S-S-P1.5 supernatant shifted the peaks to smaller wavelengths of 415 and 583 nm, this behaviour indicates that species in this solution are less highly polymerised forms. As a result, selectivity of the montmorillonite to retain specific polymerised species do occur but, if necessary, other species will be retained, as it has been demonstrated when S-M-0.5 did not show Cr species, because the montmorillonite has retained all species present in solution. After treatment of montmorillonite with the P(l/60) solution, the original interlayer cations are removed and then they are found in the supernatant solutions [77]. The different interlayer cations from montmorillonite [77] are now in the supernatants and the pH of the supernatants (Figure 7) does not allow evaluating the OH-Cr-species present in such solutions for certain. The textural characteristics of the original montmorillonite and after adding lOmmol Cr/g as monomeric (M-M10) and polymeric (M-P10) Cr-species are shown in Table 5, where the polymeric solution is P(l/60). The BET surface of the M montmorillonite increases, after intercalation with OH-Cr species, from 36 to 175 m2/g (similarly, the surface area calculated by t-method also increases from 38 to 170 m2/g). The micropore and external surfaces, calculated from t-plot [90], show that the main difference between the original montmorillonite, M, and after intercalation with OH-Cr species (M-P10), is attributed to the micropore contribution, because the external surfaces are similar (25 and 32 m2/g), whereas the micropore surfaces are 7 and 138 for M and M-P10, respectively. The BET surface of intercalated montmorillonite with monomeric species, M-M10, shows a similar value to the original smectite, 40 m2/g. The intercalation of polymeric chromium species onto montmorillonite increases textural characteristics such as micropore surface and consequently the total surface. Table 5 - Some textural characteristics of montmorillonite (M) and after treatment with polymeric (M-P10) and monomeric (M-M10) species, by addition of 10 mmolCr/g. Sine: surface from t plot, Se: external surface, Sm: micropore surface. Surface, m2/g BET surface Sme Se Sm

M 36 38 25 7

M-P10 175 170 32 138

M-M10 40 — — -

ii) Influence of the types of clays The adsorption isotherms for Cr(III) from chromium nitrate follow Langmuir equation according to Griffin et al [70], in which the chromium species is Cr(H2O)63+. They found the retention values of 79.5 meq/100 g and 15.1meq/100g for montmorillonite and kaolinite, respectively [70]. The cation exchange has been the main mechanism for cation adsorption. Similar behaviour has been observed by Volzone & Tavani [65] and Tavani & Volzone [66] after retention of chromium (III) from a tanning waste solution by kaolmite and montmorillonite clays. Cr (III) is also absorbed between

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30 and 300 times more than Cr(VI) by kaolinite and montmorillonite in the pH range of 1.5 to 4.0 as mentioned by Griffin et al [70]. The adsorption of chromium (III) from chromium nitrate solution by chlorite, illite, and kaolinite is pH-dependent [71]. The chromium is adsorbed as Cr(III) aqua at a pH value below 4, however, at a higher pH it is probable that polymerised chromium ion, Cr3(OH)45+, is present. The retention of chromium by illite has been similar to that of chlorite and both are higher those of kaolinite, in accordance with cation exchange capacity [71]. Koppelman and Dillard [72] have mentioned that hydrolysis of chromium occurs after adsorption in clays similar to what has been mentioned for smectites when they have been intercalated with OH-Cr species [20,22,23,73]. The basal spacing of a kaolinite and a vermiculite treated with polymerised chromium (OH-Cr species from the P(l/60) solution) is similar before treatment, 7.3 and 14.9 A, respectively [78], nevertheless the basal spacing for Cheto type montmorillonite (MCh), Wyoming type montmorillonite (MW), a beidellite (B), a nontronite (N) and a saponite (S) have been 20.6, 20.7, 20.4, 20.1, and 20.2 A, respectively. As a consequence, the chromium intercalated in smectites showed a separation of 10.9-11.1 A, calculated by the difference between measured basal spacing and the height of the corresponding layer (2:1 layer at 9.5 A). The chromium species are placed in these separations according the dimension of the species [73,88] and as mentioned in 3.1.2.2. The different basal spacing values between 16.8 and 27.8 A of the montmorillonite-OH-Cr complexes reported by Rengasamy and Oades [19], Brindley and Yamanaka [20], Pinnavaia et al [21], Tzou and Pinnavaia [22], and Volzone [77,79,91], may be attributed to different intercalation procedures (hydroxy-chromiumsolution, hydroxy chromium-smectite, Cr added, etc.). The N2 adsorption-desorption isotherms of the clays before and after OH-Crsolution treatment (not shown) corresponded to H3 type according to the classification by Gregg and Sing [92]. This type is a hysteresis loop with a vertical adsorption branch at a relative pressure very close to 1, and a desorption branch close to medium pressure. Such hysteresis loop may be formed due to slit-shaped pores [92], The pore shapes are preserved after treatment with OH-Cr-species, but with different volumes (although with the initial volume in the low pressure region of the isotherm), except for kaolinite which volume is unaffected [78]. Figure 8 shows the BET surface area of the clays before and after OH-Cr treatment, and also micropore surface/surface BET ratio. Micropore and the external surface area, and the micropore volume of the samples have been obtained by the t-plot method [90], and for kaolin clay the internal and external surface areas have been derived according to the method of Delon et al[93] that assumes slit shaped pores. The treatment of the clays with OH-Cr-species increased BET surface mainly for smectite clays. The intercalation of OH-Cr species in the clays increased the micropore surface in a higher proportion when Smic/SBET increased [78,91]. Micropore volumes in smectites increase, slightly affecting the micropore volume in vermiculite without affecting the micropore volume in kaolinite after OH-Cr treatment [78]. The micropore volume to total volume ratio increases after chromium species treatment in the vermiculite and the smectite clays, and remains unchanged in kaolinite (Figure 9). The amount of chromium (III) uptake (expressed as % Cr2O3) is a function of the octahedral charge of the original clays (Table 6), as shown in Figure 10. Similar correlation has been obtained after intercalating aluminium polymeric species in smectites [94]. The small amount of Cr retained by clays with no negative octahedral charge might be attributed to defects in clays.


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Figure 8 - BET (Brunauer-Emmett-Teller) surface of natural clays and after treatment with OH-Cr species solution. Smic: micropore surface, SBET-' BET surface.

Figure 9 - Total and micropore volume of the natural clays and after OH-Cr-species solutions treatment.

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Table 6 shows the composition of the half unit cell of each original clay that has been calculated from < 2 um fraction [26]. The major negative charges in smectites are originated in the octahedral sheet, whereas in kaolinite and vermiculite they are located in the tetrahedral sheet. This vermiculite shows positive charge in the octahedral sheet. Table 6 - Composition of half-unit cell of the natural clays [31,77,78].

Clay K

Si,v 4+ 3.955

V

2.890

MCh MW B N S

3.940 3.910 3.610 3.700 3.820

Te trahedral sh eet Charge A1 IV 3+ -0.046 0.046

AW +

1.100

-1.100

0.080

0.060

2.810

+0.040

0.060 0.090 0.390 0.300 0.180

-0.060 -0.090 -0.390 -0.300 -0.180

1.360 1.610 1.630 0.130 0.200

0.060 0.130 0.110 1.800 0.020

0.600 0.260 0.160 0.130 2.500

-0.600 -0.260 -0.160 -0.100 -0.190

3.852

Octahedr al sheet Mg 2 + Fe 3 + 0.018 0.003

charge 0

Figure 10 - Retention of chromium from the P(l/60) solution, expressed as a percentage ofCr2Os vs. the octahedral charge of the clays (Table 6). 3.2.2 - Effects of thermal treatment on Cr-clays The structural, textural and acidity changes in chromium clays due to thermal treatment allow, for example, the evaluation of the thermal stability of Cr-pillared clays, the possible immobilisation of chromium, etc. The larger endothermic peak of original smectites is produced by dehydration of the clay (water intercalated between layers) [74,95]. The size, shape, and temperature depend on the saturating cation. When the smectites are treated with the OH-Cr-solution,


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P(l/60) solution, the exchangeable cations are replaced by OH-Cr-species. The differential thermal analysis, DTA, diagrams of all OH-Cr- smectites show a double peak (140°C and 190°C), and correspond to dehydration of OH-Cr-species. This behaviour has been confirmed by means of a DTA diagram of the OH-Crmontmorillonite with the same montmorillonite [74]. The thermal stability of the interlayer spacing up to 600°C in air atmosphere of one intercalated montmorillonite with different chromium species such as monomeric, trimeric, and polymeric species are shown in Table 7. A typical reduction of interlamellar spacing has been observed in montmorillonite, M, which collapses from 14.8 to 9.3 A after thermal treatment. Similar behaviour is shown by the montmorillonite intercalated with monomeric species, M-M sample (15.0 to 9.4 A). The clay treated with trimeric species, M-T, is more stable than the M-M sample. A better thermal stability is shown for montmorillonite treated with chromium species with higher polymerised species, M-P(l/60), but all samples collapsed at 600cC (9.6 A). The intercalated montmorillonite with OH-Cr polymerised species by Tzou and Pinnavaia [22] and Brindley and Yamanaka [20] shiftted the d(001) spacing from 27.6 to 10.2 A, and from 16.8 to 9.8 A, after thermal treatment from 25 to 450°C, respectively (Table 7). As a consequence, the high polymeric species causes a larger spacing and better thermal stability in the smectite. Table 7 - Thermal stability of the interlayer spacing of the intercalated Cr species in montmorillonite. Temperature M-M M (°C) (A) (A) 14.8 15.0 25 14.2 14.0 200 12.5 12.0 300 9.3 9.4 450 9.3 9.4 600 * Brindley and Yamanaka [20] ** Tzou and Pinnavaia [22]

TP**

M-T

M-P(l/60)

M- P(30/20)

BY*

(A)

(A)

(A)

(A)

(A)

18.4 14.8 14.6 11.9 9.2

20.7 19.9 18.0 14.5 9.4

19.7 17.0 16.0 9.0, 14.0 9.0

16.8 16.8 10.4 9.8

27.6 24.0 22.0 10.2

i) Heating up to 450°C to obtain Cr-PILCs The PILCs are obtained by cation exchange of polynuclear hydroxy-cation species between the aluminosilicate layers followed by calcination [17]. Cr-PILCs have been prepared from smectites clay by Rengasamy and Oades [19], Brindley and Yamanaka [20], Vaughan and Lussier [17], Carr [23], Pinnavaia et al [21], Vaughan [18], Tzou and Pinnavaia [22], Drljara et al [96], Volzone [31]. Figure 11 shows that the d(001) spacing of the starting smectites (sm) is in the range of 13.5 - 15.5 A. These values are a function of natural saturation interlayer cations of the smectites [26,74]. After heating at 450°C (sm-450), the spacing of smectites decreased at around 9.5 A (the depth of the individual triple-sheet-layer) due to dehydration of smectites (water intercaled between layers) [31]. The intercalation of OHCr species increases the d(001) spacings of the smectites (OH-Cr-sm) to 20.1-20.7 A, Figure 11. The OH-Cr-sm are converted to Cr pillared smectites by heating at 450°C, at a heating rate of 2 °C/min in N2 atmosphere (Cr-PILCs/N, Figure 11), and the d(001) spacing of these products are in the range of 13.8 - 18.8 A. The difference between the

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values of the d(001) spacing of Cr-PILCs/N and the height of the 2:1 layer (9.5 A) corresponds to an interlayer separation or gallery in the range of 4.3 - 9.7 A, according to smectite members used to prepare Cr-PILCs/N. The treatment of the OH-Cr-sm at 450°C in air atmosphere (Cr-PILCs/A) originate a reduction of the d(001) spacing of around 15 A, except for pillared nontronite (N) that shows a low value (10 A) [Figure 11]. This behaviour is associated with the dehydroxylation of the 2:1 layer of the nontronite that occurs at 490°C [95]. The a-Cr2O3 phase is observed in the X-ray diffraction spectra of the OH-Cr smectites heated at 450°C in air atmosphere, and it coincides with an exothermic peak around 420°C in thermal gravimetry analysis that corresponded to the dehydration of OH-Cr-species and the crystallisation of Cr2O3 [74,77]. The Cr2O3 phase is not observed in the XRD of the OH-Cr smectites heated at 450°C in a nitrogen atmosphere [31,77] and therefore, the poly-oxo-hydroxycations may be present in the interlayer position of the Cr-PILCs/N. This agrees with Tzou and Pinnavaia [22] who have worked with Cr-montmorillonite.

Figure 11 - Interlayer d(001) spacing of the starting smectites and after different conditions. MCh: Cheto-type montmorillonite, MW: Wyoming type montmorillonite, B: beidellite, N: nontronite, S: saponite [31]. The textural characteristics of the Cr-PILCs obtained in both atmospheres are shown in Table 8. The BET surface area of the OH-Cr-sm after pillaring by heating at 450°C in N2 atmosphere (Cr-PILCs/N) reached 50 - 165 m2/g in a different way depending on the smectite members The contribution in micropore surface area, by tmethod [90], regarding total surface (micropore surface area + external surface area) of the Cr pillaring smectites in N2 atmosphere are in the range of 53 - 87 %. The total volume (Vtot) obtained at the end of the N2 adsorption isotherm branch (P/Po = 0.986), increases from 0.023 - 0.174 cnrVg for starting smectites to 0.072 - 0.304 cm3/g for Cr pillared smectites obtained in a N2 atmosphere. The micropore volume (Vmic) has been obtained by a high pressure branch extrapolated to the

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adsorption axis in the t-plot (90). The contribution in Vmic of the Cr pillared smectites (in N2 atmosphere) increases in the range of 1.37 to 10.3 times with respect to the original smectites. The micropore volume and the total volume ratio (Vmic/Vtot) increase after pillaring with respect to the ratio for the original smectites. The values of micropore surface area and micropore volume of the Cr-PILCs/N increases with the interlayer spacing [Figure 11 and Table 8]. Table 8 - Textural characteristics Cr-PILCs obtained in nitrogen (N) and air (A) atmospheres. SBET: BET surface area; Smic: micropore surface area; Se: external surface area; Vtot: total volume; Vmic: micropore volume; Vmic/Vtot: micropore volume/total volume ratio [31]. SBET

Sample MCh Cr-PILCMCh/N Cr-PILCMCh/A

m2/g 81 165 50

Smic m2/g 71 143 25

Se m2/g 7 34 12

Vtot cm3/g 0.174 0.140 0.058

Vmic Cm3/g 0.039 0.075 0.020

Vmic/Vtot % 22 54 34

MW Cr-PILCMW/N Cr-PILCMW/A

19 128 11

12 68 5

4 62 1.5

0.074 0.304 0.065

0.003 0.031 0.005

4 10 8

B Cr-PILCB/N Cr-PILCB/A

7 78 10

6 54 5.5

2 36 2

0.044 0.107 0.042

0.005 0.028 0.005

11 26 12

N Cr-PILCN/N Cr-PILCN/A

27 50 4

14 36 1.5

2 21 1.5

0.051 0.072 0.033

0.016 0.022 0.002

32 31 6

S Cr-PILCS/N Cr-PILCS/A

8 62 8

6 48 2

2 25 1.8

0.023 0.091 0.067

0.005 0.028 0.039

22 31 58

The Cr-PILCs obtained in air condition (Cr-PILCs/A) shows low textural characteristics, and this could be attributed to the reduction of interlayer spacing and/or the aggregation of Cr2O3 species in the products. The micropore surface area, the micropore volume, and d(001) spacing of the Cr pillaring smectites increases with the amounts of the chromium retained (1.33 - 2.48 mmol Cr/g) [31] Cr-PILCs obtained at 300°C and 450°C show Lewis (L) and Bronsted (B) acidity characteristics. L/(L+B) ratio of the Cr-PILCs increases with the tetrahedral negative charge of the smectite structures as demonstrated by Volzone [31]. The structural charges and compositions of the original smectites (Table 6) influence the structural, textural, and acidity characteristics of the Cr-PILCs. ii — Heating at high temperatures The dioctahedral smectites show an endothermic peak between 450-750°C in TGA diagram, a fact that proves the existence of dehydroxylation (loss of water of

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constitution) [95]. These differences must be related in some way to the energy to which the hydroxyl groups are bound in the lattice [97], and to its chemical composition, as it is also seen in Table 6. The nontronite, N shows one breadth endothermic dehydroxylation peak (490°C) below the one obtained for beidellite, B, (560°C), and for montmorillonites, MW, (640°C-710°C). Saponite, S, lost the OH at high temperature (720°C). This behaviour is characteristic of the trioctahedral smectites [95]. In montmorillonite, MW, a small S-shaped endothermic-exothermic peak at about 850°C-950oC corresponds to the destruction of the lattice and the recrystallisation into new phases respectively [95]. The endothermic peaks of nontronite, N, and beidellite, B, are absent, although the exothermic one occurs at 900°C for N and at 980°C for B. According to Kerr [98] such end-exothermic occurs simultaneously. The endothermic peak at around 490oC-710°C of dioctahedral smectites (N, B, and MW) are shifted to lower temperature after the OH-Cr-treatment, (460°C-660°C). This peak depends, among other factors, on the perfect stacking of the layers and on gross substitution [97]. Nevertheless, the peaks at high temperatures of all smectites are shifted to higher values. The absence of the original exchangeable cations, which are replaced by Cr-species, tends to shift peaks to higher temperatures [74]. The smectites phases developed at 1000°C, with and without OH-Cr treatment, are shown in Table 9. Table 9 - Phases of smectites with and without OH-Cr treatment after thermal treatment at 1000 °C in air atmosphere [74]. Sample MCh MCh-Cr

Phases at 1000 °C (3-quartz, crystobalite, anorthite P-quartz, a-Cr2O3

MW MW-Cr

a-quartz, albite a-quartz, a-Cr2O3

N N-Cr

a-quartz, hematite, anorthite a-quartz, hematite (low), a-Cr2O3

B B-Cr

mullite, a-quartz a-quartz, a-Cr2O3

S S-Cr

enstatite enstatite, a-Cr2O3

The phases of the smectites are determined by bulk composition (Table 6), and exchangeable cations. The calcium present in the interlayer position of the nontronite, N, has originated anorthite, whereas the sodium content of the MW montmorillonite has originated albite. The high structural aluminium content of the beidellite, B, favours mullite formation, whereas the high structural magnesium content in saponite, S, (Table 6) allows the presence of the enstatite phase. The iron content of nontronite has been the hematite phase. Quartz is present in these dioctahedral smectites.


311

The a-Cr2O3 phase is present in all OH-Cr-smectites treated from 420cC up to 1000°C. No alteration of the Cr(III) oxidation number has been observed throughout all thermal treatment up to 1000°C. Anorthite and albite phases are not present in the XRD diagrams of the MCh-Cr and MW-Cr smectites, respectively, treated at 1000°C. This could be attributed to the fact that the exchangeable cations (Ca,Na) present in the original smectites have been replaced during the treatment with the OH-Cr-solution, and then those phases could not develop at higher temperatures. With respect to a- and (3-quartz, hematite and enstatite phases are present in smaller proportion. The mullite is not present in the beidellite-Cr (B-Cr) smectite up to 1000°C. As to the OH-Cr(III)-species retained by the smectites, no alteration in its oxidation number has been observed when subject to an air thermal treatment. Nevertheless, if the same chromium species in solution is supported by an inert substance and subjected to the same procedure, Cr(VI) has been found after thermal treatment [99]. A comparison of the products obtained from OH-Cr montmorillonite heated in air and nitrogen atmosphere up to 1000 °C is shown in Table 10. Table 10 - Phases of OH-Cr-M at different temperature up to 1000 °C in air and nitrogen atmospheres [76]. OH-Cr-M Temperature °C 25

Air atmosphere

Nitrogen atmosphere

montmorillonite (d(001) spacing: 20.5 A) ct-quartz (impurity) a-cristobalite (impurity)

montmorillonite (d(001) spacing: 20.5 A) a-quartz (impurity) a-cristobalite (impurity)

450

montmorillonite (d(001) spacing: 14.0 A ) a-quartz (impurity) a-cristobalite (impurity) aCr2O3 (eskolaite)

montmorillonite (d(001) spacing: 18.6 A) a-quartz (impurity) a-cristobalite (impurity)

1000

u-cordierite P-quartz (impurity) P-cristobalite a-Cr2O3 (eskolaite)

P-cristobalite Mg-Cr-spinel

The a-Cr2O3 phase has been found in OH-Cr-M by heating in air atmosphere, which appears at 450°C and remains up to 1000°C (the last temperature analysed). The a-Cr2O3 phase is absent in OH-Cr-M heated in a nitrogen atmosphere. Nevertheless, at 1000 °C thermal treatment shows that the Mg-Cr-spinel is present. The thermal treatment of the intercalated OH-Cr montmorillonite at 1000°C, where the montmorillonite has already collapsed, also presented differences in the products: eskolaite (a-Cr2O3) in air, and Mg-Cr-spinel in nitrogen, where the oxidised status of the chromium has been mantained, Cr(III).

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3.3 - Retention chromium from wastewater by clays This section analyses quadrants III and IV in Figure 3 (c). Although precipitation, in a physical-chemical treatment, is the most utilized method for retaining high chromium concentration in effluents, clays could be potential adsorbents of chromium in low concentration from effluents. Common wastewater is tanning waste where concentration of sodium is higher than that of chromium because high NaCl concentrations come from the tanning process. Volzone and Tavani [65] have studied the adsorption of Na and Cr(III) ions by smectite using tanning wastewater containing 13.11 and 0.81 mmol L"1 of each cation, respectively. Nevertheless, the highest adsorptions of Na and Cr (III) have been 0.30 and 0.80 mmol per gram of smectite, respectively. Such behaviour is attributed to the different density of the electric charge of both cations. The same authors [66] have also found a similar behaviour by using kaolinite as an adsorbent, where 0.008 and 0.036 mg per gram kaolinite of sodium and chromium have been retained. An illite clay also showed a similar behaviour [67]. Exchange capacity has been the mechanism proposed and the predominant chromium (III) species in wastewater have been monomeric chromium species. Modified kaolin with varied mineralogical composition influence chromium retention from tanning waste. The main three components, in different proportions, of raw clays (A, B, C, D, and E) are kaolinite, illite, and quartz. The composition of each of the clay is shown in a triangular plot in Figure 12 [89], where the C sample is a component rich in illite.

Figure 12 - Mineralogical composition of some clays used as raw materials [89]. The clays have been modified by treatments with caustic potash solutions; stabilised at pH 7.0; and there were a thermal treatment at 500 K (Ak, Bk, Ck, Dk, and Ek).


313

The waste liquid used is a tanning waste solution provided by INESCOP as mentioned in section 3.2. Once suspended, solids have been removed by filtration. Then, this solution has been contacted with different clays at different times and at 25 °C and with a solid/solution ratio of 5.5 % in a batch system [89]. Figure 13 shows Cr retention by modified clays as a function of contact time; and Table 11 depicts retention values after 1 and 8 days. In general, extending contact time favours the uptake of chromium, although its amount increases in different proportion as a function of the mineralogical composition. A higher kaolinite content with a low quartz amount in raw clay favours retention in the modified sample (Ek). The mineralogical composition of the raw kaolin material (eg A and E), [Figure 12] influence the amount of retention chromium both from tanning and salt, as it is observed in Tables 3 and 11. The presence of illite and quartz in kaolin clays has a considerable influence on the value of retention chromium.

Figure 13 - Retention of chromium from Cr tanning wastewater by modified kaolinitic and illitic clays. Three bentonites with a high content of Wyoming (Bl) and Cheto (B2) types montmorillonites, and saponite (S), treated in similar conditions as kaolins, have also showed an important retention of chromium from wastewater [Table 11 and Figure 14]. Values are higher than those for modified kaolin, except for Ek, whose retention values after 1 and 8 days are similar to modified montmorillonite, Blk, Table 11. The pHs of the suspensions are close to 5.5 and some of the chromium (III) may precipitate on clay [71]. The B2 bentonite has been modified by OH-A1 intercalation, B2al, [15]. Cr retention from tannery waste by B2al is lower than the one by untreated bentonite (B2: 6.5 and B2al 6 mgCr/g after a one-day contact time). In general, OH-A1 intercalation reduces cation exchange capacity, thus hindering a high Cr retention [15]. To improve retention, hexametaphosphate ions (HPM) were added to B2al (B2alh). Later, the sample

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C. Volzone

has been given thermal by heating at 500°C (B2alh5). Retention of Cr has increased as shown in Figure 14 and Table 11. Adsorption capacity increased because A1-0H groups were available for reaction with HMP; and the high amount of retained Cr may be related to the presence of surface phosphate groups. After heating at 500°C, structural properties remain unchanged but there is a possible dehydration of an external Al phosphate. The high amount of retained Cr at a low pH (close to 4) may be related to the presence of surface phosphate groups [15] in accordance with phosphatic soils [4]. The retention of Cr by modified Al-bentonite with HMP after heating at 500°C increases more than twice if compared to the original bentonite, Table 11. Table 11 - Cr retention from tannery waste by modified clays.

Sample Ak Bk Ck Dk Ek Blk B2k Sk B2al B2alh B2alh5

Cr retention from tanning wastewater mg/g 1 day 12.0 9.5 7.5 6.5 16.5 16.0 17.0 18.0 6.0 10.0 10.0

8 days 17.0 15.0 8.0 7.5 22.0 30.0 26.0 35.0 8.0 13.0 18.0

The B2k sample retains more chromium than the B2alh5 (Table 11). Nevertheless, mechanical resistance in pellet form is reversed. This property is very important if the solid adsorbent is used in percolation systems. Several experiments about retention chromium in solutions have been carried out using powdered clays and in pellets to evaluate mechanical resistance in a liquid media. More retained Cr (III) from chromium salt is obtained than from tanning wastewater by clays as shown in Tables 3 and 11. It can be seen from Figure 6, a basic chromium sulphate salt contains more polymerised Cr species than tanning wastewater [Figure 6(b)] [84]. As a consequence, more species from a salt solution are required to compensate the negative charge of the clay, due to the low charge per Cr atom. In general, chromium retention is a function of type of clay, activation treatment, and chromium solution used. Clays may also be used in dye and colloid retention from a diluted tanning wastewater [100]. Figure 15 (a) shows the spectra of a diluted chromium (50 ppm Cr) tanning waste containing dye and colloids after a 24-hour contact with two different clays. A natural montmorillonite, m, and the same montmorillonite after acid treatment with sulphuric acid (ml 8) are used as adsorbents. The spectrum of the initial solution shows bands at 450, 581, and 619 nm; the band at 581nm could correspond to the presence of chromium.


315

Figure 14 - Chromium retention from Cr tanning wastewater by modified bentonites.

Figure 15 - Absorption spectra: (a). Tanning wastewater containing low chromium, dye, and colloids before and after treatments with natural montmorillonite (m) and acid montmorillonite (ml8). (b) An aniline solution prepared in laboratory after being treated with acid montmorillonite (+ml8). The high background in the spectrum is due to the colloids in the solution. Clays retain mainly the colloids. For a comparative analysis, one solution of dye, the aniline commonly used in tannery, has been reported [Figure 15 (b)]. The spectrum of the supernatant after the contact of acid montmorillonite with aniline solution is shown also in Figure 15 (b). The acid montmorillonite has been previously calcinated at 400 c C. Clays can interact with organic substances by intercalation, adsorption and cation exchange [101]. Organic substances, such as aniline, are strongly adsorbed by clay

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C. Volzone

minerals [102], The acid nature (Bronsted acid) of the 2:1 layer silicates facilities the protonation of organic species [103]. Espantaleon et al [104] have studied removal dyes from solution by using acid-activated bentonite with a sulphuric acid solution at different concentration. The removal of dyes in this case has been better than with carbon and natural bentonite [104]. Acid treatment on clays should be controlled because acid attack may break the clay structure [36-38] thus loosing the adsorptive characteristics. In general, the best textural characteristics of acid bentonites are obtained when close to 75% of the octahedral cations is released from the structure by acid treatment. 4 - Can we use clays to improve the environment? There is no easy answer to this question. However, if we analyse the variables that play an important role in the retention of metals by clays carefully, it is possible to use clays to have a better environment, and then we would also be capable of controlling the disposal of metal in soils. Nowadays, there are many researchers all over the world studying the metal retention mechanism from solutions by clays and/or soils. A contribution about different aspects of the removal of chromium (III) from solutions by clays has been described in this chapter. However, the answer is not complete as there are more questions to be tackled by an efficient and thorough study, i.e. what do we make with a metal-clay product?; how do we use the resulting Cr-clay?; is it inert forever? An answer to the use of Cr-clays may be, for example, preparing pillared clay (PILCs), as long as we take into account the influence of the variables, i.e. impurity of clays, interference of chromium in solutions, etc. As to how we may obtain an inert material from chromium clay, the answer may be, for example, after an appropriate calcination of Cr-clays. It would then be possible to obtain refractory products. These studies should be carefully analysed since, depending on the different types of atmosphere (i.e. air, oxygen, nitrogen, etc.) used during heating, it is possible to obtain different products from the same Cr-clay as explained in this chapter. Mass balance should also be evaluated to check if chromium evaporates. It is important to take into account that chromium (III) species that cannot intercalate onto solids may be oxidised to Cr (VI) when the solid is treated at high temperature, as it occurs; for example, with a-Al2O3, where the chromium (III) species is only deposited on the surface [99]. It is necessary to keep in mind that it is important not to create pollutants in the environment from industrial activities, and to replace them by using new alternative technologies. Nevertheless, there is pollution in our planet that is harmful to human, animal, and plant life. Acknowlegement It was a pleasure for me to take part in Project V.6-CYTED because interesting experience on removal of chromium from wastewater was acquired. The invitation by Editor Prof. F. Wypych to write this chapter is gratefully acknowledged. I would like to thank all the publishers that allowed me to quote the tables, figures, and pieces of information in the references. I wish to thank Lie. A. M. Cesio for our discussions about chromium chemistry. Finally, I thank my family for their support, and I am particularly grateful to my friend and translator Alicia Bernatene for helping me editing this paper and to my son, Lucas Marchel, for helping me with some figures.


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5 — References [I] H.M. Lima and A.C. Araujo, in: Mineral Precessing and Environment, IV Meeting of the Southern Hemisphere on Mineral Technology, ed. by S.Castro and F. Concha. Univesidad de Concepcion, Concepcion, Chile, 1994. [2] G. Blanchard, M. Maunaye and G. Martin, Water Res., 18 (1984) 1501. [3] J. Rubio, I.A.H. Schneider and W. Aliaga, in: Clean Technology for the Mining Industry, ed. by M.A. Sanchez, F. Vergara, S.H. Castro, Universidad de Concepcion, Concepcion, Chile, 1996. [4] S.P. Singh, L.Q. Ma and W.G. Harris, J. Environ. Qual., 30 (2001) 1961. [5] G. Rodriguez Fuentes and P. Avila Garcia, Catalizadores y Adsorbentes Iberoamericanos para la Remocion de Metales Pesados de Efluentes Industriales, Ediciones CYTED, Espafla, 2000. [6] I. Garcia, A. Sanchez, E. Alvarez Ayuso and O. Jimenez De Bias, Clay Miner., 34 (1999)460. [07] S.W. Bailey, in: Crystal Structures of Clays Minerals and their X-ray identification, ed. by G.W. Brindley and G. Brown, Mineralogical Society, London, 1980. [08] D.M.C. MacEwan and M.J. Wilson, in: Crystal Structures of Clays Minerals and their X-ray identification, ed. By G.W. Brindley and G. Brown, Mineralogical Society, London, 1980. [09] W. Matthes, F.T. Madsen and G. Kahr, Clays Clay Miner., 47 (1999) 617. [10] J.M. Zachara and J.C. Westell, in: Soil Physical Chemistry, ed. by D.L. Sparks, 2nd ed., CRC Press, USA (1999). [II] H. Triantafyllou, S.E. Christodoulou and P. Neou-Syngouna, Clays Clay Miner., 47(1999)567. [12] M.L. Occelli and R.J. Rennard, Cat. Today, 2 (1988) 309. [13] H. Ming-Yuan, L. Zhonghui and M. Enze, Cat. Today, 2 (1988) 321. [14] L. Mercier and TJ.Pinnavaia, Microp. Mesop. Mat., 20(1998) 101. [15] C. Volzone and L.B. Garrido, Ceramica, 48, 307 (2002) 153. With kind permission from Ceramica for reproducing information, ceramicafajipen.br [16] C. Volzone and L.B. Garrido, VI Reunion Anual de SETAC Latinoamerica. Buenos Aires, Argentina, 20-23 de Octubre de 2003, Abs. (2003)113. [17] D.E.W. Vaughan and R.J. Lussier, in: Proc. 5th Int. Zeol. Conf, ed. by L.V.C. Rees, Heyden Press, London, 1980. [18] D.E.W. Vaughan, in: Perspectives in Molecular Sieve Science, ed. by W.H. Flank and T.E. White, American Chemical Society, Washington, D. C.,1988. [19] P. Rengasamy and J.M. Oades, Aust. J. Soil Res., 16 (1978) 53. [20] G.W. Brindley and S. Yamanaka, Am. Miner., 64 (1979) 830. [21] T.J. Pinnavaia, M-S. Tzou and S.D. Landau, J. Am. Chem. Soc, 107 (1985) 4783. [22] M.S. Tzou and T.J. Pinnavaia, Cat. Today, 2 (1988) 243. [23] M.R. Carr, Clays Clay Miner., 33 (1985) 357. [24] R. Burch, Catalysis today: Pillared Clays, Elsevier Science Publishers, Netherlands, 1988. [25] C. Volzone and L.B.Garrido, Ceramica, 42, 275 (1996) 217. [26] C. Volzone, Doctoral Thesis, Facultad de Ingenieria. Universidad Nacional La Plata, 1997. [27] L. B. Garrido, C. Volzone and R. M. Torres Sanchez, Colloids and Surfaces A: Physic. Engineer. Asp., 121 (1997) 163.

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[28] L.B. Garrido and C. Volzone, Anais do 42° Congresso Brasileiro de Ceramica, 3-6 de Junio, Pocos de Caldas, Minas Gerais, Brasil, vol I (1998) 158. [29] C. Volzone, L.B. Garrido, J. Ortiga and E. Pereira, in: Desarrollo de adsorbentes para la separation de gases, ed. by F.Rodriguez Reinoso-P.Andreu, Ediciones CYTED, Espafia, 1998. [30] C. Volzone andN.E. Hipedinger, Clays Clay Miner., 47 (1999) 109. [31] C. Volzone, Microp. Mesop. Mat., 49 (1-3) (2001) 197. With kind permission of Elsevier for reproducing information and parts of Figure 1, Table 1 and Table 3 of the paper (Figure 11, part of Table 6, Table 8 in this chapter), www.elsevier.com [32] GJ. Churchman and C. Volzone, Proceeding of the 12* International Clay Conference, Bahia Blanca, Argentina, July 22-28, 2001, eds E.A.Dominguez, G.R. Mas, F. Cravero. Elsevier, (2003) 31 [33] A.C. Vieira Coelho, P. Souza Santos, C. Volzone and L.D.V. Abreu, Proceeding of the 12th International Clay Conference, Bahia Blanca, Argentina, July 22-28, 2001, eds E.A.Dominguez, G. R. Mas, F. Cravero. Elsevier. (2003) 655. [34] A. Ackerman, Chemie at Industrie, 61 (1949) 29. [35] K. Bruckman, F. Fijal, J. Haber, Z. Klapyta, T. Wiltowski and W. Zabiniski, Miner. Polonia, 7 (1976) 5. [36] P. Souza Santos, Tecnologia de Argilas. Ed. Edgard Bltlcher Ltda., 1975. [37] C. Volzone, J.M. Porto Lopez and E. Pereira, Rev. Latinoam. Ing. Quim. Quim. Apl., 16(1986)205. [38] G.E. Christidis, P.W. Scott and A.C. Dunham, Appl. Clay Sci., 12 (1997) 329. [39] E.F. Folleto, C. Volzone and L.M. Porto, Braz. J. Chem. Eng., 20 (2003) 139. [40] J.C. Echeverria, E. Churio and J.J. Garrido, Clays Clay Miner., 50 (2002) 614. [41] T.M. Christensen, Water Air Soil Poll., 21 (1984)105. [42] B. Zhu and A.K. Alva, Soil. Sci., 155 (1993) 61. [43] P.L. Carey, R.G. McLaren and J.A. Adams, Water Air Soil Poll., 87 (1996) 189. [44] C. Airoldi and S.A.M. Crittes, Clays Clay Miner., 45 (1997) 125. [45] K.A. Bolton and L.J. Evans, Can. J. Soil Sci., 76 (1996) 183. [46] J.M. Zachara, S.C. Smith, C.T. Resch and C.E. Cowan, Soil Sci. Soc. Am. J., 56 (1992) 1074. [47] T. Undabeytia, S. Nir, G. Rytwo, E. Morillo and C. Maqueda., Clays Clay Miner., 46(1998)423. [48] P. Di Leo and P. O'Brien, Clays Clay Miner., 47 (1999) 761. [49] J-B. Yoon, S-H. Hwang and J-H. Choy, Bull. Korean Chem. Soc, 21 (2000) 989. [50] Y. Egozy, Clays Clay Miner., 28 (1980) 311. [51] G.F. Soldatini, R. Riffaldi and R.L. Minzi, Water, Air and Soil Poll, 22 (1976) 110. [52] H.A. Gharaie, M. Maftoun and N. Karimian, 17th WCSS, 14-21 August 2002, Thailand, (2002) 1961. [53] C.A. Coles and R.N. Yong, Appl. Clay Sci., 22 (2002) 39. [54] J.C. Miranda-Trevino and C.A. Coles, Appl. Clay Sci., 23 (2003) 133. [55] L. Mercier and C. Detellier, Environ. Sci. Technol, 29 (1995) 1318. [56 ] Y. Yin, H.E. Allen, C.P. Huang, D.L. Sparks and P.F. Sanders, Environ. Sci. Technol., 31(1997)496. [57] D. Sarkar, M.E. Essington and K.C. Misra, Soil Sci Soc. Am. J., 64 (2000) 1968. [58] J.M. Zachara, C.E. Cowan, R.L. Schmidt and C.C. Ainsworth, Clays Clay Miner., 36(1988)317.


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[59] L.M. He, L.W. Zelazny, V.C. Baligar, K.D. Ritchey and D.C. Martens, Soil Sci. Soc. Am. J., 61(1997)784. [60] O. Yavuz, Y. Altunkaynak and F.Guzel, Water Res., 37 (2003) 948. [61] O. Abollino, M. Aceto, M. Malandrino, C. Sarzanini and E. Mentasti, Water Res., 37 (2003) 1619. [62] K. Naganuma, M. Okazaki, K. Yonebayashi, K. Kyuma, P. Vijarnsorn and Z.A. Bakar, Soil Sci. Plant Nutr., 39 (1993) 455. [63] J.L. Venaruzzo, C. Volzone, J. Ortiga and A. Ortiz Ricardi, VI Reunion Anual de SETAC Latinoamerica, Buenos Aires, Argentina, 20-23 de Octubre de 2003, Abs. (2003) 96. [64] K. Wada and Y. Kakuto, Clays Clay Miner., 28 (1980) 321. [65] C. Volzone and E.L. Tavani, J. Soc. Leather Technol. Chem., 79 (1995) 148. [66] E.L.Tavani and C.Volzone, J. Soc. Leather Technol. Chem., 81 (1997) 143. [67] E. L. Tavani and C. Volzone, Jornadas SAM'98-IBEROMET V, Rosario, Argentina Tomol(1998)35. [68] R.J. Bartlett and J.M. Kimble, J. Environ. Qual., 5 (1976) 379. [69] R.J. Bartlett and J.M. Kimble, J. Environ. Qual., 5 (1976) 383. [70] R.A. Griffin, A.K. Au and R.R. Prost, J. Enviorn. Sci. Health, 12 (1977) 431. [71] M.H. Koppelman, A.B. Emerson and J.G. Dillard, Clays Clay Miner., 28 (1980) 119. [72] M.H. Koppelman and J.G. Dillard, Clays and Clay Miner., 28 (1980) 221. [73] C. Volzone, A.M. Cesio, R.M. Torres Sanchez and E. Pereira, Clays Clay Miner., 41 (1993) 702. With kind permission of The Clay Minerals Society for reproducing information, Figure 1 and Table 1 of the paper (Figure 5 and Table 1 in this chapter), http://cms.lanl.gov [74] C. Volzone and A.M. Cesio, Mat. Chem. Phys., 48 (1997) 216. With kind permission of Elsevier for reproducing information and part of Table 2 of the paper (Table 9 of this chapter) www.elsevier.com [75] C. Volzone and A.M. Cesio, Anales de la Asociacion Quimica Argentina, 47 (1-2) (1999) 59. With kind permission of the Asociacion Quimica Argentina, AQA, for reproducing information and Figure 1 of the paper (Figure 4 in this chapter), anales(a)inifta.unlp.ar [76] C. Volzone and A.M. Cesio, Mat. Chem. and Phys., 79, 1 (2003) 98. With kind permission of Elsevier for reproducing information and Table 1 of the paper (Table 10 in this chapter), www.elsevier.com [77] C. Volzone, Clays Clay Miner., 43 (1995) 377. With kind permission of The Clay Minerals Society for reproducing information, Figure 5 and part of Table 2 of the paper (Figure 7 and part of Table 6 in this chapter), http://cms.lanl.gov [78] C. Volzone, Aust. J. Soil Res., 36 (1998) 423. With kind permission of CSIRO Publishing for reproducing Table 2 (part of Table 6 in this chapter), http://www.publish.csiro.au/nid/84.htm [79] C. Volzone, in: Catalizadores y Adsorbentes Iberoamericanos para la Remocion de Metales Pesados en Efluentes Industrials, ed. by G. Rodriguez Fuentes y P. Avila Garcia. Ediciones CYTED, Madrid, Espafla. l ra Edition, 2000. [80] website: www.epa.gov/OGWDW/dwh/c-ioc/chromium.html [81] M. Ardon and R.A. Plane, J. Am. Chem. Soc, 81 (1959) 3197. [82] J.A. Laswick and R.A. Plane, J. Am. Chem. Soc, 81 (1959) 3564. [83] R.W. Kolaczkoncki and R.A. Plane, Inorg. Chem., 3 (1964) 322.

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[84] M.E. Thompson and R.E. Connick, Inorg. Chem., 20 (1981) 2279. [85] J.E. Finhol, M.E. Thompson, and R.E. Connick, Inorg. Chem., 20 (1981) 4151. [86] H. Stunzi and W. Marty, Inorg. Chem., 22 (1983) 2145. [87] H. Spiccia, W. Marty and R. Giovannelli, Inorg. Chem., 27 (1988) 2660. [88] H. Stunzi, L. Spiccia, F. P. Rotzinger and W. Marty, Inog. Chem., 28 (1989) 66. [89] C. Volzone, J. Ortiga, V. Segarra and E. Montiel, Asociacion Quimica Espanola de la Industria del Cuero (AQEIC), 54, 1(2003) 16. With kind permission of AQEIC for reproducing information and Figures 2 and 3 (Figures 12 and 6 in this chapter), http://www.valles.com/aqeic [90] B.C. Lippens and J.H. de Boer, J. Catal., 4 (1965) 319. [91] C. Volzone, Mat. Chem. Phys. 47 (1997) 13. With kind permission of Elsevier for reproducing part of information of the paper, www.elsevier.com [92] S.J. Gregg and K.S.W. Sing, Adsorption Surface Area and Porosity, Academic Press, 2nd Edition, London, 1991. [93] J.F. Delon, O. Lietard, J.M. Cases and J. Yvon, Clay Miner., 21(1986) 361. [94] C. Volzone and L.B. Garrido, Clay Miner., 36 (2001) 115. [95] R.C. Mackenzie and S. Caillere, in: Data Handbook for Clay Materials and other Nonmetallic Minerals, ed. by H. van Olphen and J.J. Fripiat, Pergamon, Oxford, 1979. [96] A. Drljara, J.R. Anderson, L. Spiccia and T.W. Turney, Inorg. Chem., 31 (1992) 4894. [97] R.H. Loeppert and M.M. Mortland, Clays Clay Miner., 27 (1979) 373. [98] P.F. Kerr, J. Laurence Kulp and P.K. Hamilton, Report No 3. American Petroleum Institute, Project 49, Clay Mineral Standards, Columbia University, New York, 1949. [99] C. Volzone and A.M. Cesio, J. Mat. Sci. Letters, 14 (1995) 658. [100] C. Volzone, VI Reuni6n Anual de SETAC Latinoamerica (Sociedad de Toxicologia y Quimica Ambiental). Buenos Aires, Argentina, 20-23 de Octubre. Abs. (2003) 22. [101] G. Lagaly, Phil. Trans. R. Soc. Lond. A, 311 (1984) 315. [102] C.T. Johnston, in: Organic Pollutants in the Environment. CMS Workshop lectures, ed. Brij L. Sawhney, The Clay Miner. Soc, Boulder, USA, 1996. [103] M.M. Mortland and K.U. Raman, Clays Clay Miner., 16 (1968) 393. [104] A.G. Espantaleon, J.A. Nieto, M. Fernandez and A. Marsal, Appl. Clay Sci., 24 (2003) 105.

CATALYTIC AND ADSORPTION PROPERTIES OF MODIFIED CLAY SURFACES ALEXANDER MORONTA Centra de Superficies y Catalisis, Facultad de Ingenieria, Universidad del Zulia, Maracaibo 4003-A - VENEZUELA. E-mail: [email protected]


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1 - Introduction Clay minerals are the most abundant sedimentary mineral group. They predominate in the colloidal fractions of soils, sediments, rocks and waters and are classified as phyllosilicates (usually hydrous aluminosilicates). In Geology the word clay is used in two ways: firstly as a rock classification which generally implies an earthy, fine-grained material that develops plasticity on mixing with limited amount of water. Secondly, it is used as a particle term, which describes clays as minerals which have a particle size 2 h for both reactions. However, at short mid treatments times ( ODTMA+. Similarly, mild acid treatment alone (0.1 M HC1 for 1 h at 25 °C) did not to produce a large amount of camphene and limonene, but severe acid treatment (1 M HC1 for 1 h at 95 °C) did enhance it. Moreover, mild acid treatment on the organoclays resulted in a fourfold increase in the product yield. Additionally, the conversion was further increased as the severity of acid treatment increased. Similarly, a relative increased yield was only observed in organoclays containing the smallest amounts of DDTMA+ and 0DTMA + (0.25 CEC) and treated with mild acid conditions, the former being more active that the latter. The activity of organoclays prepared with small amounts DDMTA+ and ODTMA+ was particularly more appreciable when acid treatment became more severe. In general trend, DDMTA+ and ODTMA+ cations were more resistant than TMA+ to displacement byH + .

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The shape, charge and orientation of the organocation should be taken into consideration due to these parameters have a remarkable effect on the catalytic conversion of a particular organic molecule. This observation is shown in Figure 10, where the products distribution for the isomerization of oc-pinene is illustrated for aluminum activated organoclays prepared using tetramethylammoiun (TMA)+, the diprotonated form of l,4-diazabiciclo-(2.2.2) octane (DABCOH2)2+ and 1,5diaminopentane (DAPH2)2+. Evidently, the type of organocation utilized showed a significant effect on the total conversion obtained using the isomerisation of cc-pinene over SWy-2 clay. The lowest catalytic activity is registered in SW-A1/DAP, suggesting that the exchange with DAPH22+ cations results in a sterically restricted diffusion of pinene in the interlayer space of the catalyst and or no useful acidity. The activity in SW-A1/DABCO samples is considerably higher than for SWAl/DAP. This enhancement in activity can be ascribed to the greater openness in the interlamellar space for a-pinene to react, except for the internal surface fraction occupied by the immobilised DABCO ions. On the other hand, the vertical orientation of DABCOH22+ produces channels between the alkyldiammonium cations in the interlayer space, allowing access to reactant molecules, whereas in DAPH22+ there is a lack of height that may prevent the access of a-pinene to the interlamellar space [169]. In particular TMA+-exchanged clays, treated at room temperature with only 0.1 M HC1, proved to be effective catalysts for the conversion of a-pinene to camphene and limonene. Total conversions of 60 to 90% were obtained making them effective competitors for zeolites and pillared clays for this isomerization [170]. The enhanced activity was attributed to the spatial separation of the TMA+ ions in the interlamellar spacing, which probably controlled the effective size of the catalytic site [164]. This study indicated that (i) acid-activated organoclays (AAOCs) were more effective when the galleries were not congested with large organocations, hence AAOCs derived from TMA+-exchanged clays were found to be the most effective, (ii) the activity was influenced by the nature of the starting clay and (iii) that preadsorbedTMA+ cations appeared unexpectedly less resistant to subsequent displacement by protons. In a similar work, Breen and Watson [171] studied the influence of acid treatment on organoclays prepared using a polycation (Magnafol 206) in samples derived from a Na-montmorillonite (SWy-2) and a Ca-montmorillonite (SAz-1). The clays were first exchanged with the polycation to satisfy 0.25, 1 and 1.5 times the CEC of both clays and then acid activated using 6 M HC1 at 95 °C for 30, 90 and 180 min. Acid-activated samples, absent of the polycation, were also prepared either at 25 °C or at 95 °C. They found that hot acid treatment increased the total conversion of a-pinene for both SWy-2 (77%) and SAz-1 (65%) more than cold mild acid treatment (43% for SWy-2 and 15% for SAz-1). The highest yields were achieved using a mild acid activation treatment (90 min) in clays with the lowest polycation content (0.25 CEC), total conversions of 90% and 83% for SWy-2 and SAz-1, respectively. At more prolonged treatment time (180 min) the total conversion was catastrophically decreased (especially for SAz-1) because the samples presented a much-reduced polycation loading. Catalysts prepared with intermediate and high polycation loadings were, in general, less effective for the reaction process. The presence of polycation had a more marked influence on the activity of samples derived from SAz-1 increasing the yield from 25% for acid-activated SAz-1, in the absence of polycation, to 50% camphene for acid-activated polycation exchanged SAz-1, while the increase in percentage of camphene formed for SWy-2 was only from 42 to 52%. Breen and Watson [170] concluded that the enhancement in yield for SAz-1

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was due to the increased hydrophobicity of the polycation loaded clay, whereas the comparable yield for SWy-2 in the absence and presence of polycation may suggest that this clay disperses well in the non-polar a-pinene.

Figure 10 - Products distribution for catalysts derived from SWy-2 clay. Breen's studies [168,171] demonstrated that the amount of either organocation or polycation plays a key role as well as the severity of acid treatment and the type of clay utilized. Indeed, there is a competitive mechanism between protons and organocations for the clay surface (external and internal) that governs the hydrophobic/hydrophilic character and consequently the catalytic activity. Acid activated organoclays prepared combining TMA+ at 1 CEC of the studied clays and different concentrations of hydrochloric acid demonstrated that the catalytic activity towards the isomerization of ce-pinene is dramatically reduced when the incorporation of the organocation is >30% of the CEC the clay. The gallery surface accessible to reactant molecules in the presence of high TMA+ content is insufficient to allow access to the catalytic acid sites [172]. Similarly, the catalytic activity of aluminum activated clays (AlACs) and aluminum activated organoclays were studied to avoid depletion of the octahedral sheet of a Mg-rich saponite. The samples were prepared combining volumes of 1M TMA+ solutions and 0.1 M Al3+ in different ratios to satisfy the CEC of four clays [173]. In general the catalytic properties over AlACs and AlAOCs was very similar to those observed using H+ and H+/TMA+ [172]. Al+/TMA+-exchanged clays gave lower conversions than their Al+-exchanged counterparts when the Al3+ offered was low (10-40%), but values were similar at an Al3+ content of 50% CEC. Figure 11 shows the product distribution for one of the studied clays (SWy-2). Certainly, at high organocation content there was a reduction in acidity and therefore a decrease in activity was expected in the isomerization of a-pinene, but-1-ene and for the adsorption of hept-1-ene [172-175]. The high activity found was attributed to the ability of clays to keep the layers permanently apart thus facilitating the ingress of reactant molecules.


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Figure 11 - Products distribution for the catalysts derived from SWy-2 clay. 12 - Conclusions Clay minerals were originally very important catalysts for petroleum-cracking reaction, but they were replaced by more thermally stable zeolites catalysts. Nowadays, pillared clays have produced a cheap and competitive thermal stable solid. Additionally, pillared-organic clays have been found to be good adsorbents for the removal of organic pollutants from water. Useful catalysts are also obtained by modification of the original clay by combination acid activation with orgonocation intercalation for organic reaction. The catalytic activity of these modified materials can be carefully optimized for specific reactions. Changes in the surface are, cation exchange capacity, the nature of the exchange cation, the organic loading and the charge density influence activity in different ways. Acknowledgements The author acknowledges his postgraduate and engineering students for their continuous outstanding experimental work and for improving figures. Thanks are given to FONAC1T and CONDES-LUZ for financial support. Thanks are specially given to professor Jorge Sanchez for helpful discussions and carefully reading this manuscript. I acknowledge the contributions of the authors indicated in the citations. Finally, I would like to thanks the editor for the invitation to write the present chapter and for his forbearance. 13 - References [1] R.E. Grim, Ed., Applied Clay Mineralogy, McGraw Hill, New York, 1962. [2] S. Guggenheim and R.T. Martin, Clays Clay Miner., 43 (1995) 255. [3] N. Bondt, J.R. Deiman, P. van Troostwyk and A Lowrenberg, Ann. Chim. Phys., 21(1797)48. [4] H.H. Murray, Clay Miner., 34 (1999) 39. [5] T.A. Wolfe, T. Demirel and E.R. Baumann, J. Water Pollut. Control Fed., 58 (1986)68.

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PREPARATION OF LAYERED DOUBLE HYDROXIDES EIJI KANEZAKI Department of Chemical Science and Technology, Faculty of Engineering, The University of Tokushima, 2-1 Minamijosanjima, Tokushima 770-8506 - JAPAN E-mail: [email protected]


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1 - Preparation of layered double hydroxide with interlayer carbonate. Layered double hydroxides (LDH) have a general formula of [MaM'b(OH)2(a+b)]A3bH2O, abbreviated as MM'/A-LDH where M and M' are a divalent and a trivalent metal cation in the layer of the hydroxide, respectively, and the excess positive charge of the layer is compensated by the negative one on the interlayer monoanion A, which can be replaced by a di-, tri- or other multivalent anion or in some cases a mixture of them, thus LDH is called an anionic clay mineral. LDH is a mimic of naturally occurring hydrotalcite (MgAl/CO3-LDH), has a similar layered structure, which is called hereafter the hydrotalcite-like layered structure, and is also called the hydrotalcite-like compound. Over the last two decades, interest has been growing in the availability for the intercalation of various organic anions having flexible or rigid molecular frameworks into LDH not only from the scientific but also from industrial viewpoints [1]. LDH is usually prepared in ordinary conditions of temperature, pressure and so on as a precipitate from solutions by means of environmentally benign methods. Therefore, it is easy to synthesize the LDH which has the desired anion(s) at the interlayer region when we carefully select the combination of the metals and the organic compound. Both the facility in the synthesis and the potential variety in the combination of the components promise the usefulness of LDHs for developing new-type of materials. A lot of studies have been done on the synthesis and characterization of LDH with the variety of divalent (Zn2+, Ni2+, Fe2+, Co2+, etc.) or trivalent metal cations (Fe3+, Cr3+, Sc3+, etc.) in the layers of LDH. It is outstanding that many kinds of anion, indifferent to mono- or multivalent, are intercalated at the interlayer of LDH because the volume of the interlayer gallery is variable and the interlayer distance between the adjacent layers is enhanced or reduced in proportion to the molecular size of the anion. The interlayer anion of LDH is organic, inorganic or coordination compounds which could be introduced into LDH by means of anion-exchange, coprecipitation or rehydration the last of which is very characteristic of the intercalation of LDH and is described below in detail. The rehydration method originates in a unique nature of LDH; the hydrotalcite-like layered structure is reproduced when the precursor LDH is calcined at high temperature to collapse the layered structure followed by swelling in aqueous solution containing anions to be intercalated. Collapsing the layered structure in the precursor LDH results in only amorphous phases of the mixed metal oxides of MO and M'2O3 in the calcined solid, all of which are absent in the X-ray powder diffraction patterns (XRD). When the intercalation of anions occurs at the interlayer gallery region of LDH, the XRD pattern usually changes drastically and the explicit layered structure appears again; the magnitude of the basal spacing, calculated from the lowest 29 angle of diffraction lines, indicates the molecular size of the intercalated anion perpendicular to the normal axis of the stacking layers. Therefore, in the study of the solid-state chemistry of LDH, the XRD measurement before and after the intercalation of a particular anion is essential with rare exceptions. Furthermore, the inspection of the thermal change of the XRD pattern together with that of differential thermal analysis/thermal gravimetry (DTA/TG) allows us the fruitful discussion since the collapse of the layered structure, which results from the dehydroxylation of the double hydroxide, takes place at a moderate temperature. However, thermally metastable solid phases are sometimes missed in conventional measurements in which samples are heated and cooled outside the sample chambers. Therefore, high temperature in situ measurements are necessary in order to investigate the thermal change of the solids

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precisely. In particular, the in situ high temperature XRD measurement (in situ HTXRD) is favorable for the study of LDH since the layered structure recovers very soon in an ordinary atmosphere owing to the adsorption of water and CO2 in air. Results of these measurements are illustrated below for two LDH compounds, which have the interlayer carbonate and are frequently used as the precursor for the intercalation of the particular

1.1 - Mg and Al layered double hydroxide with interlayer carbonate Figure 1 illustrates a general view of the in situ HTXRD patterns of MgAl/CO3-LDH in the temperature range from 30°C to 1000°C. There are three regions of temperature (T), which have the common HTXRD pattern; 30 CH3COO(CH2)3CH3 + H2O

Under the same conditions, the catalytic activities of ZnAl-PWnZ (Z=V, Ti, Cr, Ni) were determined. It was found that the catalytic activity of ZnAl-PWnTi was the highest, even twice as high as that of H-type molecular sieves (HY) commonly used in industry. The sequence of esterification activities was found as follows: ZnAl-PWHTi (59.2%) > ZnAl-PWnV (48%) > ZnAl-PWnCr (42%) > ZnAl-PW,,Ni (36%), (data in the brackets are conversions for the reactions time 5 h). This indicates that these monosubstituted anions of Keggin-type create new active centres that ZnAl-NO3" precursor or PW n Z does not possess. It is considerably possible that the new active centres are derived from the interaction between the Keggin type anions and the host layers. In addition, we have performed the following experiment to test the stability and lifetime for the catalyst of ZnAl-PW n Ti. ZnAl-PW,,Ti (0.8 g) was used to catalyse the reaction for 2.5 h, then the catalyst was filtered and then put into another reaction of the same reactants, the reaction was also carried out for 2.5 h, while the mother liquor with the catalyst filtered was allowed to react under the same conditions for the same period. For the catalysts after being used for the second time, no reduction of their activities could be found, while the reaction of the mother liquor with the catalyst removed could hardly be found. The above operation was repeated for 5-8 times, and the same results were obtained. IR and XRD result for the PW n Ti being used as catalyst for 5-8 times showed that its structure was still remained in the ZnAl-PWuTi. This demonstrates that the pillared layered Keggin anions are stable in structure. The data of the catalytic activities and selectivities for the reaction of acetic acid with n-butanol, using the catalyst of ZnAl-GeWn, ZnAl-GeWuNi, and ZnAl-GeWnCu, etc. are listed in Table 3. By comparison with ZnAl-NO3 and GeW u , the catalytic activities of ZnAl-GeWn, ZnAlGeW n Ni, and ZnAl-GeWnCu are markedly higher in the reaction [21]. The selectivities of these catalysts are nearly the same, which are high for the esterification (>98.8%) and low for the etherification ( ZnAl-BWnCo > ZnAl-CoWnCo. The order shows that the catalytic activity increases with the increase in valence of the central atom, which is in agreement with the catalytic activity order of their nonintercalated POM anions [54]. When the reaction was carried out for 5 h, the selectivity

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Changwen Hu and Danfeng Li

of w-butyl acetate was almost 100% and without by-product being observed. The time courses of the esterification reaction for these POM-LDH catalysts are shown in Fig. 17. Table 3 - Esterification activity and selectivity of acetic acid with n-butanol". Catalyst*

Catalyst mass/ g

Acetic acid conversion (%)

Selectivity ("/ «-BuOAC

0.8002 0.2 99.8 66.5 ZnAl-GeWn 0.5 99.5 57.3 ZnAl-GeWnNi 0.8012 0.1 99.9 56.2 0.7995 ZnAl-GeWnU 1.2 98.8 17.2 0.8018 ZnAl-NO3 0.1 0.8021 99.9 17.8 K 8 GeW u O 39 " Reaction conditions: Acetic acid/n-Butanol/Acetophenone (solvent) = 0.05/0.05/0.25 (mol) Reaction temperature: 9(fC, reaction time: 5 h; * Catalysts were pretreated in N2for 2 h at 120"C before the reaction.

Figure 17 - Catalytic activity of ZnAl-XWuCo 1, ZnAl-GeW,,Co 2, ZnAl-BW,,Co 3, ZnAl-CoWuCo 4, HY5, GeWnCo 6, ZnAl-NO3 7, Without catalyst. The catalytic behaviour of POM-LDH for the alkylation of isobutane with butene has been investigated using ZnAl-[SiWuO39(H2O)]8", MgAl-[PW12O39(H2O)]7' and NiAl-[PCoWnO39]5~as catalysts, which were synthesized via both the reconstitution method and the LE method under microwave condition [35, 43]. The data of the alkylation activity is listed in Table 4. It is found from Table 4 that whether it is LDH or POM-LDH, both exhibit obviously high activity for the alkylation of isobutane with butene. For the LDH that are used as catalysts in the reaction, MA1-CO32" is more active than ZnAl-CO32" and MgAl-CO32\ Upon the catalyst of LDH-[SiWnO39(H2O)]8" prepared by the reconstitution method, the conversion of butene is much the same as

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upon LDH, but LDH-[SiWnO39(H2O)]8" can remain a stable conversion. It should be noticed that the catalyst of NiAl-[SiWnO39(H2O)]8~ creates a much higher conversion of butene, the C12 and C,6 content than that of the NiAl-[SiWnO39(H2O)]8". This is considered to result from the increase in acid sites of heteropolyacid on the catalyst after the interlayer water is lost. The C12 and C16 content increased are produced from the polymerised butene on the acid sites of interlayer H3PWi2O40. The conversion of butene can be catalysed by both LDH and POM-LDH, which shows that the alkylation reaction can occur on both the base sites of brucite and the acid sites of heteropolyacid. Therefore, it is realized that POM-LDH is a sort of acid base bi-functional catalyst. Table 4 - Alkylation activity of isobutane with butene".

Catalyst

Butene conversicD n (%)

Product s distribut ion (%) ( Mass fraLDHs. POM-LDH BET specific Pore volumexlO" Median diameter of Vml-g 1 surface area/m2-g"' the pore/nm 8.24 20.00 9.71 ZnAl-SiW,, 22.04 11.21 7.87 ZnAl-SiWnCo 6.24 7.02 ZnAl-SiW n Cu 3.56 ZnAl-SiW,,Ni 12.95 5.59 9.26 8.05 31.13 ZnAl-NO3 15.46 4.2. - Photocatalysis Recently, dispersed solid particles of TiO2 have been extensively used as an efficient photocatalyst in the chemical decomposition of many organic substrates [6365]. It generally leads to total mineralization of organics into CO2. But TiO2 ultra fine particles are easy to lose and it is difficult to separate them from the reaction systems due to the formation of a milky dispersion of the TiO2 particles. POM-LDHs are generally used as oxidative and redox catalysts in the reported

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references. However, their ability to undergo photo-induced multielectron transfer without changing structures is attractive for their use as photocatalysts in the oxidation of organic substrates [58]. The photoactivities of POM-LDHs originate from the unique structure of POMs [23]. They have a number of features in common with semiconductor metal oxide clusters and can be considered as the analogs of the latter [65]. It is often stated that the photochemistry of POM can be regarded as a model for the photochemical processes on semiconductor metal-oxide surfaces [66]. Both classes of materials are constituted by d° transition metal and oxide ions and exhibit similar electronic attributes including well-defined HOMO-LUMO gaps (band gaps). The "gaps" inhibit the recombination of electrons and holes that are generated by the irradiation of the surface of the photo-catalysts with the light energy higher than the band gaps. The electrons and holes thus formed are capable of initiating chemical reactions because of the formation of OH radicals resulting from the subsequent reaction of holes and OH" groups coming from water. Photocatalytic degradation of the organic pollutants by POMs such as [W7O24]6~, [W10O32]4~, [PW,iO39(H2O)]7", [SiWnO39(H2O)]8" and [P2Wi8O62]6" in homogeneous systems has been reported to be very effective [67,68]. However, it is difficult to separate them from the reaction systems for recycling. Therefore, synthesis of the insoluble POM-pillared compounds and using them as heterogeneous catalysts are of fundamental and practical significance. In the previous references, Pinnavaia et al have studied photo-oxidative dehydrogenation of isopropanol on the ZnAl-[V10O28]6", ZnAl-[BVWnO40]6, ZnAl[SiV3W904o]7" and ZnAl-[H2W1204o]6\ and the turnover numbers obtained were 4-10 [17,26]. Studies on the SM-LDHs pillared by polyoxotungstates are relatively more active than those on the SM-LDHs pillared by polyoxomolybdates or polyoxovanadates because polyoxotungstates are more stable than polyoxomolybdates or polyoxovanadates. More recently, we have used a series of synthesized POM-LDHs , namely, [W 7 O 24 f, [SiWnO39Z(H2O)]6" (Z=Cu2+, Co2+ and Ni2+), [P2W18O62f, [P2WnO61Mn(H2O)]8" and [NaP5W3o0110]14" on the organic pollutantsHexachlorocyclohexane (HCH) and pentachloroenitrobenzene (PCB) photodegradation reaction, and found those materials have high reactivity on the reactions [11]. As is well known, the two pollutants as organocholorine hydrocarbons threaten the health of human beings seriously and cannot be totally degraded by the microbiological method. Scientists have conducted a lot of practical work in order to eliminate the pollutants in water and create a green environment. By using the POMLDHs as the photocatalysts, the conversion of HCH into CO2 and HC1 was 30%-80% by irradiation of HCH slurry in the near-UV region for 4 h, and HCH can be totally mineralized by controlling the reaction conditions (see Figure 18). Figure 19 has shown the photodegradation of HCH in presence of Mg2Al[W 7 O 24 ] 6 \ Different photoactivities observed may be caused by the following reasons: (i) different amounts of the interior active sites (the bonds of W-O-W) existing in the different POM-LDH intercalates and (ii) different specific surface areas obtained for the different POM-LDH intercalates (see Table 8). Usually, a higher crystallinity or larger particle size results in a lower specific surface area. As for the LDH precursors, the photocatalytic activity was hardly observed, indicating that the photoactivity of the POM-LDHs originates from the interlayer POM ions rather than LDH precursor. In addition, it is very easy to separate the POM-LDH catalyst from the reaction systems indicating the catalysts are recyclable.


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Figure - 18 Activity patterns for the photocatalytic degradation ofHCH on the POMLDHs; [HCHJ0 =10 mg/L, [POM]= 0.5 mmol/L, irradiating time 4 h.

Figure 19 - Photodegradation ofHCH and formation ofCl ion with photocatalysis time in presence of Mg2Al-[W7O24]6'. [HCH]0=10mg/L, [HCH], and [Cl]~, represent the concentration ofHCH and CT ion at elapsed time of reaction, respectively. According to our studies, the model for the photocatalytic reaction on the POM-LDHs are proposed and shown in Figure 20. When the pillared compound is irradiated by light energy near the UV area, the photons penetrate the sheets. Subsequently, the interlayer POM molecules absorb the light energy and are photoexcited to their charge-transfer excited state (POM-LDH)*. At the same time, the

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reaction molecules (HCH) diffuse into the interlayer space and are photoactivated by the (POM-LDH)*, and then are degraded into inorganic product molecules (CO2 and HC1) which subsequently escape from the interlayer space through diffusion. Table 8 - Chemical compositions, BET surface area and average particle size for the POM-LDH intercalates. Proposed formula BET surface Average particle area (m2/g) size (nm) Mg! 8Al(OH)6(W7O24)0.18-0.7 H2O 43.5 100 Zn2.9Cr(OH)8[SiW11039Ni(H20)]o.i9-0.2H20 98.1 20 ZriLgAKOHMSiWuOjô 13-0.2 H2O 9.26 50 Zn2.8Cr(OH)8[SiWii039Mn(H20)]o.i9-0.2 H2O 93.5 45 Zn, 8Al(OH)6[P2W17MnO61(H2O)]014-0.7 H2O 34.0 100 Mg1.8Al(OH)6(NaP5W3o011o)o.o72-0.2H20 66.7 150 Zn2 oAl(OH)6(NaP5W3o0110)o 075-0.2 H2O 65.2 145

Figure 20 - Model ofphotocatalytic degradation of aqueous HCH on the POM-LDH. Our studies show that the disappearance of the HCH follows LangmuirHinselwood first order kinetics. From the results of GC-MS and IC, the intermediates produced during the process of photocatalysis were iden- phototified and detected. That is, phenylhexachloride, polyphenol, tetrahydroxybenzoquinone and benzoquinone were detected by GC-MS; acetic acid and formic acid were detected by IC. According to the distribution of the intermediates, the reaction mechanism for the photocatalytic degradation of aqueous HCH on the POM-LDH intercalate is proposed as follows: As a heterogeneous photocatalyst, the interlayer POM has the double aptitudes of adsorption of the reactants on the catalysts' surface and absorption of the photon with the light energy higher than its band gap. The photocatalytic reaction occurs in the photointerlayer space of the intercalates and the mode of activation of the catalyst is photonic activation by exciting the POM-LDH with suitable light energy, and then


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forms the excited species (POM-LDH)*. By using the semi-conductor notation, the excitation of the POM—LDH at the OMCT band can be presented by Equation 1: POM-LDH + to- POM-LDH (e"+ h+)

(Eq. 1)

In principle, h causes oxidations and e" reductions. POM-LDH(e"+ h* ) has a more powerful redox ability by forming electron-hole pairs and their recombination is inhibited by the " g a p " of the POM. The reaction of the photoholes with the surface hydroxyl groups occurs and OH" radicals are generated (Eq. 2): POM-LDH (e + h' ) + H2O non-metal oxoanions (BO33\ CO32", NO3", Si2O52", HPO42\ SO42", C1O4", AsO 4 3 \ SeO42", BrO4", etc.), > oxometallate anions (VO 4 3 \ CrO42", MnO 4 \ V10O286\ Cr2O72", Mo7O246\ PWi2O403", etc.), > anionic complexes of transition metals (Fe(CN)62", etc.), > volatil organic anions (CH3COO", C6H5COO", C12H25COO", C2O42", C6H5SO3", etc.), >anionic polymers (PSS, PVS, etc.). Acid-Basic properties Layered double hydroxides and their calcined products display unique acidbasic properties. A great number of catalytic applications involved the basic character of these compounds [17,18]. This property has obviously a great interest when looking at new materials for the uptake of acid molecules such as acid gases (CO2 or SO2).

Environmental Remediation Involving Layered Double Hydroxides

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Knoevenagel condensation, CO2 adsorption or acid titration experiments are commonly used to quantify the basicity of the materials [23-26]. It has been shown that calcined LDHs display a stronger basic character than the uncalcined precursors due to the presence strong O2' basic sites. Moreover, the basic character depend on the chemical composition of both the layer and the interlayer. Rousselot et al [23] have related this property to the electronegativity of the metal cations, showing that phases containing Mg, Ca or Ga display higher basic characters. Shape and morphology When regarding a material for its adsorption or catalytic properties, the study and the tuning of its textural properties appear necessary. Natural Hydrotalcite displays particles with a regular hexagonal shape and sizes ranging between 2 to 20 um. The morphology of synthetic LDH particles depends on the method of preparation (Figure 2). Particles with a high degree of crystal cohesion leading to well-resolved XRD patterns but with a wide range of particle sizes are obtained when using the standard constant pH coprecipitation method or the "urea synthesis" [27]. Fine grained crystals with rough surfaces, relatively high surface areas and mesopores with size in the range 50-300 A were obtained by the variable pH coprecipitation method. Edge-face crystal aggregation and cofacial layer stacking create a secondary particle aggregation with a sand-roses morphology and interparticle mesopores which provide voids for water. Morphology is also affected by the chemical composition of the structure [28].

Figure 2 - S.E.M. of Cu2Cr(OH)6Cl.2H2Oprepared by a) CuO/CrCl3 induced hydrolysis and b) coprecipitation (V. Prevot, unpublished results) and Mg2Al(OH)6(CO3)0.5-2H2O prepared by c) coprecipitation andd) urea method [27].

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Surface and porosity properties The theoretical surface areas of one LDH monolayer can be easily calculated taking into account its composition and structural property: Stheo = a 2 V3 10"18N/(F.W.) N = Avogadro number; a = cell parameter (run); F.W. = formula weigth relative to the unit formula (g.mol-1). For Zn2Cr(OH)6C1.2H2O and Mg3Al(OH)8(C03)o.5.2H20, the calculated S ^ values are respectively 817 m2/g and 1285 m2/g. Practically, such high values cannot be measured for anionic clays because the internal surface is hardly accessible. Anionic clays generally display N2 isotherms features corresponding to mesoporous or nearly non-porous materials even when large pillaring anions (POM) are intercalated. The typical values of specific surface area (s.s.a.), measured by the B.E.T. technique, range from 20 - 85 m2/g. N 2 adsorption plots indicate type II N-adsorption isotherms with a narrow hysteresis cycle, due to pores open on both ends. However, specific surface area can be greatly improved when coprecipitation is performed in mixed water/alcohol media [29]. Synthetic hydrotalcite prepared in H2O/ethylene glycol (1/1) solution exhibits a s.s.a. value of 136 m2/g, larger than for the material prepared in standard conditions (45 m2/g). 2 - Remediation of inorganic contaminants by exchange/adsorption process Miyata [30] reported for the first time in 1983 the ion-exchange isotherms of a series of hydrotalcite like compounds leading to the ion selectivities of hydrotalcite for monovalent and divalent anions: OH" > F" > Cl" > Br" > NO3" > I" and CO32" > NYS2" > SO 4 2 \ Miyata concluded this work: "by utilising their characteristic ion selectivity, HTs are expected to find application in removal of acid dyes, HPO 4 2 \ CN", CrO42", AsO43", Fe(CN)63", etc. from waste waters". This work generated a great number of studies in which the anion exchange properties of LDHs and the anion adsorption ability under LDH reconstruction process have been investigated for the removal of environmentally undesirable anions. In 1995, Parker et al [31] published extended results on the use of hydrotalcite as anion adsorbents. They confirm the initial Miyata's results for the anion sorption capacity of a commercial [Mg-Al-CO3] sorbent : SO42" > F" > HPO42" > Cl" > B(OH)4" > NO3" > I" but stressed the point that the carbonate LDH is ineffective as anion exchanger. However, the calcined materials display higher adsorption properties even after various cycles of calcinations/rehydration. As a consequence of recent interests to develop the use of anionic clays for environmental remediation, anion exchange properties of LDH have been deeply reinvestigated. The main objective of these studies is to clearly characterize the adsorption properties of the materials under various solid/liquid interface conditions. The effect of sorbent composition, adsorbate concentration, proton concentration, solid/liquid ratio and competing anions on adsorption have been examined. Physisorption and chemisorption, surface and bulk adsorption, concentration of adsorption sites have been assessed. The adsorption capacity is deeply affected by the nature of the counter-anion of the LDH layers. Inacio et al [32] showed that the adsorption capacity for the [Mg-Al-X] sorbent series varies in the order X: CO3 < Cl < NO3. The carbonate anion seems to reduce strongly the adsorption capacity. Indeed, the


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adsorption of dodecylsulfate by [Mg-Al-CO3] and [Mg-Al-Cl] leads respectively to 0 and 100% adsorption. The fluid structure at the interface and in the interlayer spaces, the effective diffusion coefficients of surface-sorbed species, their surface lifetimes, rotational and translational dynamics are controlled by the structure and the composition of the surface of the host structure [33]. In a study of internal versus external uptake of anions, J. Boclair et al [34] showed that ferrocyanide does not displace carbonate from synthetic hydrotalcite but is taken up on the outside of the particles. Anion uptake is in this case controlled by specific hydrogen bonding requirements and not by charge density alone, a feature that can be used to control whether uptake will be both internal and external, or external only. Ulibarri et al [35] have shown that both the type of interlayer anion and the crystallinity of the minerals affects deeply the adsorption capacity. Experimental adsorption isotherms are usually processed with the Langmuir or Freundlich models developed for physisorption [32]. However, in most of the studies the adsorption mechanism is identified as an anion exchange process, showing that both theoretical models are inadequate. 2.1 - Removal of oxoanions Amongst oxoanions, adsorption of phosphate species by LDHs was much more examined because of their consequences on water pollution [36-40]. Removal of phosphate by a dissolution/coagulation process using a synthetic pyroaurite ([Mg-FeCO3]) and hydrocalumite-like material ([Ca-Fe-CO3]) was effective (above 80% from studied effluents) but strongly dependent on the buffering pH effect of both the sorbent and the solution. The rate of removal was well correlated with the amount of both dissolved sorbent and release cations [36,37,38]. Simultaneous uptake of PO43" and NH4+ can be obtained by the use of non-selective nanocomposite sorbent made from granular zeolite coated with hydrotalcite and an organic binder [40]. 2.2 - Removal of heavy metals Presence of heavy metals in the environment arises from both natural and anthropogenic sources. They are present in the effluents of many industries and contamination of natural waters and soils is a major environmental concern owing to their high toxicity. The adsorption technology is the most common process used for the uptake of heavy metals from polluted reservoirs. However, depending on the pH conditions metal can exist as cations or various anionic forms. LDHs appear then as good sorbents for the removal of the anionic species but have revealed also effective for the uptake of metal cations. 2.2.1 - Removal of heavy metals as oxometallate anions Since the years 90's, researches for the development of hydrotalcite as metal adsorbents have focussed mainly on the removal of selenate [41-46], arsenate [41,43,4651] and chromate [47,52-60] anions. Selenium is an essential element for life but has a toxic effect at a very low level of concentration. Selenium has been widely used in industries, e.g., metal refineries, glass works, electronics industries, and others. It is volatile along with Hg and has received environmental concerns (maximum admitted value in aqueous environment: 100 ug.L"1). In natural waters and soils, Se exists in selenite (SeO32") and

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selenate (SeO42~) forms. You et al [44] recently reported a detailed adsorption study of both Se anionic forms by [Mg-Al] and [Zn-Al] LDHs. The L-type ion-exchange isotherms of both LDHs show a high affinity for SeO32': 123 cmol.kg"1 and 463 cmol.kg"1 for respectively [Mg-Al] and [Zn-Al] (Figure 3). No significant difference was observed between selenite and selenate adsorption. The amount of adsorbed species is directly related to the anion exchange capacity of the host structure and the adsorption mechanism involves an anion exchange process. For the [Zn-Al] material, nearly 97% of the a.e.c. was reached, less than 90% for the [Mg-Al] sorbents. The competing effect of the lattice anions increases in the adsorption order: HPO42" < SO42" < CO32" < NO3". Intercalation of SeO42" in LDHs leads to an increase in the basal spacing (from 0.773 nm to 0.905 nm), while no interlayer distance change was observed in the case of the SeO32" exchange. Das et al [45] have shown that the fraction of SeO32" uptake increases with a decrease in both pH and temperature. The negative value of the enthalpy AH°a N2 + O2


445

or by catalytic reaction with selective reductors such as H2 or volatil hydrocarbons: (3n+l)NO + CnH2n+2 -> (3n+l)/2N2 + nCO2 + (n+l)H2O The removal of SOx can proceed either via an oxidative process through the conversion of SO2 into metal sulfate by reaction with basic oxides: SO2 + l/2O2 -> SO3 MOX + SO3 -» MXSO4 or via a reductive pathway under hydrogen leading the formation of H2S: SO2 + 3H2 - H2S + 2H2O or MXSO4 + 3H2 -> MXO + H2S + 3H2O The state of the art on the catalytic decomposition of N2O has been described in some reviews [184]. For the catalytic removal of NOx, various catalysts have been used including supported and unsupported metals, pure and mixed oxides and zeolites such as perovskite oxides [185,186], spinel oxides [187], ZnO [188], CeO2 and Ce3O4 [189], Rh containing zeolites [190,191,191b] or supported oxides [190,191,191b]. Zeolitic systems are very active for decomposition at low temperatures but their main limitation is due to an effective poisoning by different other gases (mainly SO2) which can be present in the gas stream. Some pure oxides display a high catalytic activity in nitrogen oxide decomposition but their low specific surface areas drastically reduce their industrial applications. Since few years, many groups focus their research on the study of the catalytic activity of mixed oxides prepared from the calcination of hydrotalcite like compounds [192-194]. The ability of the LDH precursors to accomodate in the layers a great variety of divalent and trivalent metal cations, homogeneously distributed at an atomic scale level and their low thermal stability leads to the formation of mixed oxides with tunable chemical composition and high BET surface areas. NO and N2O decomposition. The [Cun-Mg"-AlIn] mixed oxides system was one of the first investigated for the catalyic decomposition of Sox [195] and Nox [196] from the exhaust gas of the FCC units. The active catalyst is prepared by reduction of the calcined [CuII-MgII-Alm] LDH (M'VAI111 = 3 and 5% molar Cu11) in propane [196]. It is composed of a dispersion of Cu° metallic nanoparticles embeded in an amorphous matrix of MgO. The decomposition of NO in air by the catalyst is maximum at 750°C but it never exceeds 80% of total conversion and it decreases with operating time while under reducing conditions (addition of propane in the gas stream) total reduction of NO is obtained. Cu(0) for NO decomposition and Cu(0) and/or Cu(I) for NO reduction with a hydrocarbon were identified as the active sites of the catalyst by an in situ XPS/XAES study [197]. The ability of these materials to redisperse the copper active sites without destroying the structure of the material makes them different from other catalysts such as zeolites, and

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is essential for suitable catalytic behavior at high temperatures. [Mg-Co] mixed oxides derived from LDH were also investigated [198]. In this case, decomposition of the double hydroxides into amorphous mixed oxides (MgO, CO3O4, Co3_xMgxO4) catalyst is performed at 350°C under N2O/He gas flow. Under catalytic conditions (N2O/He 1.5 dmV 1 gas flow, GHSV 3000 h"1), approximatively 6 moles of N2O per Kg of catalyst can be decomposed at 35O°C within lh, which is comparable to some of the most active catalysts. A higher activity is clearly attributed to the larger number of Mg-O-Co sites which is determined by the Mg/Co molar ratio in the LDH precursor. In order to control the Co3+/(M2++M3+) amount in the catalyst, [MgII-Co"-Com] LDH precursors can be prepared by in-situ generation of Co3+ from coprecipitation of Mg2+-Co2+ mixed salts [199,200]. From 25% up to 57% Co3+ molar content was introduced in the materials. The stabilizing effect of Mg2+ delays the dehydroxylation to higher temperature and increases the specific surface area of the calcined derivatives. Moreover, according to Perez-Ramirez et al [201,202] the presence of Mg in Ni and Co containing mixed oxides prevents the deactivation of the catalysts due to the preferential adsorption of nitrogen and sulfur oxides on MgO. The highest N2O conversion (6.2 mmol/g.h i.e. 70% at 400°C) is observed for the catalyst with the highest Co/(Mg+Co) ratio (0.75) and the lower Co3+/Co2+ ratio, prepared from the calcination (400°C) of the [Co2+2-Mg2+-Co3+] LDH precursor. This is explained in terms of variation in degree of inversion of the MgxCo3_xO4 spinel phase. In the decomposition reaction of NO and N2O, the desorption of O2 has been identified as the limiting step. In cases of cobalt and rhodium containing catalysts [203], the reaction mechanism of N2O decomposition involves several adsorbed states of oxygen as studied by TEOM microbalance. Energies of activation and rate coefficients were estimated for the main reaction steps of N2O decomposition at various N2O, O2, and water partial pressures by thermally treated CoLaAl-hydrotalcite catalysts [204]. Nano-size (5-7 run) supported ConAl-hydrotalcite-like catalysts on y-Al2O3 were synthesized [205] with a coprecipitation method in which the y-Al2O3 acts as a source of Al for coprecipitation and a support for the compound formation. Decomposition into the active supported spinel phase (CoIICo2.xmAlx04/Y-Al203) occurs then at much lower temperature (210°C). Kannan et al [194,206-209] studied the effect of the divalent metal and the M n /M ra ratio on the decomposition of N2O in industrial process streams simulated conditions. A [Mn-Mra] mixed oxides serie with M n = Mg, Co, Ni, Cu, Zn and M ln = Al, Fe, Cr was examined. Obalova et al [210] and Perez-Ramirez et al [201,202] recently confirmed the high catalytic performances of both [Mg-Co] and [Mg-Ni] materials. From kinetic data, the effect of the trivalent cation composition of the calcined LDH materials on activity was established in the following order: [M-Cr] < [M-Fe] < [M-Al] where M = Ni or Co. Over the [Mu-Alnl] serie the Ni and Co containing catalysts are the most active while the Mg and Zn analogues showed poor catalytic activities. Temperature required for 50% conversion of N2O increases in the series: Co-Al < Ni-Al < Cu-Al < Mg-Al. The results confirm the better activity of spinel like mixed oxides over pure divalent or trivalent metal oxides, which need higher temperature operating conditions for similar conversion percentages. The activity increases linearly with increase in the active surface metal ion concentration. However, the electronic states of the different metal cations are not assessed. In term of catalyst life-time, the best material ([Co-Al]) displays an unaffected activity after 175h time-onstream showing the viability of calcined LDHs catalysts for industrial exploitation.


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In order to improve the various properties (stability, life-time, activity) that makes the catalyst the ideal candidate for economical developement, it appears crucial to find the best chemical composition of the oxides in terms of active sites and adsorption properties. Because amongst the highest active materials for decomposition of nitrogen oxides, we found La, Ru, Rh, Pd, Pt, In supported on silicate or pure oxides, mixed oxides prepared from calcined LDHs containing non common metal cations [211-219] were tested for their catalytic activities on nitrogen oxides decomposition. Attention has been paid recently on the activity of Rh, Pd, La on supported Co containing mixed oxides [201,202,220]. According the authors, all precursors coprecipitated from different combinations of (Co, Mg, Al, Rh, La, Pd) salts crystallized in a single phase with a hydrotalcite structure. Addition of divalent and trivalent metal to the Co-Al catalyst leads to a strong improvement in the N2O decomposition. In particular, it is revealed that the less expensive Pd containing catalyst is more stable and as active as the Rh containing one provided its loading is 50 times higher than for the latter. However, interestingly, with such a high active sites content the catalyst becomes less sensitive to poisoning. Both Co-Rh,Al and Co,Pd-La,Al mixed oxide catalysts with Mg incorporated in the structure show any inhibition to SO2 and O2 in simulated FBClike flue-gas conditions at high temperature conditions because Mg-Al spinel acts as a SO2 scavenger. NO and N2O Selective catalytic Reduction. Catalytic decomposition of N2O has been studied more than the Selective Catalytic Reduction. However, the presence of other gases (O2, H2O, SO2) in realistic gas stream conditions often inhibited the catalysts activity and affect the nitrogen oxide decomposition rate [184,191]. N2O reduction by CO [221], hydrocarbons [222] and NH3 [223] have been examined. The NOx storage-reduction technique is developped for the treatment of exhaust gases [224-226]. Novel NOX storage-reduction catalysts for diesel-duty engine emissions with improved NOx storage and resistance to SO2 deactivation were prepared by impregnation of Pt or Cu,Pt on [Mg-Al] LDH precursor supports [225,226]. A dualbed catalytic system was developed by Perez-Ramirez et al [224] in which NOx and N2O are successively removed in two stages (i: NOX-N2O, ii: N2O-N2) from flue gas by selective catalytic reduction with propene over a activated carbon-supported Pt catalyst. Catalytic reduction of NO by ammonia as reducing agent on catalysts derived from LDHs were also investigated [227,228,228b]. Catalysts [227,228b] prepared from [Mg-Al] LDHs were Al3+ is partially substituted by V3+ and Mg2+ by Fe2+, showed up to 87% conversion at 65 8K in oxygen excess conditions. [Mg-Al] hydrotalcite-derived polyoxovanadate-intercalated (V10O286", V2O74") catalysts [228] display a satisfactory catalytic activity at high temperature complementary with classical Cu-Mg-Al hydrotalcite catalysts, whose performance prevailed in low temperature conditions. SOX decomposition. Sources of antropic sulfur oxide emissions are numerous, consequently appropriate technologies must be developped for each type. Indeed, if successful processes have been developped to reduce SOX effluent from thermal power stations [229] they are not relevant for emission removal by Fluid Catalytic Cracking (FCC) units because of very different operating conditions [222]. Corma et al have shown that Cu-Mg-Al Hydrotalcite based catalysts are not only efficient for SOX removal in FCC

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units [195] but also for the simultaneous removal of SOx and NOx at low oxygen concentration [222]. In the presence of propane, SO2 is removed as H2S, under a reductive pathway, leading to the formation of sulfur based Cu(I) catalytic sites active for the reduction of NO. Then, both contaminants are removed from the stream. In the case of Co-Mg-Al systems addition of an oxidant (CeO2) is necessary in order to oxidize SO2 to SO3 [230]. LDHs or calcined LDHs materials can be used to purify S oxides-containing gases. They can serve as S oxides binding material for the conversion of sulfur containing feddstaocks [231]. Vanadium cations supported by LDHs have been proved to be active species for SO2 oxidation [227]. Presence of both V3+ and V5+ was evidenced by XPS and the catalytic conversion rate was directly related to the amount of vanadium in the material. DeSOx additives based on closed materials (Mg-Al-V-Ce hydrotalcite derivatives) [232] display a high adsorption capacity at the FCC operating conditions and also good regeneration ability. 4.4 - Photocatalytic decomposition by LDH's Photochemical remediation is an alternative process to chemical degradation particularly for water purification or control of toxic air contaminants. The main advantage of the photocatalytic detoxification is that it leads to total degradation of organics into harmless inorganic residus under mild conditions. Polyoxometalates display similar electronic features than metal oxides semiconductors, generating under irradiation OH radicals, suitable for photooxidation of organic molecules. Immobilisation of W7O246~ anions in LDH host structures leads to insoluble photocatalysts materials with an homogeneous dispersion of active species over the solid and higher specific surface areas compared to pure POMs. Intercalation of POMs in LDHs is obtained by direct anion exchange reaction on expanded terephtalate containing LDH provided the pH of the solution is adjusted to the existing conditions of the oxometalate in solution [233,234]. Guo et al demonstrated the photocatalytic degradation of aqueous organochlorine pesticides (hexachlorocyclohexane or HCH) by Polyoxometalates (POM) intercalated [Mg2-Al-W7O24] LDH (Guo, 2001) and calcined POM containing LDH's ([Zn2-Al-SiWn039] and [Zrij-Al-SiWuOjsMnÔ)]) [235]. Complete mineralization of HCH was proved by the recovery of nearly 98% of chlorine. The mechanism of the organic molecule photooxidation involves an intramolecular O(2p)-W(5d) charge transfer, the formation of the interlayered excited state [W7O246"]* species and the generation of the strong and unselective photoactive OH radicals. The photochemical degradation is assisted by the ability of the solid catalyst to adsorb the reactants (organics and O2) and the photon. Calcination (600-700°C) of [Zn2-AlSiW u O 39 ] and [Zn2-Al-SiWi,O39Mn(H2O)]) [235] POM -LDHs systems into mixed oxides leads to an improvement of the photocatalytic activity because of the presence of photoactive ZnAl2O4 and ZnWO4 spinel compounds with lower band gaps compared to the pure products. Surface and porosity properties do not play a major role in the degradation efficiency. In a more reasonable approach, the transformation of organics in less toxic residues by heterogeneous photocatalysts can be preferred. Oxidation by soft and selective agents can be a first step through the transformation hi more valuable molecules. F. Van Laar et al [236] have evidenced the photocatalyic activity of [Mg-AlMoO4] LDHs for the peroxidation of methylcyclohexene, dimethylbutene and


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LAYERED DOUBLE HYDROXIDE / POLYMER NANOCOMPOSITES FABRICE LEROUX* and JEAN-PIERRE BESSE Laboratoire des Materiaux Inorganiques, UMR 6002-CNRS, Universite Blaise Pascal, 24 av. des Landais, 63177 Aubiere cedex, FRANCE. * E-mail: [email protected]


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I - Introduction Commonly defined as nanocomposite, the system may be described as inorganic sheets lying on top of each other in which covalent forces maintain the chemical integrity and present an interlamellar gap filled up with the polymer guest, thus giving rise to a sandwich-like structure, but it is also extended to the situation where the inorganic filler (LDH) is dispersed into polymeric matrix. The incorporation of polymer between the galleries proceeds via different pathways such as coprecipitation, exchange, in-situ polymerization, surfactant mediated incorporation, hydrothermal treatment, reconstruction or restacking. The latter method, recently effective via the exfoliation of the LDHs layers, appears to be more favourable - in terms of crystallinity - to capture monomer entities than the whole polymer. The in-situ radical polymerization recently developed for these nanocomposites present potentially the advantage to tune the tacticity and the molecular weight of the generated polymer by playing on the layer charge density and the particle size of the host structure, respectively. This process can be defined as an endotactic reaction like those encountered for other relevant assemblies. Indeed, a large variety of LDH/polymer systems may be tailored considering the highly tunable intralayer LDH composition coupled to the choice of the organic moiety. Bio-related polymers and large bio-macromolecules have been incorporated within the galleries of LDH materials, such as poly(aspartate), alginate, deoxyribonucleic acid (DNA), an obvious interest for these bio-organoceramics is the drug release aspect, but also from a fondamental point of view a better understanding of the biomechanisms and other biomimetism phenomena. The purpose of this chapter is to present the state of the art and to identify new trends in term of applications for the materials composed of the assembly between Layered Double hydroxides (LDH) and Polymers. Many reviews and chapters of books devoted to various aspects of claynanocomposites including some of them include LDH materials, may be found in the literature [1-3]. The scope of the present chapter is first to update the LDH/polymer research domain by providing as much informations as needed concerning the synthesis and characterization in addition to this academic point of view to point out the potential applications of those systems. This chapter devoted to Layered Double Hydroxide / Polymer Nanocomposites is structured as follows: - a description of LDH materials including the natural occurence, chemical composition and the aspect of the stacking sequence is briefly discussed. In addition, directly related to the building of inorganic-organic assemblies, the layer charge density and the colloidal and exfoliation properties are presented. - a second section is devoted to the synthetic pathways for the LDH / polymer assembly. Taking into account the anion affinity and stability of LDH, special cares are needed to avoid the breakdown of the host structure and also to provide the best matching between monomeric repetition and layer charge density. It is illustrated by the example of the inorganic polymer poly(silicate) LDH nanocomposite. The methods for the assembly are listed and a non-exhaustive list of compounds is supplied. - the methods of characterization for these two-components materials, i.e. the host structure, and the state of polymer, as well as the nature of the interaction, are gathered in a next section. We will see that the simple model based on a hydrophobic/hydrophilic bilayer needs sometimes to be reformulated taking into account the tacticity of the formed polymer and the free space available between the layers of the host structure. The textural properties and behaviour in temperature will also be presented.

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- a fourth section summarizes the most obvious domains of potential applications for the LDH polymer systems, namely the sensors and electrochemical devices, the preparation of carbonaceous replicla presenting high surface area with a controlled porosity, the hydrocalumite-type materials related to the MDF cements and finally the application of LDH as nanofillers dispersed in a polymeric matrix. - in the fifth section bio and bio-inspired LDH nanocomposites, the state of the art, the characterizations and processings are presented. - a final section is devoted to the perspectives and future developments. 2 - Brief description of LDH material 2.1 - Natural occurancy and chemical composition Natural hydrotalcite of composition Mg6Al2(OH)16(CO3) 4H2O was first identified in Sweden in the year 1842. Other minerals presenting different chemical natures and stacking sequence belong to this large family, such as manasseite, Mg3Al(OH)8 (CO 3 ) 05 2H2O (2H), meixnerite, Mg3Al(OH)8(CO3)0 5 2H2O (3R), sjogrenite Mg3Fe(OH)8(CO3)0,j 2,25H2O (2H), stichtite, Mg3Cr(OH)8(CO3)0,5 2H2O (3R), takovite Ni3Al(OH)8(CO3)0,5 2H2O (3R), pyroaurite, Mg3Fe(OH)g(CO3)0,5 2,25H2O (3R), hydrocalumite, Ca2Al(OH)6[(C032")on(OH)o78 2.38H2O (3R), wermlandite, Mg(Al, Fe)0,5SO4 2H2O (2H), [Fen4Fenl2(OH)12]2+.[SO4. nH2O]2", an iron hydroxysulphate commonly called green rust, etc. Thanks to synthetic procedures, the family has been largely extended, and various cations at different oxidation states were also stabilized giving rise to unusual compositions. It is the case for cations such as V(III) [4], Mn(II) [5], Co(III)Co(II) [6] or those incorporating La, Y [7]. 2.2 - Layer charge density The anionic exchange capacity (A.E.C.) is reported for some LDH compositions in Figure 1.

Figure 1 - Variation ofanion exchange capacity (meq/lOOg) as a function of the amount of trivalent cation reported per formula weight. The data for [Mgt.xGaJ samples are taken from ref. 11.

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The values range between 450 to 200 meq/lOOg, lower values are not possible. The ratio M(III) to M(II) would be too low to maintain the LDH structure. For comparison, cationic clays present exchange capacity in the vicinity of 100 meq/lOOg associated with an area per charge of 70 A2/charge, whereas it is ranging between 25 and 40 A2/charge for LDH materials with compositions M(II)xMn(III) usually in the domain 2 < x < 4. This pictures layers tightly stacked via the attractive forces with the interlayer anions filling the gallery. Vacancies in the inter-sheets domain are not great taking into account the large packing of anions balancing the layers charge. This situation is unfavourable for either an ion-exchange reaction or an exfoliation process. The smaller the exchange capacity {i.e. the layer charge density), the easier the formation of nanocomposite is. The LDH materials do not present the property to be readily exfoliated such as smectite or MS2 type-chalcogenides (MoS2, NbSe2, etc.) [89], the delamination of LDH sheets requires elaborate synthesis. The difference in the charge density may be exemplified by amino acids using either Zn2Al hydrotalcite and montmorillonite as the guest molecules, L-tyrosine and Lphenylalanine, protonated or deprotonated may be incorporated as pillars in cation or anion exchanger layered compounds [10]. When using montmorillonite as host, the basal spacing is largely increased whereas the amplitude for the hydrotalcite is much smaller which shows the difference in the swelling properties between the two clays. 2.3 - Colloidal and exfoliation properties Small particle size and low layer charge density are important features for giving colloidal and/or exfoliated systems and therefore beneficial for subsequent polymer / host structure assembly [12]. This statement may explain the relatively small number of nanocomposites reported in the literature, added to the fact that each LDH composition leads to an unique material the properties of which (exchange, reconstruction or exfoliation) are not easily transferred from one to another. Beside to the layer charge density, the diffusion of cumbersome molecule such as polymer may be a major impediment, therefore the particle size needs to be minimized if one considers the incorporation via direct exchange. To overcome diffusion problems, two main options may be depicted : either the control of the particles size during the preparation or the delamination of the host framework once formed. A method involving separate nucleation and aging steps was reported [13]. It is based on the simultaneous addition of the reactants in a colloidal mill in which the forces prevent aggregation of the nucleated particles. In contrast to conventional preparation methods giving rise to a wide dispersion in the crystallite size, this technique affords smaller crystallites associated with a very narrow distribution of crystallite size, suitable for further anion exchange reaction. The effect of synthetic conditions on tailoring the size of hydrotalcite particles was studied by Oh et al [14]. Homogeneous precipitation of uniform hydrotalcite particles are reported utilizing urea hydrolysis [15]. Small particle size may be achieved by stabilization of emulsions by heterocoagulation of clay minerals and LDH using paraffin/water emulsions stabilized by colloidal particles without surfactants [16]. LHD material was found to be stabilized by forming envelopes round the oil droplets, and the addition of bentonites creates a three-dimensional network of particles with high elasticity which impedes coalescence of the oil droplets. Lagaly et al have studied the properties of colloidal magnesium aluminum hydroxides [17].


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Other colloidal suspensions of LDH may be obtained after hydrolysis of methoxide-intercalate LDH, and are suitable to form transparent films [18]. One can also change the interlayer chemical nature, which may become more suitable for subsequent reactions with organic species. Hydrophobic LDH, prepared from hydrocalumite and anionic surfactants, adsorb n-heptane, benzene, toluene, and npropanol between the layers with considerable increase of the basal spacing [19]. Large molecules may be incorporated using a mixture of surfactants S+-S" during the exchange reaction [20]. Even if aqueous montmorillonite dispersions were the first platelet-like colloidal systems that showed nematic ordering [21], stable nematic phase of LDH was recently evidenced [22], making LDH a closer mirror image of smectites. The second option consists of delamination of LDH layers. Owing to the high layer charge, the LDH material does not present a natural tendency to exfoliate (see above), and hence many publications are found in this area. Nevertheless, few attempts have been successful with three distinguished routes to date: - delamination of Mg2Al first intercalated with dodecylsulfate anions (DDS) is achieved in polar acrylate monomer under high shear [23]. Suspensions are stable and the polymerization of 2-hydroxyethylmethacrylate (HEMA) gives poly(acrylate) with the inorganic component still in the delaminated form. - a combined use of alcohols and Zn2Al / DDS was used by Adachi-Pagano et al to achieve exfoliation [24]. As stipulated by Hibino and Jones [25], this process does not result from a strong driving force by solvent inclusion, but rather by the choice of the boiling point of the solvent. Instead of a solvent presenting high dielectric constant, known to facilitate delamination, Adachi-Pagano et al had preferred the use of reflux condition at temperature higher than water boiling point. An increase of the alkyl chain of alcohol increase the boiling point but decrease the dielectric constant. The drying process of the surfactant-modified precursor is of great importance to obtain a complete translucent colloidal solution [26]. - by the use of various combinations of amino acid anions and polar solvent, exfoliation of LDH sheets may be achieved [25]. Optimum results habe been obtained using glycine / formamide. A clear colloidal suspension with no original crystalline structure is obtained. As much as 3.5 g.L"1 of LDH can be delaminated by this way. As evidenced by Leroux et al [26], an optimum hydration state and/or water molecule / solvent displacement rate exists for successful delamination All these pathways open new promising possibilities for the incorporation of molecules as cumbersome as polymers and also for the dispersion of LDH materials. It is recently exemplified by Li et al [27]. Using the method developed by Hibino and Jones, they were able to entrap poly(vinyl alcohol) between the layers of Mg3Al. The ultrasound is found to improve slightly the crystallinity of the nanocomposite, although, a large contribution of PVA casting the crystallites cannot be avoided. Alternatively, Bubniak et al use a surfactant-modified LDH to disperse in PEG matrix [28]. 3 - Synthetic pathways for the assembly According to the type of interaction between the inorganic and organic moieties, hybrid materials may be classified in two distinct classes of materials [29]: the first concerns those for which the interactions are weak (van der Waals, electrostatic, or hydrogen bonding), the second concerns strong interactions (covalent bonding). In absence of grafting, the nanocomposite polymer / LDH may be regarded as belonging to first category.

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3.1 - Special cares In the case of conjugated polymer, when the host structure presents a strong oxidizing capacity, such as xerogel V2O5, the in-situ polymerization is concomitant with the incorporation of the monomer. Therefore the process is called reductive intercalative polymerization (RIP) [30]. Concerning LDH materials, RIP process is irrelevant, other strategies have to be undertaken to induce the polymerization. Generally, conjugated polymers are polymerized in very acidic medium and with the use of dopant such as FeCl3 or NH4S2O8. Unfortunately, these conditions cannot be employed with LDH framework as low pH medium will dissolve the inorganic structure and anions presenting great affinity toward LDH such as chloride and disulfate anions will displace the monomer. 3.2 - The methods In this section, we consider the case of polymer LDH systems, composed of sheets lying on top of each other and in which covalent forces are maintaining the chemical integrity, whereas weak interlayer interactions are present between lamellae. To be completely sandwiched, the polymeric moiety has to diffuse between the inorganic layers. There are several possible strategies to incorporate the polymer at the core of the host material as underlined by Schollhorn in a review paper [31], who considers three principal options: (a) intercalation of the monomer molecules and subsequent in-situ polymerization, (b) direct incorporation of extended polymer chains in the host lattice via exchange reaction in the case of small molecular weight or via coprecipitation method, (c) transformation of the host material into a colloid system and its subsequent restacking in presence of the polymer. An additional route consists to take advantage of the memory effect of some LDH materials and to reconstruct the lamellar framework on the polymer. Another alternative is to use the guest displacement or solvent-assisted method to compatibilize the host structure galleries to the guest molecule. These different pathways are displayed in Figure 2. The in-situ polymerization (a) is generally a highly suitable method and is employed for the incorporation of various monomers between 2D host structure. Yet, the process is limited by two factors [2]: - the distance between monomers when they are strongly anchored (or grafted) to the host matrix, ;. e. the degree of freedom, which must somehow be in agreement with the layer charge, - the condition that the polymerization (temperature, pH or redox reaction) must leave the layered structure intact. Concerning LDH host structure, different monomers can be polymerized insitu. Acrylate / Mg2Al LDH hybrid material, obtained via exchange reaction with the interlayered anions (Cl~ or NO32"), is further polymerized after a thermal treatment at 80°C. The basal spacing was found to slightly decrease to 13.4 A from 13.8 A [32]. The IR spectroscopy provides information on the polymerization with the disappearance of the C=C vibration band. It is noteworthy that carbonate LDH phase does not react with acrylate. Acrylic acid was also intercalated in the lamellar structure of an ironsubstituted nickel LDH material [33-34], In this study, potassium persulphate is used as an initiator for the polymerization process. The resulting phase was although partially exchanged by SO42" anions. Insertion of polymer leads to a phase presenting a basal spacing of 12.6 A thinner than that of acrylate intercalated monomer phase (13.6 A). This was explained by the absence of electrostatic repulsion between the C=C double bonds. Recently, Vaysse et al report the in-situ polymerization of acrylate in iron-, cobalt-, or manganese-substituted nickel hydroxydes [33]. The preparation of the


465

inorganic framework either via solid state reaction or coprecipitation depending on the nature of the trivalent cations influencing strongly the arrangement of the macromolecule. For Co and Mn-containing LDH, the intercalation and polymerization process appear to proceed concomitantly. Although, the presence of isotactic poly(acrylate) and a basal spacing for the nanocomposite of 7.8 A seem to be questionable taking into account the layer charge density of the host structure and the free-space available for the polymer chain, respectively. Styrene sulfonate was also polymerized between Zn2Al LDH sheets [35], giving rise to well-defined nanocomposites. It was found that when the layers charge density is decreased, i.e. Zri2Al -> Zn4Al, the polymerization is not completed, this will be further discussed in section 4. 3.

Figure 2 - Scheme of the preparation of LDH / polymer nanocomposites: (a) in-situ polymerization, (b) polymer direct incorporation, (c) restacking or reconstruction, and (d) guest displacement or solvent-assisted method.

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The direct polymer incorporation by coprecipitation or exchange reaction (b) may also be achieved. During the inorganic crystal growth, it is possible to form nanocomposites with polymers presenting an anionic function such as sulfonate groups. From the polymer point of view, it corresponds to an entrapment reaction, which from the inorganic seeds, it may be considered as a self-assembly assisted process. This gives rise generally to ill-defined materials such as those including poly(acrylic acid), poly(vinylsulfonate) and poly(styrenesulfonate) within layered double hydroxides [3637]. The crystallinity may be partially cured by hydrothermal synthesis as exemplified by PSS / Zn2Al nanocomposite [36]. It was found in this case that the coherence length along the stacking direction is largely increased but also that the inorganic sheets are less corrugated. Generally, the polymer is found not only to influence strongly the textural properties (see section 4.6) but also the intralayer composition of the inorganic structure. The oxide and salt method first developed by Boehm et al [38] and commonly used for the preparation of M2Cr (M = Cu and Zn) was reported to immobilize polymer by the formation of Zn2Cr layers, although, no further characterization were provided [36]. Thus for organo-modified smectite, when the chemical nature between the interlayer space and the guest is not compatible, often due to hydrophibicity, the guest displacement and/or solvent-assisted methods (d) are used. For instance, the presence of hydrophobic alkyl chain in some case makes it possible the incorporation of non functionnalized polymer like what was reported for poly(paraphenylene) (PPP) into MoO3 host structure [39], poly(ethylene oxide) into MnPS3 chalcogenide [40], poly(palanine) polycondensation and poly(vinylpyrrolidone) into ammonium acetate modified-kaolinite [41-42]. Concerning LDH material, previous studies had shown that a pre-intercalation may prepare the interlayer LDH galleries for subsequent incorporation, as evidenced by the early work of Drezdon with the pre-intercalation of terephthlate anions [43] or the swelling with glycerol for the incorporation of poly(oxometallates) (POM) such as H2W12O406" by Dimotakis and Pinnavaia [44]. Dodecylsulfate (DDS) LDH precursor was used for the incorporation of C60 without functionalizing the fullerene molecule [45]. C6o spherical molecules were introduced by dissolving the molecule into the interlayer hydrophobic phase. The solvent plays also an important role in the swelling process. Similarly, the same precursor was used for the direct incorporation of poly(ethylene oxide) [28], the organomodified LDH presents a basal-plane repeat distance of 26.2 A, which suggests a highly interdigitated situation of the alkyl chains. When incorporated PEO, the d-spacing is increased up to 38.2 A. The nanocomposite was characterized by XPS and FTIR spectroscopies. Immobilization of a guest molecule can be achieved by reconstruction of the layered framework by the so-called memory effect. The reconstruction method (c) was successfully employed by Yun et al for the preparation of silicate-intercalated LDH [46]. Meixnerite-type material of composition [MgxAl-OH] (x = 2,3,4) were calcined at 500°C under air and used as precursor for the incorporation of silicate. This was performed using tetraethylorthosilicate, Si(OC2H5)4 (TEOS). This procedure gives rise to samples better crystallized than those formed by ion-exchange [47] or direct coprecipitation methods from metasilicate source and Zn2M (M = Al, Cr) as LDH precursor [48]. An interlayer short chain silicate structure was evidenced by 29Si NMR experiments (section 4.3). Restacking of the layers on a polymer (PSS, Mw of 70000 g/mol) is reported by Leroux and Besse [2]. Unfortunately, the process gives rise to illdefined materials, and appears to be more suitable for the incorporation of smaller molecules.


467

Let us now discuss the case of conjugated polymer. When using the RIP process, from the point of view of the host structure, the final assembly may be considered like a polymer bronze, from the side of the polymer it is viewed as a dopant. Unlike the xerogel V2O5 [49], LDH materials cannot induce instantly the polymerization of poly(aniline), therefore the notion of dopant or polymer bronze is not well adapted in this case. Insertion of conjugated polymers into LDH framework was first reported by Challier and Slade [50]. Terephthalate and hexacyanoferrate exchanged Cu2Cr LDH phases are used as host matrices for the oxidative polymerization of aniline. The reaction performed under reflux condition gives rise to a rather poorly defined material with a basal spacing of « 13.5 A as well as by-products. An alternative method consists in incorporating a soluble anionic monomer such as aniline-2-sulfonate or metanilic acid (3-amino benzene sulfonic acid - H2NC6H4SO3H). Polymerization of the monomer requires less drastic conditions than that needed for aniline, giving rise to a relatively well-ordered system [51]. Indeed, the electrophilic function decreases the potential of polymerization [52-53] and the sulfonic acid ring-substituted polyaniline (PANIS) is capable of self-doping [54-56]. This is suitable since any doping using an external oxidizing agent induces preferentially an exchange with the counter-anions. The conductivity of PANIS is independent of the external protonation over a broad pH range, although, the presence of the sulfonate groups decreases the conductivity of the polymer in its conductive state [57]. It was found that the atmospheric oxygen is necessary for the ignition of the in-situ polymerization [51]. A recent work has shown that the location of the amino group, the length of the alkylsulfonate functional group or the presence of electron withdrawing group on the benzene ring, orientate strongly the rate of polymerization [58]. Polymerization of aniline carboxylic acid into LiAl2 LDH material was also reported, although, the reaction is not complete [59]. The photoinduced isomerization and polymerization of (Z, Z)-muconate anion in the gallery space of [LiAl2(OH)6]+ layers was recently reported [60]. Initially vertically orientated, the anions are polymerized in aqueous medium, while isomerization into more stable (E, E)-muconate only takes place in methanol suspension. The photoisomerization of indolinespirobenzopyran in anionic clay matrices of layered double hydroxides was also reported by Tagaya et a/ [61]. 3.3 - A non exhaustive list A growing number of LDH/polymer systems is reported in the literature. Some of them are displayed in Table 1. The preparation is indicated according to the classification (Fig. 2). 4 - Characterization of the two-components materials A combination of techniques is necessary to characterize these multicomponents assemblies, such as Infrared Fourier transform (FTIR), solid state nuclear magnetic resonance (NMR), Raman, electron spin resonance (ESR) and X-ray absorption (XAS) spectroscopies (X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS)) at a local scale, X-ray diffraction for the long range order, adsorption measurements, electronic microscopies (by electron scanning (SEM), electron transmission (TEM) or by atomic force (AFM)), and gas adsorption for the textural properties. 4.1 - The host structure The incorporation of polymer alters the order at different scale, from the atomic order to the microstructure. The breadth of the hydroxyl stretch region may be

468


explained by the presence of hydrogen bonded hydroxyl groups with organic moiety [65]. Generally, the incorporation of polymer caused the density to decrease, typically from 2 to 1.5 g/cm3. Table 1 - Polymer/LDH nanocomposites classification (see text).

Polymer /LDH

Synthetic pathiway

References

[50] Cu2Al / PANI (d) Cu2Cr /PANI (a) [51] [59] LiAl2 (a) [62] Ca2Al / PVA (b) [37] Mg3Al / PSS (b) (a) (a, b) Zn2Al / PSS [37], [51] MAI / PA, PVS (M = Mg, Co, Zn) (b) [63], [36] CaAl / PA, PVS, PSS [36] (b) NiFe / PA [32], [34] (a) M2Ni / PA (M = Mn, Fe, Co) [33] (a) (a, b, c) Zn2Al / PSS [2] [64] (b,d) M O 7 PEG-(DC and AS) (M =Cu, Zn) Polymer: (PANI) poly (aniline), (PVA) poly (vinyl) alcohol, (PSS) poly(styrene sulfonate), (PVS) poly (vinyl sulfonate), (PA) poly(acrylic acid), (PEG-DC) poly(ethylene glycol) dicarboxylic, (PEG - AS) poly(ethylene glycol) alkyl (3-sulfopropyldiether). Pathway: (a) in-situ polymerization, (b) polymer direct incorporation, (c) restocking or reconstruction, and (d) guest displacement method.

Figure 3 - X-ray diffraction patterns of two pristine materials (a), hydrotalcite-type Zn2Al and hydrocalumite, Ca2Al, and of their 4-styrene sulfonate (VBS) derivatives (b). The Miller indexing is given, Zn2Al and Ca2Al structures are described in R-3m and R-3 space group, respectively.


469

The presence of large number of harmonics [resp. small] on XRD diagram is indicative of a long-range [resp. short] ordering in the stacking sequence for the framework. From the diffraction line width and using Scherrer relationship, an average coherence length may be estimated, although one has to pay attention to the validity of the relation when the diffraction lines are ill-defined. A general trend may be drawn from the different works gathered in the literature. The host structure Ca2Al gives rise to more ordered structures then that for other LDH counterparts, independently of the polymer nature. It is generally attributed to the ordering of hydrocalumite phase (see next section). The incorporation of monomer molecule (or polymer) is proping apart the layered structure as evidenced by the shift of the harmonics in Figure 3. 4.2 - An endotactic reaction The host structure may accomodate very well a guest molecule, when the monomeric repetitions are close to specific distance found in the galleries. In this case, the incorporation may be defined as an endotactic reaction. In the first approach, one may take into account the layer charge density of the host and the projected surface area of the guest molecule. It is reminiscent of poly(pyrrole) present between the layers of host structures, the compatibility in distance is given by the comparison between 3 (NH) functions (8 A) and V 2 O 5 xerogel host in the crystallographic direction [1-30], or in the direction [201] for FeOCl [49]. Polyaniline may be accomodated into FeOCl galleries by the creation of H Cl stabilizing hydrogen bond taking into account the matching between the distances 10.2 A for (PANI) and 10.4 A for FeOCl on [101] [66].

Figure 4 - Ideal local order for a cation composition M(II) /M(III) of (a) 2:1, and (b) 3:1. The correlations between cations are indicated by P2 -} P6 (see text). Since only the average structure of LDH is known, except in few cases such as hydrocalumite and LiAl2, it is important to consider the local structure. According to an

470


ideal model based on edge-sharing octahedra [67], it is possible to define the local environment around each type of cations according to the layer charge density. Fig. 4 displays the local cation environment for M(II)XM(III), x= 2 and 3. The correlation between cations are as follows: a first correlation Me - Me at a distance of a noted as P2, then P3, P4, P5 and P6 at «n/3, 2a, 873 K, thus indicating that the reaction was not thermodynamically limited. By using EXAFS spectroscopy and combined EXAFS/XRD under the operating conditions, the active species were identified as Cu1 for NO decomposition and Cu° for its reduction. The reduction of NO with propane in the presence of different contents of O2 has also been investigated [219,220], observing complete NO conversion at T > 723 K and for O2-contents lower than the stoichiometric amount required for the complete oxidation of propane. The catalytic behaviour depended significantly on the catalyst composition, with better performances for the Cu/Cr mixed oxides obtained by calcination for 3 h at 873 K [219] in comparison with those obtained with a decavanadate-exchanged Mg/Al (2:1) LDH calcined at 823 K for 3 h [220]. Between the Cu/Cr mixed oxides, the best performances were observed for an intermediate Cu/Cr ratio (2:1), which also withstood higher O2-contents in the feed. Operating at 80% of the stoichiometric O2-content, this catalyst showed 100% conversion of NO and 75% conversion of propane at 723 K, and an almost negligible formation of N2O above 623 K. Some of these authors [221 ] have also recently claimed the use of multicomponent mixed oxides in the total oxidation of N-containing organic vapours, using dimethylformamide (DMF) as a probe molecule, this being widely used as an industrial solvent. The catalytic performances depended on the catalyst composition and calcination temperature: for example, the best results were observed for Cu/Zn/Cr/Al/V or Cu/Cr/V LDHs calcined for 3 h at 673 K or 823 K, respectively. Calcined LDHs have been widely investigated in the decomposition of N2O, a greenhouse pollutant 270 times approximately more powerful than CO2 (on a weight basis), which, furthermore, contributes to catalytic destruction of stratospheric ozone. The continuous increase of N2O in the atmosphere is mainly caused by anthropogenic activities, such as cultivated soil, biomass burning, stationary and mobile combustion, and chemical processes (for example adipic acid synthesis). However, in the short term, only the emissions associated with combustion and chemical processes can be reduced, with a reasonable goal of achieving about 60-90% reduction by 2010 [222]. Kannan and Swamy [223,224] claimed calcined Co/Al, Cu/Al or Ni/Al LDHs as effective catalysts in the decomposition of N2O, although using a recirculating static reactor and a low partial pressure of N2O (6.7 kPa), i.e. with experimental conditions very far from those of practical interest. In fact, using a flow reactor and feeding a 1% v/v N2O/He gas mixture, both Cu/Al and Ni/Al mixed oxides showed very low activity with appreciable conversion only above 773 K [222]. Even worse performances were observed for calcined LDHs containing both elements, while a relatively higher activity was obtained by CuO supported on a Mg/Al mixed oxide prepared by calcining a LDH precursor. A significant increase in activity was observed when the Ni/Al sample was prereduced at 773 K for 30 min in a 20% H2/He flow and then reoxidized with N2O before the catalytic tests. The best performances were obtained using a Mg/Al mixed oxide containing a small amount of Rh, for which the change from 10 to 90% of N2O conversion occurred in a temperature range of 100 K ca. [222]. The role of the composition of the precursors has been investigated as a function of the reaction conditions and feed composition (mainly presence of O2 and/or H2O), with the aim to optimize these materials in a simulated process stream (Table 6) [27,225,226].

520

S. Albertazzi, F. Basile and A. Vaccari

Table 6 - Catalytic activity in N2O decomposition of different mixed oxides obtained by calcination of HT precursors [27,225,226]. N 2 O conversion (%) Atomic ratio 0.5, gave rise to active and selective catalysts for the synthesis of C1-C6 alcohols, with high purity of the alcohol phase, since the presence of Co lowered the formation of esters, ketones and aldehydes, being C r C 6 hydrocarbons the main by-products obtained. It must be noted that for compositions typical of LDHs [i.e. M(I1)/M(II1) atomic ratios ranging from 2.0 and 3.0] CH3OH was the main product, while the HMW alcohol synthesis required an at. ratio ranging between 0.5 and 1.0, thus suggesting the formation also of an amorphous phase containing part of the Cr3+ ions. An interesting review of the most recent trends and developments in the science and technology of

530


catalyzed hydrogenation of carbon oxides (CO and CO2) has been published recently [299].

Figure 24 - Main products obtained as a function of the composition with the Cu/Co/Cr catalysts [298]. Among the most recent applications of CH3OH, the production of H2-rich gaseous fuels have been investigated for spark ignition engines and coupled-fuel-cells electric engines [300,301]. This latter application requires a CO-free H2 stream, since CO irreversibly poisons the Ru/Pt electrocatalyst in polymer electrolyte CH3OH fuel cells and dramatically diminishes their performances. H2 can be obtained from CH3OH via three different processes: a) decomposition (Eq. 5); b) steam reforming (SR) (Eq. 6); and c) partial oxidation (PO) (Eq. 7): CH3OH ^ /2H 2 + C0 CH3OH H H2O A/3 H2 + CH3OH H- V2 0 2

A H 298 =

co2

n2H 2 + co 2

- 92 kJ/mol = 49 kJ/mol 298 = -192kJ/mol

A H 298 AH

(Eq. 5) (Eq. 6) (Eq. 7)

To date, the SR reaction of CH3OH has been the only process used for H2 production for fuel cell applications [302],This reaction, however, produces a considerable amount of CO as a by-product, which may be removed in a second-stage catalytic reactor via either the water gas shift reaction (Eq. 4) or even CO oxidation [303]. For fuel cell technology, the PO reaction of CH3OH is advantageous in comparison with SR reaction, because it uses air instead of steam and does not require any external heat. LDHs have been claimed for the preparation of catalysts active in all the above processes [304-311]. For example, Pd and Rh-based catalysts obtained by calcination of LDHs for 3 h at 723 or 923K and subsequent reduction for 1 h at 673 K have been investigated in the decomposition of CH3OH, in order to decrease the temperature of H2 production. Higher metal dispersion and activity was obtained with a Pd/Mg/Cr catalyst, significantly better than those of analogous impregnated catalysts (Fig. 25) [304].

Catalytic Properties of Hydrotalcite-Type Anionic Clays

531

Figure 25 - Arrhenius plot of the methanol decomposition rate over various Pd catalysts reduced at 673 K: (v) 3.0 wt.% Pd impregnated on MgO; (») 2.9:2.9 wt.% Pd/Cr impregnated on MgO; (n) 3.0 wt. % Pd impregnated on Mg/Cr LDH calcined for 3 h at 698 K; (A) 3.8 wt. % Pd/Mg/Cr LDH calcined for 3 h at 698 K [304]. On the other hand, a novel catalyst with high activity and selectivity to syngas, due to the large surface area and Pd dispersion, was prepared from a composite obtained by synthesizing the LDH precursor in a suspension of hexagonal mesopouros silica (HMS) [305]. Finally, in situ production of H2 by catalytic decomposition of different alcohols in a 10% (v/v) mixture with simulated gasoline has been recently reported using a Cu/Al (72:28 as at. ratio) catalyst, derived from the corresponding LDH during the reaction [306]. The onset of H2 formation generally occurred at 473-503 K, and was related to the formation of metallic Cu during in situ modification of the initial LDH. In alcohol/fuel mixtures, dehydrogenation of the alcohols appeared to be the major mechanism, with significant irreversible catalyst deactivation above 623-633 K. Cu/Zn/Cr and/or Cu/Zn/Al catalysts, obtained either from pure LDHs or mixtures of LDH and hydroxycarbonate phases, have been widely employed in the SR and PO reactions of CH3OH [301,307,308]. A Cu/Zn/Cr LDH calcined at 573 K showed a high activity in the SR reaction at 523 K, with negligible deactivation with time-onstream and, furthermore, optimum activity and stability also in the tests carried out at high pressure [307]. Cu/Zn/Al catalysts - derived from precursors containing mainly a LDH - exhibited good catalytic activity in the PO reaction, with at 573 K a CH3OH conversion of 40-60% and high selectivity in H2 (>90%) and CO2 (>95%) [308], although the possible presence of hot spot phenomena should be considered, giving rise to higher temperatures on the catalyst surface. The catalytic activity decreased with increasing (Cu+Zn)/Al atomic ratio in the precursor, while increasing amounts of hydroxycarbonates favoured the formation of considerable amounts of dimethyl ether as a by-product. The low temperature (423-673 K) SR reaction of CH3OH to produce H2 has been investigated also over Mg/Al, Cu/Al, Co/Al and Ni/Al catalysts formed in situ during the reaction from the corresponding LDHs [309]. The reducibility of the divalent cations present in the LDH was a crucial parameter in determining the SR activity of the catalysts. The Cu/Al catalyst was the most efficient, becoming active at 503 K ca., while Ni/Al and Co/Al catalysts required significantly higher (588-593 K) activation temperatures and Mg/Al was fully inactive. Furthermore, pre-activation of the Cu/Al

532


LDH by calcination for 4 h at 673 K, followed by reduction for 1 h at 673 K in dilute H2 did not significantly change the catalytic activity (Fig. 26).

Figure 26 - Steam-reforming activity as a function of the temperature for the pretreatedCu/Al (3.1:1.0 as at. ratio) LDH-derived catalyst [309] A way of significantly reducing the hot spot phenomena is to combine the SR and PO processes by simultaneously cofeeding O2, steam and CH3OH to obtain oxidative CH3OH reforming (OSR). Furthermore, the ratio of three reactants can be chosen in order to supply the heat necessary to maintain the steam reforming reaction by the partial oxidation reaction, with an overall reaction heat near to zero [300,301]. Velu et al [310,311] have recently reported LDH-derived Cu/Zn/Al and/or Cu/Zn/Al(Zr) catalysts very active in OSR of CH 3 0H, producing CO-free H2 suitable for fuel cells, with up to 100% CH3OH conversion after 25 h of on-stream operation at 503 K ca. During the PO reaction, levels of CO above 3% are usually obtained, but the temperature and excess H2O present in the OSR process gives favourable conditions to transform CO into CO2 by the water gas shift reaction (Eq. 4) [301]. Among the Cu/Znbased catalysts, those containing Zr were the most active, with optimum O2/CH3OH and H2O/CH3OH molar ratios in the ranges 0.2-0.3 and 1.3-1.6, respectively [311]. Figure 27 compares the rates of H2 production and outlet CO concentrations in the SR and OSR reactions, showing that the latter reaction was more efficient since the rate of H2 production increased by six fold, while the outlet CO level only by a factor of two. Catalysts always obtained from LDHs (containing Ni2+, Zn2+ and/or Cu2+) have also been claimed for the steam reforming of dimethyl ether [312] or other oxygenated compounds (ethers, alcohols, C2-C4 aldehydes or ketones, although examples have been reported only for methanol) [313,314] to produce H2, together with other products (for example acetic acid), without the use of excess steam (T= 473-573 K; P = 0.5-2.5 MPa; GHSV= 2,000-3,000 h"1). Finally, Cu/Zn/Cr catalysts obtained from LDHs were investigated in the use of CH3OH for H2 storage and as a H2 carrier, showing high stability in CH3OH dehydrogenation to methyl formate. In these catalysts the formation of spinel-type mixed oxides avoided the severe deactivation observed with the Cu/ZnO based catalysts, due to the reduction of ZnO in the vicinity of Cu [315].


533

Figure 27 - Comparison of the efficiency of the CH3OH steam-reforming (SR) and oxidative-steam-reforming (OSR) reactions, operating at 503 K and a constant CH3OH conversion value of 50% [311]. 7 - Concluding remarks For the layered double hydroxides [LDHs, also called hydrotalcite-type (HT) anionic clays] a broad spectrum of catalytic applications already exists and even more will probably be found in the future in unexpected areas, due to the possibilities of designing catalysts tailored for specific reactions and/or substrates. Thus, the exponential increase in the number of publications referring to LDHs in recent years is not surprising. Even more significant is the percentage of these papers reporting catalytic applications (as such or, mainly, after thermal decomposition), thus showing the increasing academic and industrial interest for these materials and their relevance in catalysis [316]. In all cases, it must be noted the high flexibility and potential of synthetic LDHs as precursors of multicomponent catalysts, due to the large number of composition and preparation variables that may be adopted. One of these promising fields is the increase in the longevity by intercalation in LDHs of biomimetic catalysts, which have generally a very limited stability as homogeneous catalysts (see, for example, Sections 3 and 5). As previously mentioned, since the interlayer space is not accessible, the reaction occurs on the external edge-sites of the crystallites [140]. LDHs intercalated with Mn-porphyrins have been reported as suitable catalysts for many reactions [317], as the epoxidation of cyclohexene using PhIO, while very poor activity was observed in the hydroxylation of heptane, thus suggesting that the accessibility of the metal centres depended on substrate polarity [318]. Furthermore, mention has to be made of the application as biomimetic catalysts of Mg/Al or Ni/Al LDHs exchanged with tungstates (to an extent of 12% and with the WO42" anions mainly located in edge positions) [319]. These catalysts were used in selective oxidative bromination and bromide-assisted epoxidation reactions with an activity 100 times higher than the homogeneous analogue (Fig. 28). The low cost and heterogeneous nature of this system, together with its ability to operate under mild reaction conditions (298 K; 6 < pH < 8 ) using bromides rather than Br2, suggest interesting environmentally friendly routes to produce brominated chemicals and

534


drugsand epoxide intermediates. As recently reported, the latter may be considered an interesting example of "engineered solid catalysts" based on ion exchange properties of cationic or anionic clays (LDHs) [320]. The polarity of these supports can be varied by an appropriate ion exchange, tailoring the catalytic activity, such as it has been reported for tungstate (hydrophilic) or tungstate/p-toluensolphonate (hydrophobic) exchanged Mg/Al LDHs in the olefin epoxidation with H2O2 as a function of the nature of the organic substrate [321].

Figure 28 - Catalytic pathway in bromination with WO/~ LDH [319]. H2O2 binds to tungstate to form peroxotungstate at the surface. Electrostatic attraction brings bromide to the surface, facilitating transfer of the activated oxygen atom from peroxotungstate to Bf. Reaction of 2-electron-oxidized Br species in solution: electrophilic bromination of phenol red into phenol blue (1) and bromide-assisted 'O2 generation from H2O2 [319]. Finally, it must be pointed out that the concept of catalytic applications of LDHs should have a wider meaning, including all the "catalytic devices", i.e. both usual heterogeneous catalysts and materials based on the exploitation of the catalytic properties, like gas sensors, porous catalytic membranes, semiconductor surface devices, etc. In this frame, mention has been made to the applications as electrode coatings, potentiometric sensors or in electrocatalysis [322-328]. Acknowledgments Financial support from the Ministero per 1' Istruzione, 1' Universita e la Ricerca [MIUR, Roma (I)] and the EU-Growth Project G5RD-CT2001-00537 is gratefully acknowledged. 8 - References [1] S. Miyata, Industrial use of hydrotalcite-like compounds, Kagaku Gijutsushi MOL 15(10) (1977) 32 (in Japanese) [2] W.T. Reichle, Chemtech 16 (1986) 58. [3] F. Cavani, F. Trifiro and A. Vaccari, Catal. Today, 11 (1991) 173. [4] A. de Roy, C. Forano, K. El Malki and J.-P. Besse, Synthesis of Macroporous Materials, vol. 2: Expanded Clays and Other Microporous Solids, Eds. M.L. Occelli, and H.E. Robson, Van Nostrand Reinhold, New York, USA, 1992, ch. 7. [5] F. Trifiro and A. Vaccari, Comprehensive Supramolecular Chemistry, vol. 7: SolidState Supramolecular Chemistry: Two and Three-Dimensional Inorganic Networks, Eds. J.L. Atwood, J.E.D Davies, D.D. MacNicol and F. Vogtle, Pergamon, Oxford, UK, 1996, ch. 8.


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Index A Absorption - 480 Acid activation - 327 Acid-activated organoclays - 335 Acid and base properties - 428 Acid and redox catalysis - 388 Acid-base interactions - 92 Acidity - 327 Active surface sites - 75 Active substances - 277 Activity coefficients - 170 Adsorption/desorption - 126, 153, 236, 321, 333, 430, 434,435, 436, 437, 440,482 Adsorption calorimetry - 132 Adsorption enthalpy - 101, 130, 141, 144 Adsorption isotherms - 128, 132, 258 Adsorption kinetics - 439 Agricultural applications - 410, 421 Aldolic condensation - 503 Alkylation-391 Alkylammonium cations - 257 Amine cations - 104,257, 331 Animal feed-410 Anion exchange capacity (AEC) - 427,428 Anion exchange process - 430 Anionic clays - 403, 411 Antacid - 279 Antidiarrhoeal - 278 Anti-inflammatory - 279 Antiseptic - 279 Arrhenius plot - 531 B Bayer-Villinger oxidation - 506 BET (Brunauer-Emmett-Teller) - 305,393, 396,399,482 Bio-LDH hybrid - 416, 488 Biological applications - 403,480 Biomolecule - 416, 487, 488 Biopolymer - 486 Broken edges - 323 Bromination - 534 Brucite structure - 2, 121, 412

548

Index

C Calcination - 442, 499, 503, 520 Carbonaceous replica - 481 Catalysts supports - 498, 499 Catalytic properties of clays - 321,443, 444 Catalytic properties of LDH-POM - 388, 390 Cation exchange capacity (CEC) - 76, 156, 324 Cation exchange reaction - 48, 175 Cationic clays - 403, 404 Cation migration - 325 Cement related materials - 483 Chemical modification - 1 Chemical reactivity - 1, 138 Chlorite structure- 123,140, 141 Clay structure - 4, 5, 6, 119, 120, 121, 122, 123, 186,253,255,269,322,323,326,328, 330,405 Claysen condensation - 503 Claysen cyclization — 503 Colloidal properties - 462 Copper(II) hydroxide acetate micrograph -18 Copper hydroxide nitrate structure - 413 Contact angle - 93, 94, 131, 143 Co-precipitation - 364, 386 Cosmetics-267, 281, 409, 418 CPO (catalytic partial oxidation) - 523, 524, 525 CTO (catalytic total oxidation) - 525 D Dawson polyoxometalates - 383 Debye-Huckel law - 170 Debye length- 170 Decarboxylation — 503 Deintercalation -206,209,210,211,213 Differential Scanning Calorymetry (DSC) - 10, 11, 16, 22, 31, 33, 36, 45, 361 Diffuse layer - 59, 163,170 Diffuse-Reflectance Infrared Fourier Transform (DRIFT) Spectroscopy - 187, 189, 191, 192, 196, 200, 203, 204,207,210, 213 DKR (Dubinin-Kaganer-Radushkevich) Equation - 112 DLVO (Derjaguin, Landau, Verwey and Overbeek) Theory - 82, 85, 98 Double layer - 172 DR (Dubinin-Radushkevich) Equation - 103 Drug delivery system — 274, 280 DTA (Differential Thermal Analysis) - 351, 356 Dye-179,438 E Electrokinetic of clay surfaces - 57 Electrokinetic properties - 58, 62 Electronegativity - 1 3 9

Index

549

Endotactic reaction - 469,471 Enthalpy- 145 Entropy-101, 145 Environmental - 426, 443, 514 EPR (Electron Paramagnetic Resonance) - 473 Esterification - 389, 390 EXAFS (Extended X-ray Absorption Fine Structure) - 479 Exchange reactions - 8, 20, 175, 386, 435 Excipients - 273,275 Exfoliation reaction - 19, 29, 49, 462 F Flame retardant - 486 Freundlich-258 FTIR (Fourier Transform Infrared) - 10, 11, 12, 17, 24,27, 31, 37, 38,46, 352, 368 G Gastrointestinal protector - 278 GC (Gouy-Chapman) theory - 155 GCS (Gouy-Chapman-Stern) model - 156 Gibbsite structure - 121 Grafting by displacement - 41 Grafting reactions - 2, 8, 14,20, 39,49 Greenhouse gases - 434,444,445,446,447, 517, 518, 520 Green rusts - 4, 461 Guest displacement - 465 H Hardness- 139 Henry reaction - 507 Herbicide - 248 Heteropolyoxometalate - 376, Homogeneous catalysis - 520 HT-FTIR (High temperature FTIR) - 368 HTLc (Hydrotalcite-like) - 426,496 HTXRD (High temperature X-ray Diffraction) - 348, 356, 365 Humic acid - 254, 440 Hybrid materials - 463, 480 Hydrated kaolinite - 42, 43, 45, 46 Hydrocalumite structure - 412 Hydrocalumite micrograph - 477 Hydrogenation - 509, 510, 512, 513 Hydrogenolysis - 509 Hydrogen bonding - 333 Hydrophobic/hydrophilic - 97 Hydrotalcite structure - 412 Hydroxysalts - 3, 13, 413 I Impregnation — 500

550

Index

Infrared spectroscopy - 134, 184 Infrared thermography - 526 Inorganic polymer - 471 In-situ polymerization - 464, 465 Intercalation by displacement - 34 Intercalation of organic anions - 357,364, 368 Interlayer or surface grafting - 8, 14, 20, 39, 49 Interlayer inorganic cations - 229 Intercalated molecules - 34, 42, 236 Intercalated organic cations - 232 Ion exchange reaction - 13, 329, 332 Isomerization of cc-pinene - 336 Isomorphous substitution - 61, 323 Isopolyoxometalate - 376 K Kaolinite-6, 32, 33, 75, 119, 120, 122, 186, 255, 270 Kaolinite intercalation - 32, 34, 42, 192, 195, 202, 206, 213, 237, 239 Kaolinite micrograph- 185, 270 Kaolinite structure - 6, 75, 119, 120, 122, 186, 255 Keggin compounds - 375 Keggin type heteropolyoxometalates - 376 Knoevenagel condensation - 503 L Layer charge density - 62, 76, 323, 461 LDH (Layered Double Hydroxide) - 4, 20, 345, 357, 374, 380, 382, 411, 414, 416, 426, 459, 460, 465 Layered double hydroxide micrograph - 429, 477, 478, 479, 482, 484, 487 Layered double hydroxide structure - 4, 380, 411, 412, 414, 426, 433, 439, 465, 473 Lithiophorite structure - 20 LW (Lifshitz-van der Waals) forces - 92 M MAS (Magic angle spinning) - 229, 351, 357, 366, 475 Mechanical properties - 483 Mechanochemical activation - 188, 191, 192,202 Mechanochemical reactions - 42 Meerwein-Ponndor-Verley reduction - 505 Memory effect — 464 Metal nanoparticles - 26 Methanol decomposition - 531 Methylation - 504 Michael addition - 503 Mixed oxides - 499, 500, 520, 521 Montmorillonite - 5, 97, 122,175, 176, 303, 327 Montmorillonite structure - 5, 122, 327 N Nanocomposite - 19, 29, 48, 49, 409, 459, 468, 480, 484, 487

Index Nanofillers - 48, 483, 484 Natural gas exploitation - 522 NMR (Nuclear Magnetic Resonance) - 216, 351, 357, 366,475 Noble metals supported catalysts - 500 Nuclear isotopes - 218 Nuclear spin interactions - 217 Nuclear wastes - 433 O Organic synthesis - 501 Organic-anion-pillared — 384 Organic pollutants - 435 Organoclay - 257, 331, 332, 440 OSR (Oxidative-steam-reforming) - 533 Oxidation - 393, 503 Oxidation-reduction reactions — 12 Oxometallates - 431 P Palygorskite micrograph - 270 Particle size - 124 PDI (Potential Determining Ions) - 60 Pesticide - 247, 249, 250, 251, 256, 259, 410, 436, 437 Pharmaceutics - 267, 271, 407, 414 Pharmaceutical denomination - 271 Pharmaceutical specification - 271 Pharmacokinetic parameters - 408 Phase-transfer method - 386 Photocatalysis - 395, 396, 398, 448, 516 PILCs (Pillared clays) - 49, 293, 306, 309, 329, 330, 440, 509 Pillared anionic clays - 388, 440, 509, 516 Polymer - 19, 29, 240, 242, 459, 468 Polymer direct incorporation - 465 POM (Polyoxometalate) - 374, 382, 385, 388, 393, 396 Porosity-430 Potentiometric titration - 74 Preyssler compounds - 377,378 PZC (Point of Zero Charge) - 69 Q Quadrupolar splitting — 220 Quaternary amine cations - 104 R Raman Spectroscopy - 184, 194, 195, 197, 198, 209, 211 Reconstruction method - 385, 442, 465 Reduction-505, 517 Reduction of oxides - 300 Reductive intercalative polymerization - 464 Reforming of methane - 527 Rehydration method - 357

551

552

Index

Relaxation - 219, 224 Remediation - 430, 435, 443 Removal of dyes - 438 Removal of heavy metals -290,292, 431,433 Removal of nuclear wastes - 433 Removal of oxoanions - 431 Removal of oxometallates - 431 Removal of pesticides - 436 Restacking- 19,465 S SAED (Selected area electron diffraction) - 28 SEDOR (Spin-echo double resonance) - 230 SEM (Scanning electron microscopy) - 18, 31, 33, 185, 186, 270, 419, 429, 477, 478, 487 Self diffusion-219, 223 Sensors - 480 Sepiolite structure - 79 Siloxane sheet-5, 121 Smectite structure - 75, 253, 255, 292, 323, 326, 328, 330, 334, 405 Smectite micrograph - 270 Solvation reaction - 1 2 Solvent assisted method - 465 Solid dosage - 273 Solvent exchange reaction - 13 Sorbent-157,440 Sorption process - 250 Sorption site - 253 SR (Steam-reforming) - 530, 532 SSA (Specific surface area) - 156, 393 Stern layer- 160 Surface acidity - 68, 327 Surface adsorption - 7, 20, 101, 108, 126, 128, 130, 132, 141, 144, 157 Surface area- 124 Surface charge - 60 Surface energy - 125, 136,143 Surface exchange reaction - 8 Surface potential - 60, 167 Surface properties - 430 Surface sites - 1 2 3 Surface tension - 91, 94 Surface thermodynamics - 90 Surfactants - 30, 108, 233,386, 441 Swelling - 146, 326 Swelling agents - 385 Swelling pressure - 59 T Talc micrograph - 270 Talc structure -123

Index

553

TEM (Transmission Electron Microscopy - 28, 482, 484 Textural properties - 476 Thermal reactions - 26, 48, 306, 309, 310, 311,499 Thermodynamics-91, 125 Thermogravimetry (TG) - 10, 11, 16, 22, 31, 33, 36, 45, 351, 356, 361 Topical use - 279, 282 TSDC (Thermal Simulated Depolarization Currents) - 134 Two-dimensional multiple quantum magic angle spinning (2D MAS) - 230 U UV-VIS (Ultraviolet-visible spectrophotometry) - 298, 300, 302 V Van der Waals forces - 333 Vibrational spectroscopy - 187 Vitabrid-C powder - 419, 420 W Wastewater-290, 292, 312,431, 433 Water adsorption - 118 Wettability-130 X XAS (X-ray Absorption) - 470 XANES (X-ray Absorption Near Edge Structure) - 479 XRPD (X-ray Powder Diffraction) - 9, 15, 21, 28, 30, 33, 35, 40, 41, 42, 43, 206, 349, 358, 359, 370, 381, 415, 419, 439, 470 Z Zeta potential - 58, 64, 67, 70, 71, 77, 78, 80 Zinc hydroxide nitrate structure - 3,413


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