Photochemical Purification of Water and Air: Advanced Oxidation Processes (AOPs): Principles, Reaction Mechanisms, Reactor Concepts

Thomas Oppenländer Photochemical Purification of Water and Air Thomas Oppenländer Photochemical Purification of Wate...

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Thomas Oppenländer

Photochemical Purification of Water and Air

Thomas Oppenländer Photochemical Purification of Water and Air

Thomas Oppenländer

Photochemical Purification of Water and Air

Author Thomas Oppenländer University of Applied Sciences (Fachhochschule) Furtwangen Department of Mechanical and Chemical Engineering Jakob-Kienzle-Str. 17 78054 Villingen-Schwenningen Germany Email: [email protected] José Franco-Pereira, who received the degree of a Magister Artium (M.A.) in film and media sciences at the University of Frankfurt (Germany), kindly designed the cover picture.

n This book was carefully produced. Nevertheless, author and publisher do not warrant the information contained therein to be free of errors. Readers are advised to keep in mind that statements, data, illustrations, procedural details or other items may inadvertently be inaccurate. Library of Congress Card No.: Applied for. British Library Cataloguing-in-Publication Data: A catalogue record for this book is available from the British Library. Bibliographic information published by Die Deutsche Bibliothek Die Deutsche Bibliothek lists this publication in the Deutsche Nationalbibliografie; detailed bibliographic data is available in the Internet at . 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim All rights reserved (including those of translation in other languages). No part of this book may be reproduced in any form – by photoprinting, microfilm, or any other means – nor transmitted or translated into machine language without written permission from the publishers. Registered names, trademarks, etc. used in this book, even when not specifically marked as such, are not to be considered unprotected by law. Printed in the Federal Republic of Germany. Printed on acid-free paper. Composition K + V Fotosatz GmbH, Beerfelden Printing betzdruck GmbH, Darmstadt Bookbinding Litges & Dopf Buchbinderei GmbH, Heppenheim ISBN 3-527-30563-7

V

Foreword The field of Advanced Oxidation Processes (AOPs) or Advanced Oxidation Technologies (AOTs) is of relatively recent vintage. However, AOPs/AOTs are proving to be important contributions to the treatment of water and air to remove harmful and toxic contaminants. Thus, it is gratifying to see this very comprehensive and authoritative treatise appear covering all aspects of AOPs/AOTs. These range from the treatment of industrial wastewaters, drinking water (both contaminant removal and disinfection), ultrapure water to the purification and deodorizing of air. Light sources include low pressure and medium pressure UV lamps, flash lamps, incoherent excimer lamps and sunlight itself. This treatise includes 146 figures, 17 worked out examples and 37 tables (many of which contain valuable collections of data and references). Each of the nine chapters cites extensive references, including very recent papers where the reader can follow up information in more detail. Professor Oppenländer is well qualified to write about AOPs/AOTs, since he has contributed to this literature in a very significant manner. It is thus particularly valuable to see the application of incoherent excimer lamps covered so well, a subject that has been a central part of his research. Finally, it is good to see careful attention being made to consistent terms, definitions and units in accord with IUPAC recommendations. This book will be of considerable value to graduate students, science and engineering faculty, scientists, process engineers and sales engineers in industry, government regulators and health professionals and all others with an interest in purifying our water and air. James R. Bolton President, Bolton Photosciences Inc. 628 Cheriton Cres., NW, Edmonton AB, Canada T6R 2M5

VI

Curriculum vitae Thomas Oppenländer studied chemistry at the University of Würzburg (Germany). He received his doctoral degree (Dr. rer. nat.) in 1984 working in the research group of Waldemar Adam on 185-nm-photochemistry of azoalkanes in solution. The following year was spent as a postdoctoral fellow at Colorado State University (Fort Collins, USA) in the group of Albert I. Meyers being involved in the asymmetric synthesis of dihydropyridines and NADH mimics. From 1985 to 1991 he was affiliated with F. Hoffmann-La Roche in Basel (Switzerland). Here he was engaged in research on drug phototoxicity and on the photochemical production of fine chemicals. Since 1991 he has been a full professor at the University of Applied Sciences (Fachhochschule) of Furtwangen (Germany) in the Department of Mechanical and Chemical Engineering, where he has been involved in photoinitiated advanced oxidation technologies. His main interests concentrate on the potentials of novel incoherent excimer lamps for photochemical treatment of water and air.

VII

Preface This book is aimed at researchers, engineers, and students who deal with problems in the expanding field of photo-initiated process technologies of water or air treatment. It lays claim to be a mine of information related to these interdisciplinary topics and presents a selection of the most important primary and secondary literature in this area of research and development. Clearly not all references of the vast research literature could be incorporated. The detailed discussion of many examples taken from the current scientific literature facilitates access to research topics of particular interest from different but interrelated point of views. This is supported by tables that collect together interesting studies, which deal with new developments of photochemical advanced oxidation processes and technologies allowing easy access to the primary scientific literature. A most important feature of this book is that it presents many detailed figures for the purpose of visualization of photochemical and photophysical phenomena, for the presentation of the underlying basic principles and for the interpretation of important research results. Additionally, several helpful example calculations are presented to demonstrate the use of quoted equations dealing, for example, with photons and reaction kinetics. This idea is thought to be very helpful in the understanding of the mathematical relationships presented Thus, the reader will become acquainted with the basic knowledge that is essential for an understanding of photochemical processes. This will enable him/her to carry out laboratory and large-scale experiments with the aim of developing and improving AOPs. There are many researchers and engineers in water and air industries who require more extensive and novel information than that which is obtainable from textbooks that have their emphasis placed on tradition. Therefore, this book intends to supply such missing information to as great an extent as is possible. To avoid any misinterpretations concerned with the digit style of numbers, the decimal point is used throughout the book instead of a comma (i.e. computer notation: 1.03 instead of 1,03, except for some graphical representations). In representative molecular structures, spin-paired non-bonding electrons around an atom of a molecule are represented (if necessary) by a bold line “7”, in accord with commonly used Lewis-structures. Single electrons are represented by a dot “•”. A full arrow (?) indicates shifts of electron pairs, whereas single electron shifts are

VIII

Preface

shown by using a half arrow (*). In photoscience an asterisk (*) usually denotes an electronically exited state of a molecule or atom, whereas in molecular orbital (MO) theory it marks an anti-bonding molecular orbital. The occasional use of commercial names to identify materials and instruments in no way implies endorsement or recommendation by the author. Thomas Oppenländer, November 2002

IX

Contents Introduction

1

1

AOPs and AOTs 5

2

Why UV and Oxidation/Disinfection?

19

2.1 Global Water Resources and Resulting Water Market 19 2.2 Present and Historical Dimensions of the Radiation Concept 24 2.3 Some Historical Landmarks of the Development of AOTs 29 References 32 3

Interaction of UV/VIS Radiation with Matter 37

3.1 3.2 3.3 3.4 3.5 3.6 3.7

Photoscience in Research and Development 37 Physical Constants and Standard Values Used in Photochemistry 40 The Electromagnetic Wave 40 The Photon Stream and Planck’s Equation 41 Electromagnetic Spectral Ranges of Interest in Photochemistry 44 Conversion of Energy Units and Other Useful Conversions 46 Photon Energies, Bond Dissociation Energies, Threshold Wavelengths and Absorption Onset of Molecules 51 Absorption of UV Radiation by Molecules 55 The Beer–Lambert Relationship 56 The Nature of Electronically Excited States 60 The Jablonski Diagram 64 Absorption of UV/VIS Radiation by Solids 66 UV/VIS Radiation as a Specific Reagent: Quantum Yield, Quantum Efficiency, Actinometry and Photoreaction Kinetics 68 Terms Associated with the Emission and Receipt of Electromagnetic Radiation 73 Safety Precautions for Radiant Sources 75

3.8 3.9 3.10 3.11 3.12 3.13 3.14

3.15 References 76 4

VUV and UV Radiant Sources and their Characteristics 79

4.1 4.2

Types of Lamps used in AOP Research and Development 79 Specific Properties of Mercury Arc Lamps 83

X

Contents

4.3 Development of Incoherent Excimer Lamps 87 4.4 Typical Photon Flow of VUV or UV Lamps 94 4.5 The Sun as Radiation Source 94 References 97 5

5.1 5.1.1 5.1.2 5.1.3 5.1.4 5.2 5.2.1 5.2.2 5.3 5.3.1 5.3.2 5.3.3 5.3.4 5.4

Photochemical Processes of Water Treatment 101 Description of Aqueous Systems 101 Classification of Water Constituents 101 Analytical Parameters of Water Quality 105

Synopsis: Water and Wastewater Treatment Technologies 112 Synopsis: Oxidative Water Treatment Technologies and Methods of Hydroxyl Radical Production 114 Nomenclature of Photochemical AOPs 118 Photooxidation Reactions 119 Photocatalytic Reactions 120 General Reaction Schemes 122 Photo-Initiated Oxidations 122 Heterogeneous Photocatalysis 123 Homogeneous Photocatalysis 124 Photolysis of Water 126 Status of Technical Realization of Photo-initiated AOPs and Photochemical Treatment Strategies 127 Photodegradation of Water Pollutants 130

5.5 References 136 6

6.1 6.2 6.3

Properties, Reactivity and Photochemistry of Auxiliary Chemicals 145 Electronic Structures of Oxygen Species Involved 145 Reduction Potentials 149

UV Absorption Properties of Auxiliary Oxidants, Catalysts and of Reactive Intermediates 149 Physical-chemical Properties of Ozone and Hydrogen Peroxide 155 Ozone 155 Hydrogen Peroxide 155 Photochemistry of Hydrogen Peroxide 157 Photocatalysts 159 Quantum Yields 162 Primary and Secondary Reactive Species 166 Hydrated Electrons 167 Hydrogen Atoms 168 Hydroxyl Radicals 168 Oxidative Degradation of Organic Matter by Hydroxyl Radicals 172 Aliphatic and Aromatic Hydrocarbons 172 Chlorinated Hydrocarbons 177 Rate Law and Rate Constants of Hydroxyl Radical Reactions 178

6.4 6.4.1 6.4.2 6.4.3 6.5 6.6 6.7 6.7.1 6.7.2 6.7.3 6.8 6.8.1 6.8.2 6.9 References 183

Contents

7

Photooxidation and Photomineralization of Organic Matter in Water and Air 189 Aqueous Systems 189 Substrate Oxidation and Substrate Mineralization 189 Reaction and Kinetic Modeling of H2O2-UV AOPs 191 Vacuum-UV Oxidation: The H2O-VUV AOP 200 Comparative Studies of Photo-Initiated AOPs 213 Biodegradability and Toxicological Studies 215 Gaseous Systems 219 Applicability 219

7.1 7.1.1 7.1.2 7.1.3 7.1.4 7.1.5 7.2 7.2.1 7.2.2 Chemical and Physical-Chemical Aspects of Air Pollution 221 7.2.3 Process Technologies of Air Treatment 225 References 231 8

8.1 8.2 8.3 8.4 8.5

Process Engineering and Reactor Concepts 239 Chemical Engineering 239 Photoreactor Concepts 243 Reactor Materials 250

Influences on the Process Performance and Design Criteria 254 Economic Factors and Design Parameters (Figures-of-Merit) of PhotoInitiated AOPs 257 Selected Industrial Applications 268 Aqueous Systems 268 Gaseous Systems 269

8.6 8.6.1 8.6.2 References 273 9

UV Disinfection 279

References 290 10

Alternative Glossary 295

References 297 Glossary of Terms Used in Photochemistry 299 Index 353

XI

XIII

Abbreviations and Symbols

Abbreviations/Symbols a)

Units

Definition

[C], c 2,4-DCP A a A10, e abs (superscript) ACM ACO ads (subscript) AO AOP AOT a-Ox aq (subscript) c0 cb CFBR CFC CFU CHC CSTR DBE DBP DDT DO DOM E e (subscript) Ebg EEA EEM EEO el (subscript) EOP Ep

mol L–1 or g L–1 – m2 m–1 (SI) or cm–1 – – m2 kg–1 m2 m–3 order–1 – – – – – – m s–1 – – – m–3 (SI), L–1, cm–3 – – – – – – – V – eV – kW h kg–1 kW h m–3 order–1 – – m–2 s–1 or mol m–2 s–1

Concentration b) 2,4-Dichlorophenol Area (Decadic) absorption coefficient Absorbance (base 10 or base e) Absorbed Collector area per mass Collector area per order Adsorbed Atomic orbital Advanced oxidation process Advanced oxidation technology Auxiliary oxidant Aqueous, hydrated Speed of light in vacuum Conduction band Contact-free batch reactor Chlorinated fluorocarbon Colony forming units Chlorinated hydrocarbon Continuously stirred tank reactor Dielectric-barrier discharge Disinfection by-product Dichlorodiphenyltrichloroethane Dissolved molecular oxygen Dissolved organic matter Reduction potential Energetic Band gap energy European Environment Agency Electric energy per mass Electric energy per order Electric Enhanced oxidation processes Photon irradiance (cf. Tab. 3-9)

XIV

Abbreviations and Symbols Abbreviations/Symbols a)

Units

Definition

Ep eV FWHM g (subscript) h H0 HC Hp,0 IC IC ISC IUPAC

J eV – – Js J m–2 – mol m–2 ppm, mg L–1 – – –

kM kN,M

s–1 L mol–1 s–1

l

m

LP Hg lamp m M (MM)

– kg kg mol–1 (SI) or g mol–1

MO MP Hg lamp NA NOM Np; Nk (k specified) OM ou P P (subscript) PAHs PCBs PCDDs PCDFs PCO PCP in Pel PIEP

– – mol–1 – mol – – W – – – – – – – W –

POP ppb (10–9 parts/unit) ppm (10–6 parts/unit) ppmv (obsolete) Qk R r Rf

– ppb ppm ppmv J – e) –

Energy of a photon Electron volt Full width at half maximum (half-width) Gaseous Planck constant Fluence (cf. Tab. 3-9) Hydrocarbon Photon fluence (cf. Tab. 3-9) Inorganic carbon Internal conversion (Jablonski diagram) Inter system crossing (Jablonski diagram) International Union of Pure and Applied Chemistry Rate constant (first order) Rate constant (second order) of reaction between molecules N and M Lengthc), penetration depth, optical pathlength, photoreaction zone Low-pressure mercury lamp Mass Molar mass (the subscript M specifies a substrate molecule M)d) Molecular orbital Medium-pressure mercury lamp Avogadro constant Natural organic matter Amount of photons Organic matter Odor unit Radiant power Photon Polycyclic aromatic hydrocarbons Polychlorinated biphenyls Polychlorinated dibenzo-p-dioxines Polychlorinated dibenzo-p-furanes Photocatalytic oxidation Pentachlorophenol (C6Cl5HO) Electric input power Production-integrated environmental protection Persistent organic pollutant Parts per billion Parts per million Parts per million by volume Radiant energy Alkyl-skeleton of an organic molecule Reaction rate Reduction factor

Abbreviations and Symbols Abbreviations/Symbols a)

Units

Definition

ROS S SC Si SOC SUWOX t T T TC TOC trans (superscript) UNEP UV V v (subscript) vb vib VIS VOC VUV X Up U V˙ g) ek

– – – – – – h, min, s K (SI) or 8C – ppm, mg L–1 ppm, mg L–1 – – – m3 (SI) or L – – – – – – – mol s–1 (Einstein s–1) – m3 s–1 or L h–1 m2 mol–1 (SI) or L mol–1 cm–1, or M–1 cm–1 – – nm Hz, s–1 cm–1

Refractory organic substances Singlet state (Jablonski diagram) Semiconductor Scavenger molecule Semivolatile organic compounds Super critical water oxidation Time Temperature Triplet state (Jablonski diagram) Total carbon Total organic carbon Transmitted United Nations Environment Program Ultraviolet (200 nm ≤ k ≤ 400 nm) Volume Visible Valence band Vibrational Visible electromagnetic radiation = light Volatile organic compound Vacuum-UV (100 nm ≤ k ≤ 200 nm) Halogen Photon flow Quantum yield f) Volume flow rate Molar (decadic) absorption coefficient b)

g g k m m

Efficiency Quantum efficiency Wavelength Frequency Wavenumber

a) Additional terms used for the description and measurement of electromagnetic radiation are collected in Tab. 3-9, abbreviations of water analytical parameters are summarized in Tab. 5-1, and abbreviations and definitions of design parameters (“figures-of-merit”) are given in Tab. 8-4. b) Many authors prefer the symbol “M” for molar concentration, rather than mol L–1. c) To avoid any misinterpretations, the physical quantity length is symbolized by l in contrast to the numerical value 1. d) M is also used in chemical equations to denote a molecule or substrate. e) The unit depends on the reaction order. f) To distinguish the symbol for quantum yield U from photon flow Up, it is printed italic (if possible). g) Many authors use other symbols e. g., F for flow rate.

XV

1

Introduction Browsing the vast primary and secondary literature that deals with water or air treatment processes and technologies very quickly leads to a point of dissatisfaction and confusion concerning vague interpretations of experimental results, reaction mechanisms, or descriptions of reactor specifications. Because water and air are essential to our life, they are of special interest not only to researchers and engineers but also to economists and many others. Thus, extra-scientific considerations such as media attention are enormous. Many of these activities generate considerable confusion related to water and its treatment technologies, especially in the case of photochemical methods. From the chemist’s point of view, however, water in its purest form does not exist in the environment. It is very difficult and costly to produce and has anomalous solvent properties, so that it is contaminated immediately by dissolving trace elements or compounds from its environment. On the other hand, “pure” air is simply a mixture of several gaseous compounds of well-known composition. By thinking of nature as a huge photochemical reactor with the sun as an intensive source of electromagnetic radiation, the air as the gaseous component and water as the liquid medium, the bridge to photochemical reactions in water and air is built. This led to the investigation of photo-induced and photo-initiated processes within these media. As a result of these understandings, photochemical technologies for water and air treatment were developed over recent decades. Because of the interdisciplinary nature of this research area, which connects aqueous and gaseous chemistry and photochemistry with engineering concepts, an almost Babylonian expansion of terms, definitions, designations and abbreviations has been observed, such as AOPs, AOTs, EOTs, EEO, EEM, ACM, ACO, H2O2-UV, H2O-VUV, photocatalysis, photooxidation, photo-initiated oxidation, fluence, “UV light”, “UV dose”, etc. Consequently, it is one objective of this book to outline, strictly define, and unify the corresponding nomenclature used in the literature to describe the various processes and techniques (in accord with recommendations of the International Union of Pure and Applied Chemistry, IUPAC), and to address severe misinterpretations that abound throughout the general literature, such as for example the misleading, often used and fundamentally incorrect term “UV catalyzed” oxidation. Another important goal is the elaboration, clarification and discussion of the underlying chemical, photophysical and photochemical processes and mecha-

2

Introduction

nisms, with distinct emphasis on the last, based on the historical development of the photon concept. Hence, the complexity arising from the diversity of reactions in aqueous or in gaseous media with or without catalysts or auxiliary oxidants, in the presence or absence of microorganisms, will be structured and classified on an accurate basis. In addition, this task requires knowledge of the language of a chemist who is usually wild about chemical formulas and structures that are composed of line drawings. Hoffmann and Laszlo (1991) enthusiastically outlined this concept. These authors stressed that the language of chemistry parallels the grammar of our natural language (with several restrictions of course). This is also true if we introduce the specialized language of photochemistry, so that the scheme of Hoffmann and Laszlo may be extended to a “photochemical sentence” (excluding photophysical deactivation processes): Language: Chemistry: Photochemistry:

subject substrate substrate

–– transitive verb –– object –– reagent –– product –– absorbed photon –– photoproduct

The interpretation of an absorbed photon as a specific reagent that transforms a substrate under special conditions to a distinct photoproduct, i.e. which causes a photochemical reaction, has an enormous impact on the understanding and design of photochemical processes and their scaling-up. In addition, it clearly demonstrates that light or UV radiation can never act as a catalyst, because the energy of the absorbed photon is used to produce an electronically excited state of a molecule, and therefore its energy is dissipated and the photon is lost. Furthermore, one should notice that the product of a chemical (thermal) reaction is not usually identical with the photoproduct that is produced in a photoreaction. This is a direct consequence of the higher energy introduced by photons into a molecular entity compared with thermal chemistry, i.e. excited state versus ground state reactivity. This again has an important influence on the understanding of photochemical processes used for the purification and detoxification of water and air. After having discussed the fundamentals of the interaction of UV/VIS radiation with matter, the photochemical processes of water treatment will be analyzed in relation to the photochemistry of the oxidative auxiliaries. The behavior of the primary and secondary reactive species (mainly oxygen containing radicals and carbon radicals, respectively) is outlined in detail. After a systematic classification of photo-initiated oxidations, reactions related to the used oxidants, catalysts and applied excitation wavelengths, a detailed description of photooxidation and photomineralization of organic matter from a chemistry point of view follows. It includes the analysis of the level of reactive intermediates, the level of oxidation products and a discussion of toxicological studies. Additionally, several recent research activities and potential applications of novel incoherent excimer lamps including the expanding field of vacuum-UV (VUV) photochemistry of water are reported in detail. Another section of the book covers the engineering concepts based on the underlying photochemical and chemical processes. It assesses the different types of

Acknowledgements

photoreactors and the various influences on the performance of the process. Most importantly, the economic factors of photo-initiated oxidations are worked out according to the Bolton concept of design parameters (Bolton’s “figures-of-merit”) that are directly related to the observed degradation kinetics, and thus to the cost of the advanced oxidation process (AOP). Short reviews of the recent research activities into UV disinfection of water and air and of the UV treatment techniques of polluted air are enriched by the description and discussion of several selected examples of published industrial applications. Many questions related to the selection of UV processes will be answered and common pitfalls referring to UV applications addressed. Until now, many technical and research papers concerning photo-initiated treatment processes of water or air are almost useless for synoptic discussions due to the lack of precise data describing the photoreactor, its components and a comprehensive set of the analytical and engineering parameters of the system. Thus, a detailed analysis of the experimental information necessary for the correct description of photoprocesses (for example in technical publications) will help to facilitate future work and communication in this interdisciplinary field of research and development. Thus, the motto of this book is, “to purify, irradiate!”

Reference Hoffmann R, Laszlo P (1991) Darstellungen in der Chemie – die Sprache der Chemiker, Angew. Chem. 103: 1–16; similar article in French: Hoffmann R, Laszlo P (1989) La Représentation en Chimie, Diogène, No. 147: 24–54.

Acknowledgments

I wish to express my sincere gratitude to several companies and institutions that provided information concerning their products and research efforts. The following companies, institutions, and persons provided specific information for me: • a.c.k. aqua concept GmbH (Karlsruhe, Germany), M. Sörensen: case studies and a photograph of a photoreactor. • ABB (Baden, Switzerland), U. Kogelschatz: incoherent excimer lamps and corresponding emission spectra. Many stimulating discussions with Dr. Kogelschatz initiated research concerning water and air treatment with VUV/UV excimer lamps. • C & C Consult (Schleiden/Eifel, Germany): R. S. Chatty: technical information, investigations, and a photograph of a mobile air treatment system. • Calgon Carbon Inc. (Pittsburgh, PA, USA), W. Lem: photograph and description of a photoreactor and treatment examples. • Dr. Hönle AG UV-Technologie (Planegg/München, Germany), P. Wind: information and a photograph concerning the surface irradiation system used for bottle screw cap disinfection.

3

4

Introduction

• GMBU e.V. Gesellschaft zur Förderung von Medizin-, Bio- und Umwelt-Technologien, Fachsektion Sensorik (Jena, Germany), D. Faßler: electronic release of the Conference Proceedings of the 2nd International Conference on Oxidation Technologies for Water and Wastewater Treatment, May 28–31 (2000), Clausthal-Zellerfeld (Germany). • HDN-Technik GmbH (Rednitzhembach, Germany), K. W. Hofmann: information concerning odor abatement (gaseous systems) and waste air treatment and a photograph of a photoreactor. • Heraeus Noblelight AG (Kleinostheim, Germany), J. P. Schulz: product information and spectra of the low-pressure and medium-pressure mercury vapor lamp types. • Institute for Sanitary Engineering, Water Quality and Waste Management, University of Stuttgart (Germany), E. Thomanetz: descriptions and photographs of photoreactors, and treatment examples. • UMEX GmbH (Dresden, Germany), S. Johne: electronic release of the Conference Proceedings of the First International Congress on Ultraviolet Technologies, IUVA, June 14–16 (2001), Washington, DC; photoreactor specifications and photographs of UMEX reactors. • VitaTec UV-Systeme GmbH (Freigericht, Germany), M. Trageser: photographs of photoreactors and treatment examples. • WaCo Wassertechnik Consult GmbH (Hamburg, Germany), O. Debus: supplementary information regarding the AOX elimination with a contact-free batch photoreactor. The editorial assistance of Wiley-VCH is also acknowledged. In addition, I wish to thank the Willy-Hager-Stiftung (Stuttgart, Germany) for their financial support. This foundation arranged contacts with E. Thomanetz at the Institute for Sanitary Engineering, Water Quality and Waste Management, University of Stuttgart. We had close discussions on the internal project results concerning the development of AOTs. The exchange of stimulating ideas and the numerous scientific discussions with Prof. Thomanetz during the past few years were extremely fruitful and encouraging to the planning and realization of the current book project. He made many practical suggestions, and fascinating insights into the technical applications and problems of advanced oxidation technologies were opened up. Finally, yet importantly, I wish to acknowledge the direct support of James R. Bolton. He read the entire manuscript and offered critical and constructive comments, and made detailed suggestions concerning mechanistic interpretations and technical applications. James Bolton, Professor Emeritus of Chemistry, University of Western Ontario, Department of Chemistry (London ON, Canada), is currently the Executive Director of the International Ultraviolet Association (IUVA), Adjunct Professor at the Department of Civil & Environmental Engineering, University of Alberta (Edmonton AB, Canada), and he is the President of Bolton Photosciences Inc. (Ayr ON, Canada).

5

1

AOPs and AOTs Students and experts who concentrate seriously on research and development strategies that are related to the interdisciplinary topics of UV radiation/light, the environment and technology are increasingly confronted with an enormous flood of information that is spread throughout the scientific and technical literature. This is especially the case since the introduction of the Internet, which enables easy access to journalistic, and primary and secondary scientific literature. Hence, Tab. 1-1 summarizes several introductory and sophisticated links to Internet pages of special interest to these research areas. The link collection of Tab. 1-1 should facilitate the entry of newcomers into the complex field of ultraviolet (UV) radiation or light-induced environmental technologies and photochemical or photophysical research concepts. In addition, it should help the researcher and developer to specify their interests. In this connection, it is worth mentioning that often scientists and engineers do not differentiate correctly between the terms light and ultraviolet radiation in everyday usage. The term “light” strictly stands for the visible part (VIS) of the electromagnetic spectrum and it covers only a short wavelength segment between 380 nm ≤ k ≤ 780 nm. Nonetheless, the term “UV-light” is used in the common vocabulary of (photo)scientists and engineers, but for the sake of clarity, it should be avoided. In this book, the abbreviation UV/VIS is used to describe UV radiation and light. However, the term “radiation” has a negative public reaction (it means “high-energy radiation”, something strange and dangerous) and it should be used carefully in the public domain. The cross sections of the subject areas UV radiation/light and technology, UV radiation/light and environment, and environment and technology (Fig. 1-1) are interrelated and extraordinarily versatile with respect to their future development potentials. Therefore, the use of light and UV radiation in diverse applications represents one of the advanced technologies of the 21st century. This is particularly the case for the utilization of solar energy and any research activities that are related to climate protection technologies. This book concentrates on the basic principles, reaction mechanisms and engineering concepts of technologies that use UV radiation (or light) as a selective reagent for the cleavage of chemical bonds and hence for the destruction of unwanted chemicals or microorganisms. Accordingly, these methods include UV

6

1 AOPs and AOTs Tab. 1.1 Link collection of Internet pages comprising research, development, and general infor-

mation in the field of UV radiation/light, environment and technology (all links have been tested regularly for their actual status) Index

Source/description

Link

Air quality guidelines, drinking water quality, water resources

World Health Organization (WHO)

http://www.who.int/home-page

Bibliography on heterogeneous photocatalysisa)

National Renewable Energy Laboratory

http://www.ott.doe.gov/coolcar/ pdfs/water_ air2.pdf

Chemical nomenclature

International Union of Pure and Applied Chemistry

http://www.iupac.org

Chemical structure database, structure viewing support by Chemscape Chime

U.S. National Library of Medi- http://sis.nlm.nih.gov/Chem/ cine, National Institute of ChemMain.html Health

Chemical-physical data, litera- Database catalog for chemists http://www.chemieture datenbanken.de Data collections of organic and small inorganic compounds

National Institute of Standards and Technology (NIST)

http://webbook.nist.gov

Drinking water standards of the U.S.

EPA Office of Water

http://www.epa.gov/safewater/

Economy, technology, profes- German Machinery and Plant http://www.vdma.de sion and other online services Manufacturers Association (VDMA) b) Environmental policy and reports

Federal Environment Ministry http://www.bmu.de/ (BMU, Germany)

Environmental products, tech- ECO Services International nology and services, global di- (Switzerland) rectory

http://eco-web.com

European Union Law and other activities

European Union On-line

http://europa.eu.int

Federal Legislation of Germany

Federal Law Gazette (Germany)

http://www.bundesgesetzblatt. de

http://www.rbv-koeln.de/ Gas and water relevant topics, Federal Association of Gas technical press releases of and Water Companies/Asso- figawa/index.php4 ciation of Pipework Construc- http://www.rbv-koeln.de brbv c) tors (Germany) German Standards

German Institute of Standardization (DIN)

http://www.din.de

Glossary of terms used in photochemistry, full text in pdf format

Report of the IUPAC d) Photo- http://www.unibas.ch/epa/ chemistry Commission (Ver- texts.html hoeven, 1996)

1 AOPs and AOTs Tab. 1.1 (continued)

Index

Source/description

Hydrogen peroxide: applicaUS Peroxide tions to water, wastewater and hazardous waste treatment

Link http://www.h2o2.com

International chemical safety cards

National Institute for Occupa- http://www.cdc.gov/niosh/ tional Safety and Health ipcs/icstart.html

International conference

Advanced Oxidation Technolo- http://www.aotsconference.com gies for Water and Air Remediation (AOTs)

International conference

Solar energy research and ap- http://photoenergy.org plied photochemistrye)

International conference, research

Clausthaler UmwelttechnikInstitut GmbH

International standards and guidelines

International Organization for http://www.iso.ch/iso/en/ Standardization (ISO) ISOOnline.frontpage

Journal finder and journal search, journal service

Wiley Interscience

http://www3.interscience. wiley.com

Journal tables of contents: regular email service

Elsevier

http://www.elsevier.nl

Journals and books, subject search and alerting service (email)

Wiley-VCH

http://www.wiley-vch.de

Journals and publishers

ChemPort

http://www.chemport.org

Laser development and applications, optics, photonics and imaging

The International Society for Optical Engineering (SPIE)

http://www.spie.org

Laws, regulations, information

Environmental Protection Agency (EPA)

http://www.epa.gov

Light and lighting, radiation

International Commission on http://www.cie.co.at/ciecb Illumination (CIE)f)

Light measurement handbook g)

International Light, Inc.

Literature search, general

KVK-virtual catalog: books and http://www.ubka.uni-karlsjournals ruhe.de/kvk.html

http://www.cutec.de

http://www.intl-light.com/ handbook

Literature search: photochem- Photochemistry/photobiology istry and related fields database, access to photochemistry literature (Grabner and Kuti, 1997)

http://www.chemres.hu/ pchem

Odor assessment, annoyance and research

http://odournet.com

OdourNet

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8


Index

Source/description

Link

Organic photochemistry

Home page of Turro’s h) research group

http://turmac13.chem. columbia.edu

Original publications of photo- International Journal of Photoe- http://www.photoenergy.org/ ijp science, full text of papers in nergyi) pdf format, online journal Ozone and its applications

International Ozone Association (IOA)

http://www.int-ozoneassoc.org

Persistent organic pollutants (POPs)

POPs related United Nations Environment Program

http://irptc.unep.ch/pops/

Photochemistry activities

European Photochemistry As- http://www.unibas.ch/epa sociation (EPA)

Photochemistry activities

Inter-American Photochemical http://www.chemistry. Society (I-APS) mcmaster.ca/~iaps

The Radiation Chemistry Data http://www.rcdc.nd.edu/icabr/ Radiation chemistry databases: radical and excited state Center at the Notre Dame Ra- RCDC.html processes, extensive data com- diation Laboratory Database pilations Radical reactions and rate con- The Radiation Chemistry Data http://www.rcdc.nd.edu/ stants (Buxton et al., 1988): Center at the Notre Dame Ra- Solnkin2 e–aq, H•, •OH, O•–, 1O2 diation Laboratory Database Regulations and standards re- Environmental legislation lated to environment and en- (Germany) vironmental technology Research projects, environment

http://www.umwelt-online.de/ regelwerk/index.htm

Federal Ministry of Education http://www.bmbf.de and Research (BMBF, Germany)

Search engine: only web sites Elsevier Science containing scientific information

http://www.scirus.com

http://www.bgvv.de Security data of chemicals etc. Federal Institute for HealthOriented Consumer Protection and Vetenary Medicine in Berlin (Germany) Solar radiation, relevant glossaries

Renewable Resource Data Center (RReDC)

http://rredc.nrel.gov

Technical standards for indus- American Society for Testing tries and Materials (ASTM)

http://www.astm.org

Technical standards, gas and water treatment

German Association of Gas and Water Experts (DVGW)

http://www.dvgw.de

Toxicological profile information sheet

Agency for Toxic Substances and Disease Registry

http://www.atsdr.cdc.gov/ toxpro2.html


Index

Source/description

Link

Toxicology, environmental health, chemical information

Specialized Information Ser- http://sis.nlm.nih.gov/ vices, National Library of Med- index.html icine

Ultraviolet technologies, appli- International Ultraviolet Asso- http://www.iuva.org cations, research ciation (IUVA) Ultraviolet technologies: exper- Bolton Photosciences Inc. tise

http://www.boltonuv.com

UNEP chemicals

United Nations Environment Program (UNEP)

http://irptc.unep.ch

Water activities in Europe

European Water Association (EWA)

http://www.ewaonline.de

Water activities in USA

American Water Works Association AWWA Research Foundation Water Environment Federation

http://www.awwa.org http://www.awwarf.com http://www.wef.org

Water activities, international, International Water Associapublications tion

http://www.iawq.org.uk

Water and wastewater industry news

U.S. Water News, Inc.

http://www.uswaternews.com

Water management industries, search engine

Dutch journal H2O

http://www.h2o-scanner.com

Water programs

EPA’s Office of Water

http://www.epa.gov/ow

Water technology online

National Trade Publications Inc.

http://www.watertechonline. com

Water, marketplace for the water industry

VerticalNet web site

http://www.wateronline.com

Water, photochemistry, and chemistry in general

German Chemical Society and http://www.gdch.de divisions (GdCh)

Water, versatile information j)

Water quality

http://www.waterquality.de

Water, wastewater and waste treatment

German Association for Water, Wastewater and Waste

http://www.atv.de

a) M Blake (1999) Bibliography of Work on the Heterogeneous Photocatalytic Removal of Hazardous Compounds from Water and Air, update No. 3, January 1999: 154 pp, also available from the National Technical Information Service, Springfield, VA 22161. b) VDMA: Verband Deutscher Maschinen- und Anlagenbau e. V. c) brbv: Berufsförderungswerk des Rohrleitungsbauverbandes. d) IUPAC: International Union of Pure and Applied Chemistry. e) Photoenergy Center, Ain Shams University, Cairo. f) CIE (French title): Commission Internationale de l’Eclairage. g) by A Ryer (1997), International Light, Inc. h) NJ Turro, Columbia University, New York. i) edited by MSA Abdel-Mottaleb (since 1999). j) collected by H Willmitzer.

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1 AOPs and AOTs

Some interdisciplinary research and development areas related to the terms UV radiation/light, environment and technology.

Fig. 1.1

Fig. 1.2

Main objectives of photochemical technologies for the UV treatment of water and air.

techniques for the treatment of contaminated media water and air, mostly in the presence of oxidative auxiliaries or photocatalysts, leading to their efficient detoxification, purification or remediation, and disinfection, including odor abatement strategies and surface treatment (Fig. 1-2). However, the most obvious goal of these photochemical technologies is to attain water and air quality standards that fulfill government regulations according to emission ordinances. Ultimately, if necessary the complete mineralization of the organic carbon content of water or air can be achieved according to the general Eq. 1-1. In general, the process of photomineralization of organic contaminants describes the oxidation of organic carbon atoms to carbon dioxide or carbonate species (CO2, H2CO3, HCO–3, CO2– 3 ) initiated by electromagnetic radiation. Simultaneously, hydrogen atoms are converted into water and heteroatoms X of organic molecules are trans-

1 AOPs and AOTs

formed into the corresponding mineral acids HX = HCl, HBr, HNO3, HNO2, H2SO4, etc. Cn Hm Xz

hm;O2

! nCO2 m

z=2H2 O zHX

(1-1)

For example, 2,4-dichlorophenol (C6H4Cl2O) mineralizes stoichiometrically according to Eq. 1-2 (Bolton and Cater, 1994). Even pollutants containing inorganic carbon such as cyanide (CN–) can be oxidized to less toxic compounds by adequate irradiation methods.

(1-2)

All of these photochemical processes and related technologies are usually classified as Advanced Oxidation Processes (AOPs) (Glaze et al., 1987) and Advanced Oxidation Technologies (AOTs) or sometimes as Enhanced Oxidation Processes (EOPs). The underlying oxidative reaction mechanisms are basically imitations of natural photo-initiated processes that take place in sunlit surface waters or in the Earth’s atmosphere. They rely mainly on the formation of reactive and short-lived oxygen containing intermediates such as hydroxyl radicals (•OH) and they exploit the high reactivity of these species. The hydroxyl radical is a powerful oxidant and a short lived, highly reactive, and non-selective reagent that is easy to produce (Fig. 1-3). It has electrophilic properties and its reactions with appropriate substrate molecules are kinetically controlled usually exhibiting very high second order rate constants, which are often close to (or even above) the diffusion-con-

Some characteristic features of the hydroxyl radical.

Fig. 1.3

11

12

1 AOPs and AOTs

Fig. 1.4

The family of advanced oxidation technologies for water and air remediation.

trolled limit (von Sonntag, 1996). Kinetic reaction control refers to competing irreversible reactions in which the product composition is determined by the relative rates of product formation. Furthermore, the •OH radical is a ubiquitous transient species in nature and it is a very important agent in numerous human diseases or disorders and in the aging process (Knight, 1998). The transfer of such natural processes into technical dimensions requires methods for the efficient production of the so-called “free” hydroxyl radicals at sufficient concentrations from appropriate precursors. Besides photochemical and solar photocatalytic processes many other innovative technologies have been proposed for treating liquid or gaseous wastes around abandoned sites (Freeman and Harris, 1995) and are currently under intensive investigation (Fig. 1-4), for example catalytic processes (e.g. Hofmann et al., 2000), electrochemical oxidation of organic water contaminants (Galla et al., 1999) and non-thermal plasma treatment methods for exhaust air processing. Further AOPs of special interest include sonolysis (Ince et al., 2001, Hua and Hoffmann, 1997), X-ray irradiation and -radiolysis (Ferradini and Jay-Gerin, 1999), electron beam irradiation (Pikaev, 2000, Cooper et al., 1998, Getoff, 1993) of contaminated water, radiation chemistry in general (Jonah and Rao, 2001) and the SUWOX process (Schmieder and Abeln, 1999) that uses super critical water conditions (Kritzer and Dinjus, 2001) for the oxidation of organic water contaminants, and many other remediation technologies (Freeman and Harris, 1995). Over several years of extensive research and development efforts in these fields a wide-ranging knowledge of environmental applications has accumulated, including analytical methodologies and process engineering of photochemical reactions, e.g. for the generation of •OH radicals. This fact manifests itself in the countless scientific and technical publications in various journals that deal with environmental technologies and sustainable development.

1 AOPs and AOTs

At this point it is advisable to note that all the AOTs mentioned above are “endof-the-pipe” technologies that are usually applied to the subsequent elimination of undesirable production side or waste products. However, in the sense of production-integrated environmental protection (PIEP) the process dependent pollution of media should be avoided by ecologically friendly process optimizations at an early stage of chemical production (Christ, 1999 a, b). In this connection, photochemical AOPs are perfectly placed for the decontamination of tributary wastewater streams to avoid and to reduce residues that accumulate directly during the production process. This is also true for the UV disinfection of process water, demonstrating great relevance to water recycling strategies and for the prevention of point source pollution of water. Because even PIEP cannot be realized without the formation of wastewater and waste gases (cf. Christ, 2000) and because biotechnological processes depend extensively on biochemical reactions in the water phase there seems to be a positive future for photochemical (and other) AOTs for industrial water purification and/or disinfection, for example in synergistic operation with biological treatment installations. The search for comprehensive book publications concerning photochemical water and air treatment technologies turns out, surprisingly, to be without very much success, especially in the case of the German technical literature. Usually, textbooks on environmental chemistry (Bliefert, 1997, vanLoon and Duffy, 2000) cover the basics of UV-induced processes in water or in the Earth’s atmosphere only briefly. The modern technologies of photochemical water and air treatment are not covered. Even in books on environmental technology the techniques of UV oxidation or UV disinfection are only treated superficially (e.g. Bank, 2000, Hartinger, 1995, Kunz, 1995). This lack of comprehensive information is extremely disappointing in textbooks concerning wastewater treatment technologies (Neitzel and Iske, 1998, Pöppinghaus et al., 1994, Hancke, 1991) or in books on waste gas or air purification (Ebeling, 1999, Fritz and Kern, 1992). On the other hand, the physico-chemical principles and basic mechanistic considerations of the ozone UV and of the hydrogen peroxide UV process have been reported recently by Gottschalk et al. (2000) within the context of ozonation of water and wastewater. The disinfection of drinking water by UV treatment has a long history and it was covered by Bartz and Wippler several years ago (1989). In Europe today, over 3000 drinking water facilities use UV disinfection technology (Rice, 2001). Most encouraging in this context is the fact that the world’s largest UV disinfection plant was recently built in Germany to disinfect the water of the Wahnbachtalsperre located southeast of Cologne (König, 2000) supplying about 720 000 residents with 110 to 120 million m3 of drinking water per year. This plant is designed for volume flow rates V˙ of water of between 13 000 and 14 000 m3 h–1. It consists of seven independent photoreactor modules with a total of 462 UV lamps, i.e. special high-intensity low-pressure mercury lamps (König, 2001). In the United States and Canada even biologically pretreated municipal waste waters (secondary sewage effluents) are usually UV disinfected by passing through huge irradiation channels before they are drained into the surface water and released into natural cycles. It has been shown that UV irradiation is a power-

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1 AOPs and AOTs Tab. 1.2 Selected candidates, applications and products of UV treatment technology

UV disinfection

UV oxidation

Air purifiers (apartment, vehicle, hospitals etc.)

Air stripper ventilation treatment

Air-conditioning installations

Chemical and biochemical laboratories

Aquaculture for hatcheries

Degradation of toxic explosives

Clean room air supply, air disinfection

De-odorization of air, odor abatement

Community water systems

Galvanic wastewater: cyanide oxidation

Drinking water supply

Ground water remediation

Food and beverage industry

High purity water production

Hospital water supply

Landfill leachate water treatment

Indoor or closed system air treatment

Oxidation of non-biodegradable compounds

Legionella control in warm water systems

Pharmaceutical wastewater treatment

Municipal water systems

Reuse of wastewater

Pharmaceutical and cosmetic industry

Semi-conductor manufacturing industry

Process water treatment

Textile industry wastewater treatment

Rain water utilization

Treatment of recalcitrant organics a)

Sewage water treatment

Ultra pure water production

Sterile boxes

Process water treatment

Surface disinfection units

Drinking water treatment

Under sink UV systems (households etc.)

Mineralization of toxic contaminants

a) PAHs, PCP, PCBs, Pesticides, POPs etc.

ful alternative to chemical disinfection by conventional chlorination avoiding hazardous disinfection side products. Ho et al. reported in 1998 on the existence of over 500 UV facilities for wastewater disinfection operated in the United States with water flow rates V˙ between 158 m3 h–1 and 3200 m3 h–1. Since 1986, UV technology has been used in Québec (Canada) for the disinfection of municipal wastewater treatment effluents in irradiation plants with up to 4640 (!) low-presin of 290 kW (Babineau et sure mercury lamps with a total electric input power Pel al., 1999). Large-scale UV systems with over 5000 low-pressure mercury lamps are commercially available (Schneider et al., 2001). At the same time, even in Europe some installations for the UV disinfection of secondary sewage effluents by the German North Sea, by the Baltic Sea and in Austria have been in operation for several years (Popp et al., 1996). At present, a huge wastewater disinfection plant has been installed in Manukau, Auckland in New Zealand. This UV installation consists of 12 open channels each of 17 m length containing almost 8000 high-efficiency low-pressure mercury lamps (type: Spectrotherm). A maximum water flow of 57 600 m3 h–1 can be effectively disinfected (WEDECO, 2001).

References

Furthermore, pretreated ground or process waters can be decontaminated efficiently and purified in the presence of hydrogen peroxide by powerful irradiation units that consist of 12 or more modules each containing a medium-pressure in of 30 kW (Bircher et al., 1996). mercury lamp with an electric input power Pel Hence, that is why the tiny and inconspicuous •OH radical has emerged as an economically and ecologically important “industrial chemical” (cf. vanLoon and Duffy, 2000) over the past two decades. It still attracts a growing interest of water consuming or wastewater producing industries around the world. Finally, an Internet search for term combinations such as UV/oxidation, UV/disinfection or UV/technology leads to a multitude of products and companies that offer a variety of interdisciplinary applications to water and air treatment in industry or in private households (see Tab. 1-2). All of the facts mentioned above and the observation that several photochemical AOTs for water and air remediation have been successfully commercialized during recent years, justify a comprehensive description of photochemically driven advanced oxidation processes for water and air treatment in this book, which includes a brief description of UV disinfection techniques.

References Babineau D, Payment P, Chartray D, Laporte A, Fournier-Martin A (1999) Ultraviolet Disinfection of Municipal Wastewater Treatment Effluents: Pilot Studies with a Low-Pressure System Using Thermotolerant Coliforms and Clostridium perfringens as Indicators, Vecteur Environ. 32, No. 6: 34–43. Bank M (2000) Basiswissen Umwelttechnik: Wasser, Luft, Abfall, Lärm und Umweltrecht, 4th ed., Vogel Buchverlag, Würzburg. Bartz WJ, Wippler E (eds.) (1989) Desinfektion von Trinkwasser durch UV-Bestrahlung, 2nd ed., Expert-Verlag, Ehningen. Bircher KG, Simms K, Lem W (1996) Rayox UV/Oxidation-An Integrated Approach, in: Proceedings of the Symposiums on Chemical Oxidation: Technology for the Nineties, Eckenfelder WW, Bowers AR, Roth JA (eds.), Vol. 6: 288–297. Bliefert C (1997) Umweltchemie, 2nd ed., Wiley-VCH, Weinheim. Bolton JR, Cater SR (1994) Homogeneous Photodegradation of Pollutants in Contaminated Water: An Introduction, in: Aquatic and Surface Photochemistry, Helz G R (ed.), Lewis Publishers, Boca Raton, pp 467–490.

Buxton GV, Greenstock CL, Helman WP, Ross AB (1988) Critical Review of Rate Constants for Reactions of Hydrated Electrons, Hydrogen Atoms and Hydroxyl Radicals (•OH/•O–) in Aqueous Solution, J. Phys. Chem. Ref. Data 17: 513–886. Christ C (1999 a) Integrated Environmental Protection Reduces Water Pollution, Chem. Eng. Technol. 22, No. 8: 642–651. Christ C (2000) Umweltschonende Technologien aus industrieller Sicht – Verfahrensverbesserungen und Stoffkreisläufe, Chem.-Ing. Tech. 72, No. 1+2: 42–57. Christ C (ed.) (1999 b) Production-Integrated Environmental Protection and Waste Management in Chemical Industry, WileyVCH, Weinheim. Cooper WJ, Curry RD, O’Shea K (eds.) (1998) Environmental Applications of Ionizing Radiation, Wiley, New York. Ebeling N (1999) Abluft und Abgas, WileyVCH, Weinheim. Ferradini C, Jay-Gerin J-P (1999) La Radiolyse de l’Eau et des Solutions Aqueuses: Historique et Actualité, Can. J. Chem. 77: 1542–1575. Freeman HM, Harris EF (eds.) (1995) Hazardous Waste Remediation, Innovative

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1 AOPs and AOTs Treatment Technologies, Technomic Publishing, Lancaster/Basel. Fritz W, Kern H (1992) Reinigung von Abgasen, 3rd ed., Vogel Buchverlag, Würzburg. Galla U, Kritzer P, Bringmann J, Schmieder H (1999) Verfahren zum Abbau von organischen Schadstoffen durch indirekte Elektrooxidation, Chem.-Ing. Tech. 71, No. 6: 628–633. Getoff N (1993) Purification of Drinking Water by Irradiation. A Review, Proc. Indian Acad. Sci. (Chem. Sci.) 105, No. 6: 373–391. Glaze WH, Kang J-W, Chapin DH (1987) The Chemistry of Water Treatment Processes Involving Ozone, Hydrogen Peroxide and Ultraviolet Radiation, Ozone Sci. Eng. 9: 335–352. Gottschalk C, Libra JA, Saupe A (2000) Ozonation of Water and Waste Water, Wiley-VCH, Weinheim. Grabner G, Kuti Z (1997) Literature Access and Search with the Photochemistry Database, Europ. Photochem. Assoc. Newslett. No. 61: 104–107. Hancke K (1991) Wasseraufbereitung, Chemie und chemische Verfahrenstechnik, 2nd ed., VDI-Verlag, Düsseldorf. Hartinger L (1995) Handbuch der Abwasser- und Recyclingtechnik, 2nd ed., Hanser, München. Ho C-F H, Pitt P, Mamais D, Chiu C, Jolis D (1998) Evaluation of UV Disinfection Systems for Large-scale Secondary Effluent, Water Environ. Res. 70, No. 6: 1142– 1150. Hofmann J, Tauchnitz H, Vanselow H (2000) Catalytic Oxidation – A New Method for the Degradation of Pollutants in Wastewater, Chem. Eng. Technol. 23, No. 2: 125–127. Hua I, Hoffmann MR (1997) Optimization of Ultrasonic Irradiation as an Advanced Oxidation Technology, Environ. Sci. Technol. 31, No. 8: 2237–2243. Ince NH, Tezcanli G, Belen RK, Apikyan IG (2001) Review: Ultrasound as a Catalyzer of Aqueous Reaction Systems: the State of the Art and Environmental Applications, Appl. Catal. B: Environ. 29: 167– 176.

Jonah CD, Rao BSM (eds.) (2001) Radiation Chemistry, Present Status and Future Trends, Elsevier, Amsterdam. Knight JA (1998) Free Radicals: Their History and Current Status in Aging and Disease, Ann. Clin. Lab. Sci. 28, No. 6: 331– 346. König R (2000) Trinkwasserentkeimung mit ultraviolettem Licht, Talsperrenverband beschreitet neue Wege, Umwelt 30, No. 10/ 11: 42–44. König R (2001) WEDECO AG, personal communication. Kritzer P, Dinjus E (2001) An Assessment of Supercritical Water Oxidation (SCWO) – Existing Problems, Possible Solutions and New Reactor Concepts, Chem. Eng. J. 83, No. 3: 207–214. Kunz PM (1995) Behandlung flüssiger Abfälle, Vogel Buchverlag, Würzburg. Neitzel V, Iske U (1998) Abwasser, WileyVCH, Weinheim. Pikaev AK (2000) Mechanism of Radiation Purification of Polluted Water and Wastewater, Conference Proceedings of the 2nd International Conference on Oxidation Technologies for Water and Wastewater Treatment, May 28–31 (2000), ClausthalZellerfeld (Germany); electronic release. Popp W, Fenn B, Roth D, Schade M (1996) Mikrobiologische Untersuchungen zur Abwasserdesinfektion mit UV-Strahlung, Forum Städte-Hygiene 47: 250–257. Pöppinghaus K, Fresenius W, Schneider W (eds.) (1994) Abwassertechnologie, 2nd ed., Springer-Verlag, Berlin. Rice RG (2001) Practical Applications of UV in Combination with Ozone, Conference Proceedings of the First International Congress on Ultraviolet Technologies, International Ultraviolet Assoc. (IUVA), June 14– 16, Washington, DC; electronic release. Schmieder H, Abeln J (1999) Supercritical Water Oxidation: State of the Art, Chem. Eng. Technol. 22, No. 11: 903–908. Schneider OD, Malanchuk D, Nickols D, Ashe CR (2001) Comparison of LargeScale UV Systems for Disinfection of an Unfiltered Drinking Water Supply, Conference Proceedings of the First International Congress on Ultraviolet Technologies, International Ultraviolet Assoc. (IUVA), June 14–16, Washington, DC; electronic release.

References Sonntag C von (1996) Degradation of Aromatics by Advanced Oxidation Processes in Water Remediation: Some Basic Considerations, J. Water SRT-Aqua, 45, No. 2: 84– 91. vanLoon GW, Duffy SJ (2000) Environmental Chemistry, A Global Perspective, Oxford University Press, Oxford.

Verhoeven JW (1996) Glossary of Terms Used in Photochemistry, 2nd ed., Pure Appl. Chem. 68, No. 12: 2223–2286. WEDECO (2001) WEDECO’s weltgrößte UVAbwasserdesinfektionsanlage geht in Betrieb, October: Internet, http:// www.wedeco.de.

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2

Why UV and Oxidation/Disinfection? 2.1

Global Water Resources and Resulting Water Market

Water is one of the essential chemicals for all forms of life on Earth and its quality is crucial for the future of mankind. However, resulting from the high population density and the level of industrialization the hydrosphere is increasingly polluted with inorganic and organic matter. The sensitive hydrosphere is a central part of the global ecosystem and it covers about 73% of the Earth’s surface. Its total water content is distributed between the main compartments of the atmosphere, the biosphere, the oceans and the continents. The mass of water in oceans has been estimated to be around 1.37 × 1021 kg (vanLoon and Duffy, 2000). This value corresponds to 97% of the Earth’s water (Fig. 2-1). The mass of freshwater resulting from 3% of the total global water resource is mostly bound as ice in polar caps and glaciers (79%). About 20% of the freshwater resources are found as groundwater and only 1% is thought to be easily accessible surface water located in biomass, rivers, lakes, soil moisture and distributed in the atmosphere as water vapor.

Distribution of global water resources: composition of the hydrosphere (data from vanLoon and Duffy, 2000: p. 187).

Fig. 2.1

20

2 Why UV and Oxidation/Disinfection?

However, this immense mass of freshwater is unevenly distributed throughout the world and its quality is highly variable with respect to chemical and microbiological aspects. Furthermore, ground- and surface-water resources are strongly influenced by anthropogenic processes. Hence, enormous efforts are necessary to preserve the quality of surface waters by development and improvement of municipal and industrial wastewater treatment technologies and by strengthening water quality monitoring and environmental legislation policy. Accordingly, the European Environment Agency (EEA) stresses approaches to adequate water management and sustainable water use in Europe that are unequivocally of great importance to the future development of European countries (see Smith and Scott, 2002, van de Kraats, 2001). A worldwide ban of persistent organic pollutants (POPs) will be implemented by the POP convention. It has been negotiated since 1998 by mandate of the Governing Council of the United Nations Environment Program (UNEP) (Schlottmann and Kreibich, 2001). At the international convention on POPs in Stockholm in May 2001 more than 90 countries signed a global treaty that controls the production, import, export, disposal and use of these chemicals (van de Kraats, 2001). POPs are compounds that are already prohibited or strictly regulated in industrialized countries. Their dangerous environmental potential is established, for example, by their bioaccumulation in the arctic food chain. They are distributed all over the world by seasonally dependent deposition and evaporation processes. The “dirty dozen” (Fig. 2-2) covers the chloro-organic compounds aldrin, dieldrin, endr-

The dirty dozen of persistent organic pollutants (POPs) and effects of photo-initiated processes (and of AOPs/AOTs in general) generating a convenient sink.

Fig. 2.2

2.1 Global Water Resources and Resulting Water Market

in, chlordane, mirex, toxaphene, heptachlor, hexachlorobenzene, DDT, polychlorinated biphenyls (PCBs), polychlorinated dibenzo-p-dioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs) (Schlottmann and Kreibich, 2001, see also Tab. 1-1, entry: persistent organic pollutants). These ubiquitous compounds are characterized by their persistence, bioaccumulation, toxicity and long-range transport potential (Koch and Wagner, 1989). They mostly belong to the class of “semivolatile organic compounds” (SOC) and are used as insecticides, pesticides or for their formulation, respectively (Kempter and Jumar, 1983). They pose a severe risk of causing adverse effects to the environment and to human health. However, photochemical degradation reactions are known to be sinks of pesticides in environmental compartments (Klöpffer, 1992). Many polychlorinated aliphatic and aromatic hydrocarbons are photolabile (Legrini et al., 1993) and halogenated dioxins are easily dehalogenated and hence detoxified by UV irradiation (Tschirley, 1986, Crosby, 1983). Furthermore, many POPs and similar compounds may be effectively degraded by photochemical AOPs applying auxiliary oxidants or photocatalysts (Zaleska et al., 2000, 1999, Wenzel et al., 1999, Silva et al., 1992, Slein and Sansone, 1980). Therefore, photo-initiated AOPs seem to be techniques that can annihilate POPs and many other pesticides (Chiron et al., 2000) in micropolluted aqueous solutions or as trace contaminants in waste gases by an effective POPs sink at their point of origin, for example in rinse waters resulting from washing pesticide containers (cf. Fallmann et al., 1999). There is still a tremendous need for research related to photo-initiated AOPs concerning the formation of bio-available by-products, and investigations at industrial scales (Chiron et al., 2000). According to the origin of the water from natural cycles and depending on the water quality requirements, the market for water processing mainly covers three important sectors (WEDECO, 2000), namely the treatment technologies of drinking water, wastewater and of industrial process water (Fig. 2-3). In addition, the desalination and purification of seawater and the use of rainwater are of increasing interest for future water management and the development of adequate treat-

Main segments of the water processing market with potentials for photochemical AOTs (modified according to WEDECO, 2000).

Fig. 2.3

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22


ment technologies. Owen (2002) has presented a comprehensive overview of the European water and wastewater services market including a description of regulatory and legal structures and management strategies. Accordingly, there exists a wide variety of process routes by which water can be treated in order to satisfy different specifications. Apart from biological treatment techniques these systems may incorporate desorption processes (air/steam stripping), adsorption processes (activated carbon, polymeric resins), membrane separation processes (e.g. Rosenwinkel et al., 2000) and alternative technologies such as evaporation, distillation, flocculation, precipitation, and others, and last but not least photochemical AOPs (Mersmann et al., 1995). The detailed design of the appropriate treatment system depends mainly on the raw water composition and of course on economic considerations. For example, swimming pool water, house and operational water require mainly UV disinfection treatment. On the other hand, drinking water is being increasingly micropolluted by fertilizers, pesticides and other biologically resistant pollutants, thus producing the demand for new and efficient treatment technologies. It has been shown, however, that many micropollutants can be photochemically degraded (Getoff, 1993). Furthermore, increasing drinking and wastewater charges defined by fiscal policy will favor concepts for the use of greywater in households in future. Greywater is defined as washwater excluding toilet wastes, i.e. bath, laundry and dishwater. These household waters may be collected and can be used after appropriate treatment as a valuable resource for toilet flushing systems or for operating washing machines, so reducing the consumption of valuable drinking water considerably. However, greywater in storage tanks soon becomes malodorous and anaerobic microbiological processes will prevail. For this purpose UV disinfection represents a rational treatment option. The selection of a treatment process for industrial and municipal wastewater and for industrial cooling, purging or product waters depends mainly on the specific use and the desired water quality. If necessary, it should include UV disinfection, UV detoxification, or UV purification. On the other hand, the production of pure and ultra pure water for pharmaceutical purposes or of cleaning water for computer chip manufacturing industries requires disinfection and total mineralization of the organic matter content (Bendlin, 1995). In addition, photochemical technologies are well suited for the disinfection and purification of contaminated air streams (Sebold, 1994, Ollis and Al-Ekabi, 1993). They are being increasingly applied to air pollution prevention caused by relatively small point sources, e.g. malodor treatment by UV processing (e.g. Schneider, 1997/98). Competing air purification technologies include thermal waste gas treatment, adsorption, membrane and catalytic processes, biological treatment and others (Börger et al., 2000, Ebling, 1999). The special features of UV treatment technologies of water and air that are favored by manufacturers, researchers and developers are collected in Tab. 2-1 (cf. FIGAWA, 1997, Prengle Jr et al., 1996, Freeman and Harris, 1995, von Sonntag, 1995, Meyer and Fischer, 1994). From this table it is evident that a remarkable difference in the numerical handling of UV disinfection and photo-initiated oxidation techniques ex-

2.1 Global Water Resources and Resulting Water Market Tab. 2.1 Several special features of photochemical water and air treatment technologies

UV disinfection

Photo-initiated oxidation techniques

• Inactivation of a broad range of microorganisms by adequate fluence H0/J m–2

• Cleavage of chemical bonds by adequate photon fluence Hp,0/mol m–2

• Simple and reliable disinfection procedure with high performance, safe and easy handling, storage and shipping

• Oxidative degradation of bactericidal or non-biodegradable compounds and of a broad range of toxic contaminants accompanied by diminution of water toxicity

• No concentration and accumulation of viruses, bacteria or other microorganisms

• Partial oxidation usually leads to the formation of innocuous and biodegradable low molecular weight products, such as carboxylic acids and aldehydes

• Minimum formation of chemical side prod- • Detoxification of persistent organic pollutants (POPs) possible ucts and absence of mutagenic or carcinogenic compounds • No hyper-salination effects • No influence on odor and taste of water or air

• No sludge formation, minimal treatment residues, no subsequent disposal

• No threat to maintenance staff due to use of aggressive chemicals • Point-of-use systems on household scale and for camping sites • Applications in health risk zones such as hospitals etc. • Use in food processing industries

• Photomineralization generates a real pollutant sink • Implementation of synergistic treatment concepts with conventional technologies (hybrid systems), e. g. carbon adsorption, biological treatment • Several alternatives in the selection of the auxiliary oxidant (H2O2, O3, or their mixtures) • High efficiency in removing micro-pollutants by oxidation or mineralization

Modular engineering facilitates scaling-up and systems are easily automated. Batch or continuous flow modes and treatment of large volume flows. Low preventative maintenance and operating requirements together with reliable safety during operation. Design of photocatalytic solar systems based on titanium dioxide. Decentralized and remote installations possible.

ists, i.e. the former is analyzed in terms of the fluence H0/J m–2 of germicidal UV radiation (Sommer et al., 1999), whereas the latter processes depend unequivocally on the absorbed photon fluence Hp,0/mol m–2. In this case, an absorbed photon is treated as a selective reagent used for the cleavage of chemical bonds enabling stoichiometric calculations. To avoid any confusion it must be stressed that several authors use the term “UV dose” in UV disinfection literature (Leuker, 1999, Chiu et al., 1999) instead of

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UV fluence. However, this is incorrect and misleading because the SI unit of the absorbed dose of radiation is gray/J kg–1 (Gy) (Mills et al., 1993). The unit Gy is related to a specific mass and it was defined for reasons of safeguarding human health in radiology. It is used in radiation protection. However, the overall positive statements of Tab. 2-1 succeed only by adapting optimal design and use to photochemical reactors and by taking into account the photochemical reaction mechanisms (cf. Braun et al., 1993). Hence, several disadvantages, problems and limitations related to the application of photochemical AOPs must be addressed at this point: the initial capital costs are usually higher than with competing technologies and often a pretreatment of the aqueous stream is necessary to enhance the optical properties of the water (Freeman and Harris, 1995). Furthermore, the lamps (or quartz sleeves containing the lamps) can foul substantially by formation of UV absorbing films. This fouling results in a severe decrease in the UV intensity that is received by the medium. The finite lifetime of UV lamps requires their periodic replacement and hence produces additional costs (Prengle Jr et al., 1996). Further implementation considerations, evaluation and design criteria will be discussed in later sections (cf. Chapters 8.4 and 8.5). The question in the title of the present chapter deals not only with mechanistic and technical aspects of photochemical AOPs and AOTs but also with the regulation of environmental policy. Federal authorities are responsible for concurrent legislation concerning waste management and air quality control and outline water resources management in Germany (Bosenius and Rechenberg, 1998). The Environment Ministry of the European Union (EU) prepares draft regulations and directives for the council of Environment Ministers of the member states of the EU. This complex topic, however, goes beyond the scope of this book. The amount of German literature that concerns environmental legislation is enormous: for example, over 2700 entries were collected by Lohse et al. (1996) during the period of July 1995 to June 1996! Stober (1998) collected important environmental laws. Updated versions can be found in the federal law gazette of Germany (for the Internet reference refer to Tab. 1-1).

2.2

Present and Historical Dimensions of the Radiation Concept

“In the beginning God created the heaven and the earth. And the earth was without form, and void; and darkness was upon the face of the deep. And the Spirit of God moved upon the face of the waters. And God said, let there be light: and there was light. And God saw the light, that it was good: and God divided the light from the darkness.” (Genesis, verses 3–4). The historiographers of the Old Testament knew about the predominant role of light. They categorized light as a vital factor for the development of life on our planet after the creation of the Earth, water and the atmosphere. Even today, re-

2.2 Present and Historical Dimensions of the Radiation Concept

Fig. 2.4

Processes in everyday life associated with UV/VIS radiation.

search argues that UV radiation had an essential role during the prebiotic development phase on Earth, concerned with the formation of “life molecules”, i.e. during chemical evolution (Mason, 1991). Thus, the Bible writers pronounced the positive effects of light very early (Fig. 2-4). They demonstrated with their statements that UV radiation, light and life are interrelated and that positive effects should dominate. Hence, life on Earth is only possible due the ozone shield that is formed in the stratosphere, i.e. the region of the atmosphere between 12 and 50 km above the surface. Here the intensity of the high-energy solar radiation is strong enough to produce a photostationary equilibrium between molecular oxygen and ozone (vanLoon and Duffy, 2000). This steady-state situation is very sensitive to catalytic processes leading to ozone decomposition, e.g. by intervention of chlorinated fluorocarbons (CFCs). Owing to its broad and intense absorption band between k of 200 nm and 300 nm, ozone absorbs the harmful solar radiation below k of 300 nm. Therefore, the natural and toxic pollutant ozone protects life from dangerous solar radiation and acts as the “sun glasses” of the Earth (Feister, 1990). The decrease in the ozone concentration in the stratosphere, the so called “ozone hole” (Solomon, 1999), may lead to an increase in biologically active UV radiation on the Earth’s surface (Hester and Harrison, 2000, Madronich et al., 1998). On the other hand, ozone is one of the active constituents of photochemical smog observed on hot sunny days in cities with heavy traffic (vanLoon and Duffy, 2000, Heintz and Reinhardt, 1990). The photochemical production of ozone in a polluted troposphere (the region of the atmosphere between the surface and the stratosphere) is accomplished through a complex series of reactions involving nitrogen oxides (NOx) and many other volatile organic compounds (VOCs) (Kondratyev and Varotsos, 2001).

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Most of the photo-initiated processes in the atmosphere are radical reactions. However, the hydroxyl radical (•OH) is of special significance for the chemistry of the atmosphere (Ehhalt, 1999). This reactive species is mainly responsible for the photooxidation of trace organic chemicals in the troposphere and hence for the oxidative cleansing of the atmosphere (Fabian, 1989). It is nature’s atmospheric detergent (Comes, 1994, Ravishankara, et al. 1998). Furthermore, several years ago it was well established that the interaction of UV/VIS radiation and environmental pollution seems to be responsible for the dramatic forest decline that has been observed, for example in the higher areas of the Black Forest or the Ore Mountains in Germany (Schenck, 1985). Several photobiological processes are fairly well understood and they are essential for life on Earth. For example, the processes of photosynthesis, phototropism (orientation of organisms to light) and photomorphogenesis (growth of plants as a function of light intensity) (Singhal, 1999) are important positive light-induced effects, which are even of industrial relevance. The process of vision, sun tanning of the human skin as an immanent sun protection system and photochemical vitamin D production in the skin of humans are further examples of the positive effects of UV/VIS radiation on life. One of the most fascinating photobiological processes is bioluminescence, the biological or biochemical production of light (Adam and Cilento, 1982). Typical negative effects of UV radiation on humans are the development of sunburn, the formation of skin cancer or eye damage. Additionally, phototoxicity of naturally occurring compounds and UV/VIS radiation-induced adverse effects of drugs (cf. Oppenländer, 1996, 1988) represent severe problems to human health. However, the controlled use of phototoxic mechanisms plays an important role in the photomedicinal treatment of several cancers (photodynamic therapy, PDT) or of skin diseases such as psoriasis (PUVA therapy). Knowledge of the healing capacity of plant extracts in combination with sunshine was known even in ancient times, as is exemplified by Fig. 2-5. Finally, human health may be influenced negatively, if the light intensity is too low. This phenomenon usually occurs during the winter season. The resulting psychological or physical status of individuals is designated as seasonal depression. At the end of this discussion, it has to be stressed that the beneficial and detrimental categorization of UV radiation- and light-induced effects is of a subjective nature and is only related to the needs and wishes of humans (Suppan, 1994). This assessment would read completely differently from the point of view of a microorganism that is to be inactivated by irradiation with germicidal UV radiation in a UV disinfection treatment plant. The historical development of the term light demonstrates its versatility and its significance for current developments related to AOPs (Tab. 2-2). It has been characterized by controversial discussions about the contradictory wave and particle nature of light and UV radiation from its very early beginnings. At the beginning of the 20th century, it culminated in the theory of wave/particle duality. The light or UV radiation particles are called photons (symbol: c; photochemists usually use hm). Simple physical experiments verify the dual nature of UV radiation and light (Fig. 2-6): interference and diffraction are typical attributes of electromagnetic

2.2 Present and Historical Dimensions of the Radiation Concept

Fig. 2.5 Ancient Egyptian papyrus demonstrating the combination of solar radiation and skin ointment for the treatment of skin disease (reproduced from the front page of the invitation to the 4th International Conference and Workshop on Solar Energy Storage and Applied Photochemistry, January 1997 by permission of M. S. A. Abdel-Mottaleb, Ain Shams University, Cairo, Egypt).

waves, whereas the Compton Effect and the photoelectric effect are experimental evidence for the corpuscular nature of electromagnetic radiation. In Compton scattering a photon of energy Ep = hkc collides with an electron and transfers some of its energy to the latter. Thus, the scattered photon has a lower frequency m (i.e. a greater wavelength k) than the incoming photon. This phenomenon is comparable to the balancing of energy during the collision of two billiard balls. The collision of a photon with an electron (i.e. of two bodies) can be described by the fundamental laws of physics, namely the conservation of energy and the conservation of momentum. The photoelectric effect is observed during irradiation of metals or metal alloys by electromagnetic radiation of suitable wavelength. It shows that the number of released electrons is proportional to the number of incident photons and that the kinetic energy of the ejected electrons is only dependent on the energy of the photons. The number of incident photons (i.e. the intensity of the ra-

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2 Why UV and Oxidation/Disinfection? Tab. 2.2 Some outstanding historical developments related to the terms of light and UV radia-

tion Scientist

Period

Theme

Reference

Heisenberg, W. 1901–1976

Uncertainty principle and quantum me- Mason (1961) chanics

Jablonski, A.

1898–1980

Mechanisms of fluorescence and phosphorescence, Jablonski diagram (1933)

Nickel (1998)

Einstein, A.

1879–1955

The photon as quantum of the electromagnetic field

Mason (1961)

Planck, M.

1858–1947

Quantum theory of light and radiation: wave/particle duality

Mason (1961)

Hertz, H.

1857–1922

Experimental verification of the electro- Mason (1961) magnetic theory of light

Maxwell, J. C.

1831–1879

Electromagnetic theory of light

Faraday, M.

1791–1867

Search for a connection between electri- Bugge (1929) city, magnetism and light

Mason (1961)

Berzelius, J. J.

1779–1848

Electrochemical theory of light and heat Bugge (1929)

Ritter, J. W.

1778–1810

Discovery of chemically active radiation Schwedt (1999), beyond violet light, UV radiation Mascia and Tausch (2000)

Goethe von, J. W.

1749–1832

Experiments related to the chemical effects of colored illumination

Scheele, C. W.

1742–1786

Influence of parts of the solar spectrum Bugge (1929) on silver chloride

Pristley, J.

1733–1804

Paper on the history and the discoveries Bugge (1929) in the field of light and color research

Römer, O.

1644–1710

Determination of the speed of light from astronomical observations

Newton, I.

1642–1727

Particle theory mixed with elements of Mason (1961) wave theory for the explanation of opti- Wickert (1985) cal phenomena

Huygens, C.

1629–1695

Wave properties and speed of light

Mason (1961)

Grimaldi, F.

1618–1663

Wave theory of light

Mason (1961)

Descartes, R.

1596–1650

Particle character of light

Mason (1961)

Snellius, W.

1591–1626

Law of light refraction

Mason (1961)

Hero of Alexandria

125 BC

Linearity of a light beam and the princi- Von Laue (1958) ple of fastest optical pathway

Platon

427–348/347 BC

Theory of colors by interaction of a vi- Grassi (1959) sion beam with particles emitted by objects (dialog Timaios)

Schwedt (1999)

Von Laue (1958)

2.3 Some Historical Landmarks of the Development of AOTs Experiments verifying the wave/particle duality of electromagnetic radiation, and the bridge between corpuscular and wave physics: relation of the momentum p to the wavelength k.

Fig. 2.6

diation or the radiant power P) has no influence on the kinetic energy of the ejected electrons. Thus, the connecting bridge between corpuscular and wave physics is represented by the fundamental equation of De Broglie (Eq. 2-1). p h=k mc

(2-1)

Eq. 2-1 is generated by combination of Planck’s relationship Ep = hm = hc/k with Einstein’s equation Ep = m c2. This leads to the momentum p of a photon that is now related to the wavelength k (Suppan, 1994). Eq. 2-1 associates the wavelength k with the mass m of a photon and with its velocity c, the speed of light. Hence, the photon has no rest mass. Although it appears puzzling to our minds each photon is characterized by a wavelength k and a frequency m.

2.3

Some Historical Landmarks of the Development of AOTs

Only during the past three decades, have UV-based technologies emerged as an important class of treatment processes for water and air purification. They include advanced oxidation in the homogeneous and heterogeneous phase [UV oxidation, vacuum-UV (VUV) oxidation, photocatalysis] and disinfection. Their progress is definitely coupled to the breakthroughs in exploratory photochemistry, which had barely developed as a distinct science at the beginning of the 20th century. In addition, the development status of efficient UV lamps was in a rudimentary state (refer to Skaupy, 1924 and Vogl, 1937). Although the “first law of photochemistry” had already been recognized by Grotthus in 1817 and Draper in 1843, the quantum nature of UV/VIS radiation and their consequences for photochemistry were established only decades later by Planck and Einstein (see Tab. 2-2). Nowadays, the observation by Grotthus and Draper that only electromagnetic radiation that is absorbed by a molecule leads to a

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2 Why UV and Oxidation/Disinfection? Tab. 2.3 Selected historical landmarks related to the development of AOPs and AOTs and their

mechanisms Year

Research group

Specification of AOP or AOT

Reference

1874

Downes and Blunt

Bactericidal effect of the radiant energy Rice (2001) a), Ullmann’s (1954) a) of sunlight

1879

Downes and Blunt

H2O2 decomposition by sunlight

1894

Fenton

Fe2+ catalyzed oxidation of tartaric acid Fenton (1894) (Fenton reaction)

1900

Kistiakowsky

Effect of radiation on H2O2 decomposi- Lunak and Sedlak tion (1992)a)

1907

Thiele

H2O2 decomposition by UV light

Gmelin (1966)a)

1910

Henri et al.

Large scale UV disinfection of water and pilot plant installation

von Sonntag (1987) a)

1917

Smith

First full-scale UV disinfection of water Rice (2001) a) in the USA

1922

Kornfeld

H2O2 photolysis and reaction products Gmelin (1966) a)

1931

Herzberg

Water photolysis

1932

Haber and Weiss

Radical chain mechanism of the photo- Gmelin (1966)a) lysis of H2O2

1935

Weiss

HO•2 radical in solution

Glaze et al. (1987) a)

1936

Fricke and Hart

Photomineralization by VUV irradiation

Fricke and Hart (1936)

1949

Lea

Quantum yield of H2O2 photolysis as a Gmelin (1966) a) function of UV radiant power

1953

Seitz

Surface and air disinfection by UV radiation

Ullmanns (1954) a)

1956

Taube

Photoreactions of O3 in solution, formation of H2O2 and O2

Taube (1956)

1957

Baxendale and Wilson Photolysis of H2O2


1957

Chen and Taylor

Photolysis of water vapor

Chen and Taylor (1957)

1964

Eisenhauer

Fenton’s reagent and waste water treat- Eisenhauer (1964) ment

1968

Getoff and Schenck

H2O-VUV, vacuum-UV photolysis of water

1971

Formenti et al.

Photocatalytic oxidation of single com- Sauer and Ollis pounds in the gas phase (1996) a)

1975

Garrison et al.

O3-UV, pilot scale applications


1975

Hoigné and Bader

O3 in H2O: identification of •OH radicals as intermediates by competition experiments

Hoigné and Bader (1975)

Gmelin (1966) a)

Herzberg (1931)

Getoff and Schenck (1968)

2.3 Some Historical Landmarks of the Development of AOTs Tab. 2.3 (continued)

Year

Research group

Specification of AOP or AOT

Reference

1976

Oliver and Carey

UV disinfection as an alternative to chlorination

Oliver and Carey (1976)

1976

Carey et al.

TiO2 Photocatalytic oxidation of biphe- Jakob et al. (1993) a) nyl and chlorobiphenyls

1977

Farrell et al.

O3-UV, pilot scale applications

1977

Koubek

H2O2-UV, oxidation of refractory organ- Glaze et al. (1987) a) ics in aqueous waste streams

1977

Frank and Bard

TiO2 Photocatalytic oxidation of CN–

Fujishima et al. (2000) a)

1979

Leitis et al.

O3-UV, purification process


1979

Nakayama et al.

O3-H2O2-UV, waste water treatment


1979

Berglind et al.

H2O2-UV, oxidation of contaminants in Glaze et al. (1987) a) water

1980

Glaze et al.

O3-UV, refractory species


1980

Malaiyandi et al.

H2O2-UV, TOC diminution in tap water

Malaiyandi et al. (1980)

1981

Hango et al.

O3-H2O2-UV, waste water treatment


1982

Staehelin and Hoigné Mechanism of O3 decomposition in water


1982

Forni et al.

Mechanism of O3 decomposition in aqueous solution


1983

Bothe et al.

•

OH and secondary carbon radical chemistry

Peyton (1996) a)

1983

Hustert et al.

TiO2 photocatalytic oxidation

Matthews (1991) a)

1983

Pruden and Ollis

TiO2 photocatalytic oxidation

Matthews (1991) a)

1984

Glaze et al.

O3-UV, pilot scale application


1984

Brunet et al.

O3-H2O2-UV, drinking water treatment Glaze et al. (1987) a)

1985

Duguet et al.


1985

Matsunaga et al.

TiO2-UV, bactericidal activity

Rajeshwar (1996) a)

1986

Peyton and Glaze

O3-UV, mechanistic investigations


1987

Bollyky


1990

Braun

H2O-VUV, vacuum-UV photolysis of water with xenon excimer lamps

Braun et al. (1993) a)

1992

Glaze

H2O-VUV, vacuum-UV photolysis of water (xenon excimer lamp)

Legrini et al. (1993) a)

a) Cited within this reference.


31

32


distinct photoproduct (see Pfoertner, 1986) seems trivial. However, in combination with the Stark–Einstein “quantum equivalence law” (the “second law of photochemistry”) that “each absorbed quantum of radiation causes one equivalent of a chemical reaction” (Roth, 2001) the highly sophisticated photosciences of today have emerged. Two major exceptions to the Grotthus–Draper and the Stark–Einstein laws are the chemi-excitation process and the multiphoton excitations with laser radiation (VUV, UV, VIS and IR), respectively (Kopecky, 1992). The actual beginning of photochemistry as an independent scientific discipline is related undoubtedly to the work of Giacomo Ciamician and Paul Silber in Bologna who, between 1900 and 1915, initiated systematic investigations related to the interaction of light with matter. In a remarkable lecture at a conference in New York in 1912, Ciamician had already postulated the extraordinary concept of solar energy usage (cf. Roth, 1989), which, hopefully in the 21st century, will experience its broad realization. A historical review of photochemical research activities in the 20th century was presented by Heimgartner and Hansen (1993). Here it was demonstrated that synthetic organic photochemistry has enormous potentials for the design of novel molecular structures. However, a priori this review does not mention any developments related to AOPs or AOTs, which in contrast to photochemical synthesis are of a destructive nature with respect to organic contaminants. Unfortunately, this lack of information is characteristic for the photochemical and technical literature in this field. It is symptomatic for organic photochemistry that degradation pathways of organic chemicals in water have only been systematically investigated over the past few years. However, this field of photochemical research is at least as exciting as synthetic photochemistry with respect to the understanding and illustration of complex degradation pathways! Several years ago, Glaze et al. (1987) surveyed the research concepts and the historical backgrounds related to the ozone UV and hydrogen peroxide UV processes in water. Halmann (1995) reviewed and discussed the photodegradation of water pollutants. Some selected landmarks of the development of AOPs and AOTs are collected in Tab. 2-3. It is astonishing to recognize that several basic concepts and methods of modern AOTs are more than 100 years old!

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Management in Germany, Federal Ministry for the Environment, Nature Conservation and Nuclear Safety, Bonn. Braun A M, Jakob L, Oliveros E (1993) Advanced Oxidation Processes – Concepts of Reactor Design, J Water SRT-Aqua 42, No. 3: 166–173. Bugge G (1929) Das Buch der großen Chemiker, Vol. 1 and 2, 1st ed., unveränderter Nachdruck, Verlag Chemie, Weinheim 1974.

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Fricke H, Hart EJ (1936) Studies of Reactions Induced by the Photoactivation of the Water Molecule. I, J. Chem. Phys. 4: 418– 422. Fujishima A, Rao TN, Tryk DA (2000) Titanium Dioxide Photocatalysis, J. Photochem. Photobiol. C: Photochem. Rev. 1: 1–21. Getoff N (1993) Purification of Drinking Water by Irradiation. A Review, Proc. Indian Acad. Sci. (Chem. Sci.), 105, No. 6: 373–391. Getoff N, Schenck GO (1968) Primary Products of Liquid Water Photolysis at 1236, 1470 and 1849 Angstrom, Photochem. Photobiol. 8: 167–178. Glaze WH, Kang J-W, Chapin DH (1987) The Chemistry of Water Treatment Processes Involving Ozone, Hydrogen Peroxide and Ultraviolet Radiation, Ozone Sci. Eng. 9: 335–352. Gmelin (1966) Gmelins Handbuch der Anorganischen Chemie, Sauerstoff, 8th ed., Verlag Chemie, Weinheim, Part 3: 2275–2285. Grassi E (1959) Platon, Sämtliche Werke 5, Politikos, Philebos, Timaios, Kritias. Griechische Philosphie, Bd. 6, Rohwolt, Hamburg. Halmann MM (1995) Photodegradation of Water Pollutants, CRC Press, Boca Raton. Heimgartner H, Hansen H-J (1993) Organische Photochemie im Spiegel von Helvetica Chimica Acta, Helv. Chim. Acta 76: 1027–1114 Heintz A, Reinhardt G (1990) Chemie und Umwelt, Friedr. Vieweg & Sohn, Braunschweig. Herzberg G (1931) General Discussion, On Part I, Paper 3, Trans. Faraday Soc. 27:402– 403. Hester RE, Harrison RM (2000) Causes and Environmental Implications of Increased UV-B Radiation, Royal Society of Chemistry, Cambridge. Hoigné J, Bader H (1975) Ozonation of Water: Role of Hydroxyl Radicals as Oxidizing Intermediates, Science 190: 782–784. Jakob L, Oliversos E, Legrini O, Braun AM (1993) TiO2 Photocatalytic Treatment of Water. Reactor Design and Optimization Experiments, in: Ollis DF, Al-Ekabi H (eds.) Photocatalytic Purification of Water and Air, Elsevier, Amsterdam, pp 511–532.

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2 Why UV and Oxidation/Disinfection? Kempter G, Jumar A (1983) Chemie organischer Pflanzenschutz und Schädlingsbekämpfungsmittel, 2nd ed., VEB Deutscher Verlag der Wissenschaften, Berlin. Klöppfer W (1992) Photochemical Degradation of Pesticides and other Chemicals in the Environment: a Critical Assessment of the State of the Art, Sci. Total Environ., 123/124: 145–159. Koch R, Wagner BO (1989) Umweltchemikalien, VCH, Weinheim. Kondratyev KY, Varotsos CA (2001) Global Tropospheric Ozone Dynamics, Environ. Sci. Poll. Res. 8, No.1: 57–62. Kopecky J (1992) Organic Photochemistry: A Visual Approach, VCH, Weinheim. Laue von M (1958) Geschichte der Physik, Ullstein, Frankfurt, No. 222. Legrini O, Oliveros E, Braun AM (1993) Photochemical Processes for Water Treatment, Chem. Rev. 93, No. 2: 671–698. Leuker G (1999) Description and Application of Biodosimetry – a Testing Procedure for UV Systems, J Water SRT – Aqua 48, No. 4: 154–160. Lohse S, Dörner E, Hesterberg B (eds.) (1996) Neue Literatur zum Umweltrecht, No. 69, Umweltbundesamt, Berlin. Lunak S, Sedlak P (1992) Photoinitiated Reactions of Hydrogen Peroxide in the Liquid Phase, J. Photochem. Photobiol. A: Chem. 68: 1–33. Madronich S, McKenzie RL, Björn LO, Caldwell MM (1998) Changes in Biologically Active Ultraviolet Radiation Reaching the Earth’s Surface, J. Photochem. Photobiol. B: Biol. 46: 5–19. Malaiyandi M, Sadar MH, Lee P, O’Grady R (1980) Removal of Organics in Water Using Hydrogen Peroxide in the Presence of Ultraviolet Light, Wat. Res. 14: 1131– 1135. Mascia M, Tausch MW (2000) Ein historisches Experiment 200 Jahre danach. Die Entdeckung der UV-Strahlen durch J. W. Ritter, Naturwissenschaften im Unterricht 11, No. 57: 17–21. Mason SF (1961) Geschichte der Naturwissenschaft in der Entwicklung ihrer Denkweisen, Alfred Kröner Verlag, Stuttgart. Mason SF (1991) Chemical Evolution, Origin of the Elements, Molecules and Living Systems, Clarendon Press, Oxford.

Matthews RW (1991) Environment: Photochemical and Photocatalytic Processes. Degradation of Organic Compounds, in: Pelizzetti E, Schiavello M (eds.), Photochemical Conversion and Storage of Solar Energy, Kluwer Academic Press, pp 427– 449. Mersmann A, Kutzer S, Kajszika H, Wintrich H (1995) Nichtbiologische Reinigung organisch verschmutzten Wassers, Chem.-Ing.-Tech. 67, No.1: 41–49. Meyer A, Fischer R (1994) Moderne apparative Entwicklungen zur UV-Oxidationsmittelbehandlung, Gewässerschutz-Wasser-Abwasser 143: 261–274. Mills I, Cvitas T, Homann K, Kallay N, Kuchitsu K (1993) Quantities, Units and Symbols in Physical Chemistry, 2nd ed., Blackwell Scientific Publications, Oxford. Nickel B (1998) Pioneers in Photochemistry, From Wiedemann’s Discovery to the Jablonski Diagram, Europ. Photochem. Assoc. Newslett. No. 64: 19–72. Oliver BG, Carey JH (1976) Ultraviolet Disinfection: an Alternative to Chlorination, J Water Pollut. Contr. Fed. 48, No. 11: 2619– 2624. Ollis DF, Al-Ekabi H (eds.) (1993) Photocatalytic Purification and Treatment of Water and Air, Elsevier, Amsterdam. Oppenländer T (1988) A Comprehensive Photochemical and Photophysical Assay Exploring the Photoreactivity of Drugs, Chimia 42, No. 10: 331–342. Oppenländer T (1996) The Contribution of Organic Photochemistry to Investigations of Phototoxicity, in: Tonnesen H (ed.) The Photostability of Drugs and Drug Formulations, Taylor & Francis, London, Chap. 11: 217–265. Owen DL (2002) The European Water Industry – Market Drivers and Responses, CWC Publishing, London, UK. Peyton GR (1996) Kinetic Modeling of FreeRadical Water Treatment Processes: Pitfalls, Practicality, and the Extension of the Hoigné/Bader/Staehelin Model, J. Adv. Oxid. Technol. 1, No. 2: 115–125. Pfoertner KH (1986) Practical Photochemistry: Scale-up, in: Photochemistry in Organic Synthesis, Coyle JD (ed.), The Royal Society of Chemistry, Burlington House, London, Chap. 13.

References Prengle Jr HW, Symons JM, Belhateche D (1996) H2O2/VisUV Process for Photo-Oxidation of Waterborne Hazardous Substances- C1–C6 Chlorinated Hydrocarbons, Waste Management 16, No. 4: 327–333. Rajeshwar K (1996) Photochemical Strategies for Abating Environmental Pollution, Chem. Ind. June: 454–458. Ravishankara AR, Hancock G, Kawasaki M, Matsumi Y (1998) Photochemistry of Ozone: Surprises and Recent Lessons, Science 280: 60–61. Rice RG (2001) Practical Applications of UV in Combination with Ozone, Conference Proceedings of the First International Congress on Ultraviolet Technologies, June 1416, Washington, DC, electronic release. Rosenwinkel K-H, Wagner J, Nagy J (2000) Membranverfahren in der industriellen Abwasserbehandlung, Chem.-Ing.-Tech. 72, No. 5: 433–440. Roth HD (1989) Die Anfänge der Organischen Photochemie, Angew. Chem. 101: 1220–1234; Angew. Chem. Int. Ed. Engl. 28: 1193–1207. Roth HD (2001) Selected Developments in the 20th Century Photochemistry: Historical Sketches, Europ. Photochem. Assoc. Newslett. No. 71: 37–57. Sauer ML, Ollis DF (1996) Photocatalyzed Oxidation of Ethanol and Acetaldehyde in Humidified Air, J. Catal. 158: 570–582. Schenck GO (1985) Is the New Forest Decline a Photodynamic Disease Caused by Light and Peroxyacetylnitrate, Ozone, Halocarbons and Others?, Europ. Photochem. Assoc. Newslett., No. 23/24: 15–41. Schlottmann U, Kreibich M (2001) Aus für das dreckige Dutzend, Nachr. Chem. 49, No. 5: 608–614. Schneider H (1997/98) Licht frißt Gestank, Gerüche kostengünstig ohne Rückstände beseitigen, Chemie Umwelt Technik, Special Issue: 16–18. Schwedt G (1999) Goethes Experimente mit ultraviolettem Licht, Phys. Unserer Zeit 30, No. 5: 207–210. Sebold M (1994) UV-Oxidation in der Gasphase – Verfahrensprinzip und Einsatzbereiche der UV-Oxidation zur Abreinigung kontaminierter Luft, Report No. 13/94, Landesanstalt für Umweltschutz, BadenWürttemberg.

Silva de M, Dullin J, Thiemann W (1992) Versuche zur Elimination von Lindan und Fluoranthen in Sickerwässern durch kombinierten Einsatz von UV-Licht und Wasserstoffperoxid, Vom Wasser 79: 333–353. Singhal GS (1999) Concepts in Photobiology: Photosynthesis and Photomorphogenesis, Kluwer, Boston. Skaupy F (1924) Die Entwicklung der elektrischen Lichtquellen seit der Zeit der Nernstlampe, Z. Angew. Chem. 37: 445– 447. Slein MW, Sansone EB (1980) Degradation of Chemical Carcinogens, An Annotated Bibliography, Van Nostrand Reinhold, New York. Smith PG, Scott JS (2002) Dictionary of Water and Waste Management, IWA Publishing, London, UK. Solomon S (1999) Stratospheric Ozone Depletion: A Review of Concepts and History, Rev. Geophys. 37, No. 3: 275–316. Sommer R, Cabaj A, Sandu T, Lhotsky M (1999) Measurement of UV Radiation Using Suspensions of Microorganisms, J. Photochem. Photobiol. B: Biol. 53: 1–6. Sonntag C von (1987) Disinfection with UVRadiation, in: Stucki S (ed.) Process Technologies for Water Treatment, Plenum Press, New York: 159–177. Sonntag C von (1995) Chemische Grundlagen der Oxidationsverfahren und neuere Erkenntnisse bei der Bildung von Zwischenprodukten, CUTEC-Schriftenreihe, Vogelpohl A (ed.) No. 20: 1–11. Stober R (ed.) (1998) Wichtige Umweltgesetze für die Wirtschaft, Verlag Neue Wirtschafts-Briefe, Herne/Berlin. Suppan P (1994) Chemistry and Light, Royal Society of Chemistry, Cambridge. Taube H (1956) Photochemical Reactions of Ozone in Solution, Trans. Faraday Soc. 53: 656–665. Tschirley FH (1986) Dioxin, Spektrum der Wissenschaft, April: 38–45. Ullmanns (1954) Encyklopädie der technischen Chemie, Urban & Schwarzenberg, München, 3rd ed., 5: 737. van de Kraats JA (ed.) (2001) Eur. Water Managem. News, April 18th and May 22nd. vanLoon GW, Duffy SL (2000) Environmental Chemistry, A Global Perspective, Oxford University Press, Oxford.

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2 Why UV and Oxidation/Disinfection? Vogl K (1937) Lichtquellen für photochemische Arbeitsverfahren, Die Chem. Fabrik 10, No. 27/28: 296–299. WEDECO (2000) Water Technology, Annual Report, Düsseldorf, http://www.wedeco.de. Wenzel A, Gahr, A, Niessner R (1999) TOC-Removal and Degradation of Pollutants in Leachate Using a Thin-Film Photoreactor, Wat. Res. 33, No. 4: 937–946. Wickert J (1985) Isaac Newton, Ansichten eines universalen Geistes, 2nd ed., R. Piper & Co, München.

Zaleska A, Hupka J, Silowiecki A, Wiergowski M, Biziuk M (1999) Destruction of Chlorinated Pesticides in TiO2-Enhanced Photochemical Processes, Int. J. Photoenergy 1: 79–84. Zaleska A, Hupka J, Wiergowski M, Biziuk M (2000) Photocatalytic Degradation of Lindane, p,p’-DDT and Methoxychlor in an Aqueous Environment, J. Photochem. Photobiol. A: Chem. 135: 213–2220.

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3

Interaction of UV/VIS Radiation with Matter 3.1

Photoscience in Research and Development

Photoscience covers a broad spectrum of interdisciplinary and interrelated subjects and it may be subdivided into photomedicine, photobiology, photochemistry and photophysics (Fig. 3-1). Photochemistry, in general, studies the reactions that occur through electronically excited states of molecules. Specifically, photochemistry studies the change of substance quality and characteristics by the influence of UV/VIS radiation. The mechanistic interpretation of the formation of photoproducts and their characterization and identification are typical domains of photochemistry. This research concept is strictly based on photophysics, which investigates the primary event of photon absorption by a molecule, the properties of electronically excited states and their deactivation mechanisms, such as for example fluorescence, phosphorescence and energy or electron transfer reactions, and non-

Modern photoscience and its subdomains, and the major tasks of photophysics.

Fig. 3.1

38

3 Interaction of UV/VIS Radiation with Matter

Fig. 3.2

Some technological applications of UV/VIS radiation.

radiative deactivation processes. During recent decades a multitude of industrial applications of photoscience have been developed using sophisticated photochemical engineering based on specialized process technology (Braun, 1997, Braun et al., 1993, Böttcher, 1991). The fundamental uses of UV radiation and light can be classified according to the functional behavior as an energy or information carrier or as a specific and ecologically clean reagent in the form of photons (Böttcher, 1991) (Fig. 3-2). The term “photochemical technology” includes preparative techniques for the synthesis of valuable fine chemicals (Pfoertner, 1998), solar photochemistry (Mattay, 2002, Esser et al., 1994) and AOPs or AOTs acting destructively for water and air remediation, or surface treatment and disinfection. Further challenging applications of photochemical technology comprise UV radiation-induced chemical vapor deposition (photo-CVD), stereo-lithographic computer aided design and manufacturing (CAD, CAM) which apply polymer-based technologies using laser irradiation techniques, and many others (Sahyun, 1996), such as optical information recording and solar energy conversion (Böttcher, 1991). A major industrial application of UV radiation is UV curing of inks and varnishes. In addition, physical and chemical analysis using the broad spectrum of optical sensors and biosensors (Göpel et al., 1995) and the quantification of UV/VIS radiation via analysis of calibrated photochemical reactions, so called actinometric procedures (Braun et al., 1991, Kuhn et al., 1989), are of vital importance to photoscientific research and technical developments. It is evident that photochemical engineering involves specific problems that are related to the generation of an optimal UV/VIS radiation distribution within a photochemical reactor system. In contrast to well established chemical engineer-

3.1 Photoscience in Research and Development

Fig. 3.3 Influence of interactive domains on photochemical engineering (modified according to Braun, 1997).

ing concepts (Levenspiel, 1996) that describe mass and energy balances in conventional process development, the major tasks of photochemical engineering include technology for the tuning of the lamp and of the reaction system (Fig. 3-3) (Braun, 1997, Braun et al., 1993). The absorbance spectrum of the reactant dictates the choice of effective UV/VIS radiation sources: the emission spectrum of the lamp used must match the absorbance spectrum of the molecule to a maximal extent. Additionally, the lamps must have sufficient power and appropriate geometry for successful photoreactor design (Braun et al., 1991). Owing to absorption phenomena the photoreaction will only take place in a restricted volume of the photoreactor where UV radiation (or light) is actually absorbed, the so-called photoreaction zone (Pfoertner, 1986). This specificity of photochemical processes leads to an inherent inhomogeneity of the product distribution within a photoreactor and hence a variety of different photoreactor concepts has developed. Again, in drastic contrast to conventional chemical engineering, it is in no way (through use of mixing) possible to achieve a uniform concentration profile of electronically excited molecules within the photoreactor due to their extremely short lifetimes. Furthermore, the optical properties of transparent materials (glass, natural quartz or synthetic fused silica quartz of high purity and transmittance, i.e. Suprasil quartz) used for the construction of the photochemical reactors are of primary importance in photochemical engineering (Braun et al., 1991) and are often a limiting cost factor.

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40


3.2

Physical Constants and Standard Values Used in Photochemistry

The most important physical constants of interest in photochemistry and their units as used in this book are collected in Tab. 3-1. They are taken from Kurzweil (2000) and from Lide (2000). In some cases, these values differ slightly from those reported by Mills et al. (1993).

3.3

The Electromagnetic Wave

According to McCluney (1994) “radiation is the propagation of energy in a variety of forms through space”. One possibility of energy propagation is depicted by a perpendicularly coupled oscillating electric field (E) and magnetic field (H) that travels as a transverse wave with the constant speed of light c0 and that is characterized by the wavelength k. The wavelength k is inversely proportional to the frequency m of the oscillating electromagnetic field and is described by the classical relation k = c0/m. The sinusoidal E-wave and the perpendicular H-wave are in phase at a large distance from an oscillating dipole due to the non-static nature of the latter (Shortley and Williams, 1961) (Fig. 3-4). The interaction of UV/VIS radiation with matter, i.e. atoms or molecules, takes place within their electron shell, i.e. between the atomic or molecular orbitals (AOs or MOs) by promoting one electron from its ground to an electronically excited state. By this process (i.e. energy absorption), the electron is transferred to

Schematic drawing of an electromagnetic wave with the dependence on propagation time t and propagation direction x at a large distance from the generating source.

Fig. 3.4

3.4 The Photon Stream and Planck’s Equation Schematic representation of the coupling of two electronic states by an oscillating electric field (q represents an electron in an excited state and q' an electron in its ground state).

Fig. 3.5

an AO or to an antibonding MO of higher energy, respectively. The electron transition is mainly triggered by resonance of the oscillating electric field of an electromagnetic wave with the electron representing an oscillating electric dipole (Turro, 1978, 1991). Consequently, an electron in its excited state is able to emit energy in the form of an electromagnetic wave. This fact is used to produce UV/ VIS radiation. The corresponding process is called emission of electromagnetic radiation. Thus, two electrons q and q' in different energetic states are coupled by an oscillating electric field (Fig. 3-5).

3.4

The Photon Stream and Planck’s Equation

According to the wave/particle duality (see Fig. 2-6) electromagnetic radiation may be considered as a stream of photons that has a constant velocity c0 and travels in a straight line through space. Each photon carries a discrete package of radiant energy Qk that has a defined frequency and wavelength k (Planck’s equation, Eq. 3-1). Therefore, a photon represents a “quantum of electromagnetic energy at a given frequency” (Verhoeven, 1996). This means that a photon of low frequency and hence with long wavelength possesses little radiant energy in contrast to a high frequency (short wavelength) photon. Qk =J hm hc0 =k hc0m

(3-1)

Example 3-1: The frequency m of an electromagnetic wave is 4.4970 × 1014 Hz. Calculate the corresponding wavelength k in nm and the radiant energy Qk of this photon! k c0 m

1

299792458 m s 4:4970 1014 s

Qk hm 6:62606876 10

34

1 1

6:666 10 7 m 666:6 nm

J s 4:4970 1014 s

1

2:980 10

19

J per photon

41

42

3 Interaction of UV/VIS Radiation with Matter Tab. 3.1 Most important quantities, units, symbols and standard values used in photochemistry

(Kurzweil, 2000, Lide, 2000) a) Quantity

Symbol or definition

Value

Unit

Atomic mass constant

mu = 1/12 m (12C)

1.660 538 73(13) × 10–27

kg

Avogadro constant

NA

6.022 141 99(47) × 1023

mol–1

Boltzmann constant

k = R/NA

1.380 650 3(24) × 10–23 8.617 342(15) × 10–5

J K–1 eV K–1

Electronvolt

eV (= e × V)

1.602 176 462(63) × 10–19

J

Elementary charge

e

1.602 176 462(63) × 10

C

Molar gas constant

R

8.314 472(15)

J mol–1 K–1

Molar mass

M

10–3

kg mol–1

Molar Planck constant

NAh NAhc0

3.990 312 689(30) × 10–10 0.119 626 564 92(91)

J s mol–1 J m mol–1

Molar volume, ideal gas, p = 101325 Pa (1 atm), T = 273.15 K

Vm = RT/p

22.413 996(39) × 10–3

m3 mol–1

One mol of photons

Np

6.022 141 99(47) × 1023

einsteinb)

Planck constant

h

6.626 068 76(52) × 10–34 4.135 667 27(16) × 10–15

Js eV s

Speed of light in vacuum

c0

299 792 458

m s–1

Standard atmosphere

p0

101 325

Pa

Standard temperature

T0

273.15 (ideal gas, T0 = 0 8C) 298.15 (chemical kinetics, T0 = 25 8C

K

–19

a) The values in parentheses represent the standard deviation uncertainty in the least significant digits. Example: R = 8.314472(15) J mol–1K–1 reads R = 8.314472 ± 0.000015 J mol–1 K–1; b) this is not an IUPAC sanctioned unit and therefore its use is discouraged. It is, however, widely used throughout the photochemical literature.

The energy of the 666.6 nm photon is extremely low, compared with the energy content of particular materials. For example, one barrel of petroleum (1 bbl = 158.9873 L, US petroleum) is equivalent to 5.8 × 109 J, and the annual commercial energy consumption of the world in 1996 was estimated to be about 327 × 1018 J (vanLoon and Duffy, 2000). Using the molar Planck constant NAh (Tab. 3-1) it is possible to calculate the radiant energy of one mol of photons, i.e. of NA photons, according to Eq. 3-2: Qk =J mol

1

NA hm NA h c0 =k

(3-2)

3.4 The Photon Stream and Planck’s Equation The radiant energy Qk of 1 mol of photons as a function of the wavelength in the range between k of 100 nm and 1000 nm. Fig. 3.6

This inverse proportional relationship between wavelength k and radiant energy Qk of one mol of photons (Eq. 3-2) is graphically illustrated by Fig. 3-6. The radiant energy of one mol of photons is sometimes called the molar photon energy. Example 3-2: Calculate the radiant energy Qk of 1 mol of 172 nm photons. Q172 nm

6:02214199 1023 mol

1

6:62606876 10 172 10 9 m

34

Js 299792458 m s

1

= 695503.3 J mol–1 = 695.503 kJ mol–1 The energy unit of photochemical reaction kinetics is one mol of photons (i.e. NA photons). In the photochemical literature it is often designated as “one einstein” (Bolton, 1999), but this is not an IUPAC sanctioned unit. Therefore, the use of the unit “einstein” is discouraged. The concept of electromagnetic radiation represented by a particle (i.e. photon) stream allows a photon to be treated as a reagent, that “collides” and “reacts” with a molecule or an atom by promoting an electron of a ground state molecule (or from the electron shell of an atom) to an excited state (denoted by *), i.e. to a

Fig. 3.7 Photoexcitation of a molecule by absorption of a photon: the primary process in photophysics followed by secondary processes.

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44


higher energy level (orbital). This “reaction” is the primary step in a photochemical (or photophysical) event and it is called absorption (Fig. 3-7). A ground state molecule represents a molecule with an electronic configuration of the lowest energy, and electronically excited states represent electronic configurations of a molecule with electrons in antibonding molecular orbitals (Turro, 1978, 1991). Photon absorption is the fastest process observed in nature. It occurs within a time scale of 10–15 s, i.e. within one femto second! The electronically excited molecule M* can get rid of its energy either by photophysical deactivation processes or by photochemical or photobiological pathways.

3.5

Electromagnetic Spectral Ranges of Interest in Photochemistry

The wavelength range generally utilized in photochemistry lies between k of 170 nm and 1000 nm. This is mainly due to the fact that electronically excited states M* of organic or inorganic molecules M are usually generated by photoexcitation within this wavelength range. This bandwidth is determined by the absorption characteristics of inorganic and organic molecules in a liquid or gaseous phase. The photochemically active region of the electromagnetic spectrum has been divided into five sub bands: the vacuum-UV (VUV), UV-C, UV-B, UV-A and VIS (Fig. 3-8). The subdivision of the UV spectral domain is mainly due to phenomenological reasons (Fig. 3-9) that are related to physical, biological or medicinal effects. The UV-B region is usually defined between k of 280 nm and 315 nm (van-

Classification of electromagnetic radiation in the wavelength range below k of 1200 nm as bands with specific names, and the interaction with molecules M (VUV: vacuumUV).

Fig. 3.8

3.5 Electromagnetic Spectral Ranges of Interest in Photochemistry

Loon and Duffy, 2000, McCluney, 1994), or sometimes between k of 280 nm and 320 nm (Sonnemann, 1992). Other authors define it between k of 290 nm and 320 nm because no solar radiation between 280 nm and 290 nm can penetrate through the atmosphere to reach the ground level (Moore, 1996). Ionizing electromagnetic radiation, such as deep VUV or X-ray and c-radiation, is of such high energy, that electrons are ejected from molecules, ultimately leading to photoionization (cf. Fig. 3-8). VUV radiation is absorbed by almost all substances including water, carbon dioxide and oxygen. Therefore, spectroscopic measurements below k of 200 nm have to be performed under vacuum or at least under non-absorbing gas conditions (cf. Hind, 2001). This special physical condition led to the nomenclature of this wavelength band. The absorption of VUV radiation by organic molecules in solution produces selective rearrangements of the carbon skeleton, mainly initiated by Rydberg state photochemistry (cf. Oppenländer et al., 1992, Adam and Oppenländer, 1986). However, the importance of VUV radiation to AOPs is related to the fact that it is efficiently absorbed by water in a liquid or gaseous phase. This causes the homolysis of water molecules with the formation of hydrogen atoms and hydroxyl radicals. This effect is used in the VUV oxidation process for the mineralization of organic water pollutants (cf. Oppenländer and Gliese, 2000) applying novel incoherent xenon excimer lamps at k of 172 nm (cf. Chapter 7.1.3). Absorption of UV-C radiation by cellular constituents of organisms may lead to severe cell mutations and cell death. Therefore, this wavelength band is often called the germicidal range. It is used in UV disinfection processes to inactivate bacteria, viruses and other microorganisms (cf. Chapter 9). At the Earth’s surface sunlight contains no VUV and UV-C radiation because of their efficient absorp-

Fig. 3.9 Phenomenological subdivision of ultraviolet radiation (according to DIN 5031-7) into four sub-bands and their characteristic effects.

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46


tion by oxygen (absorption at k below 200 nm) and by ozone (absorption at k below 330 nm) in the upper atmosphere, respectively (Finlayson-Pitts and Pitts, 1986). On the other hand, solar UV-B and UV-A radiation reach the Earth’s surface and may induce sunburn, skin cancer and sun tanning. The VIS range is biologically active in photosynthesis. It is absorbed by dyes, drugs and many other photosensitizers that are used for example in photomedicine for photodynamic cancer therapy (PDT). The energy of photons with a wavelength k greater than 1000 nm is too small to reach electronic states of atoms or molecules. By absorption of radiation in the infrared (IR) region of the electromagnetic spectrum molecules M are vibrationally excited (Mvib). The atoms connected by chemical bonds in a polyatomic molecule behave like mass centers and acquire vibrational motion by IR absorption, thus generating different vibrational energy levels. These play a most important role in the photophysical processes of electronically excited molecules because every electronic state of a molecule couples with vibrational levels (Suppan, 1994). This interaction between electronic and vibrational levels is called vibronic coupling (Verhoeven, 1996).

3.6

Conversion of Energy Units and Other Useful Conversions

The conversion of energy units is based on the relationships presented in Eq. 3-3 (Kurzweil, 2000). 1 Ep h m h c0 m m c02 mkm2 k T m v2 e U 2

(3-3)

This set of equations connects Planck’s photon energy Ep with Einstein’s mass/energy equivalence, with Boltzmann’s kinetic energy, with the kinetic energy of a particle and with the kinetic energy of an electron in an electric field of a voltage U of 1 V. The most important conversion factors used in photochemistry and photophysics are collected in Tab. 3-2. The spectral domains of UV radiation discussed in Fig. 3-9 and the corresponding energy conversions according to Eq. 3-3 are summarized in Tab. 3-3. In addition, this table contains information about the emission lines of several types of lamp that are of interest to photochemical AOPs. Photochemists usually use the energy unit kJ mol–1, whereas physicists prefer the unit eV (electron volt), and spectroscopists are used to wavenumber r = 1/k in cm–1 instead of wavelength. Example 3-3: Convert the radiant energy Qk of 1 eV of a photon into the unit J and calculate the corresponding wavelength of the photon! Multiplication of the elementary charge e (Tab. 3-1) with the voltage U of 1 V yields:

3.6 Conversion of Energy Units and Other Useful Conversions

Qk e U 1:602176462 10

19

C 1V 1:602176462 10

19

J

The wavelength k that corresponds to this photon energy calculates as follows: k h c0 =Qk

6:62606876 1034 J s 299792458 m s 1:602176462 10 19 J

1

1:2398 10 6 m 1239:8 10 9 m 1239:8 nm This corresponds to the wavelength k of a photon in the IR region of the electromagnetic spectrum. Several useful equations and conversions that are of general importance in photochemistry are collected in Tab. 3-4. Some illustrative examples are calculated below to demonstrate the usage of the simplified and practical conversion equations of Tab. 3-4. Example 3-4: Calculate the radiant energy of 265.4 nm photons in kcal mol–1 using the appropriate equation from Tab. 3-4. Qk

m 2:8592 10 5 kcal mol 107:73 kcal mol 9 265:4 10 m

1

Example 3-5: Calculate the wavelength k that corresponds to the energy of 436 kJ mol–1 by using the corresponding equation of Tab. 3-4. k

m 1:1963 10 4 kJ mol 2:744 10 7 m 274:4 nm 1 436 kJ mol

Example 3-6: a) Calculate the photon flow Up in mol h–1 at k = 253.7 nm of a lowpressure mercury lamp (type NN 50/81, Heraeus) that has an electric input power in × 100 = 40% at this wavePel of 50 W and a radiant power efficiency g = Pk/Pel –1 length (Pk: spectral radiant power in W nm ); b) compare this value with Up at k = 253.7 nm of a medium-pressure mercury lamp with a spectral radiant power P253.7 nm of 1.035 kW (refers to a 20 kW lamp). –9 a) Up = 30094 Wmol m = 0.153 mol photons per hour m h × 50 W × 0.4 × 253.7 × 10 –9 mol b)Up = 30094 W m h ×1035 W × 253.7 × 10 m = 7.90 mol h–1

These specific calculations yield the amount of 253.7 nm photons (mol photons) that are produced (under optimal conditions) during 1 h of irradiation with common industrial low-pressure or medium-pressure mercury lamps, respectively. Also, Up may be expressed in mol photons per second which is equivalent to the older unit einstein s–1, or the number of photons per second emitted by the lamp can be calculated (Tab. 3-4).

47

L mol L mol–1 cm–1

–

Molar (decadic) absorption coefficient

Molar absorption coefficient a) ek, liquid phase at 298 K, (logarithmic base 10)

Napierian logarithm (ln)

cm

–1

kW h

Kilowatt hour –1

gal

eV

Electronvolt

in

eV

Electronvolt

Gallon (U.S.)

–

Decadic logarithm (log)

Inch

cal

Calorie

atm

Atmosphere (standard) atm

Å

Ångstrom

bar

Å

Ångstrom

Bar

cm2

Absorption cross section, r (logarithmic base e)

Atmosphere (standard)

Unit

To convert from

Tab. 3.2 Selection of useful conversion factors

2

m mol

J

m

m

3

–1

–1

kJ mol

J

–

J

kPa

kPa

Pa

nm

m

L mol–1cm–1

Unit

Decadic logarithm (log)

–

Absorption coefficientb), ak gas cm–1 atm–1 phase at 298 K, (logarithmic base e)

Molar (decadic) absorption coefficient

Joule

Meter

Cubic meter

Energy per mol

Joule

Napierian logarithm (ln)

Joule

Kilopascal

Kilopascal

Pascal

Nanometer

meter

Molar absorption coefficient, ek (logarithmic base 10)

To

Lide (2000)

2.3026

10.619

0.1

3.6 × 10

6

–

Bolton and Cater (1994) c)

–

Lide (2000)

Lide (2000)

Lide (2000)

3.785412 × 10 2.54 × 10–2

Lide (2000)

96.4853 –3

– 1.602177 × 10–19

Lide (2000)

Kurzweil (2000)

Kurzweil (2000)

Lide (2000)

0.43429

4.184

100

101.325

101325

Lide (2000)

Lide (2000)

1 × 10–10 0.1

Finlayson-Pitts and Pitts (1986)

Reference

3.82 × 10–21

Multiply the latter by

48


ppmv ppm ppm Torr Ws

Parts per million (gas phase)

Parts per million (gas phase, 1013 hPa, 25 8C)

Parts per million d) (gas phase, 1013 hPa, 298 K)

Torr

Watt second

Joule

Pascal

Mass density

number of molecules, atoms or radicals per cm3

Volume-%

To

40.9 × M e)

lg m–3

J

1

133.3224


2.46 × 1013

cm–3

Pa

vanLoon and Duffy (2000)

10–4

%

Lide (2000)

Kurzweil (2000)


Reference

Multiply the latter by

Unit

a) Absorbance A in the liquid phase is defined as A = log(P0k/Ptrans ) = ek ci l. k b) absorbance A in the gas phase is defined as A = ln(P0k/Ptrans ) = akpi l with P0k: incident spectral radiant power at wavelength k, Ptrans : transmitted spectral radiant k k power at wavelength k (Verhoeven, 1996). c) several other useful conversion factors for changing absorption coefficients from one set of units to another (either logarithmic base e or base 10) can be found in Finlayson-Pitts and Pitts (1986), and conversion factors for chemical kinetics (second order rate constants) are collected in Lide (2000). d) in aqueous solutions the unit ppm equals “grams of solute per million milliliters of solution“: ppm = lg ml–1 or mg L–1, implying the density of water is 1.00 kg L–1 (vanLoon and Duffy, 2000). e) M: molecular mass in g mol–1.

Unit

To convert from

Tab. 3.2 (continued)

3.6 Conversion of Energy Units and Other Useful Conversions 49

50

3 Interaction of UV/VIS Radiation with Matter Tab. 3.3 Spectral domains of electromagnetic radiation (VUV, UV, VIS and IR), energy conversions,

principle emission lines of mercury arc lamps, and of some novel incoherent excimer lamps Domain

Wavelength a) k/nm

Wavenumber m/cm–1

Frequency m/1014 Hz

Molar photon energy Photon energy Qk/eV Qk/kJ mol–1

VUV

100

100000

29.98

1196.27

12.40

172 b) (Xe2*) 184.9 (LP-S) 194.2 (LP-S)

58140 54083 51493

17.43 16.21 15.44

695.50 646.98 616.00

7.21 6.71 6.38

200

50000

14.99

598.13

6.20

222 c) (KrCl*) 248 (MP) 253.7 (LP, MP) 265.2 (MP) 265.4 (MP) 270 (MP) 275 (MP)

45045 40323 39417

13.50 12.09 11.82

538.86 482.37 471.53

5.58 5.00 4.89

37707 37679 37037 36364

11.30 11.30 11.10 10.90

451.08 450.74 443.06 435.01

4.68 4.67 4.59 4.51

280

35714

10.71

427.24

4.43

280.4 (MP) 289 (MP) 292 (MP) 296.7 (MP) 302.1 (MP) 308 d) (XeCl *) 312.6 (MP) 313.1 (MP)

35663 34602 34247 33704 33102 32468 31990 31939

10.69 10.37 10.27 10.10 9.92 9.73 9.59 9.57

426.63 413.93 409.68 403.19 395.98 388.40 382.68 382.07

4.42 4.29 4.25 4.18 4.10 4.03 3.97 3.96

315

31746

9.52

379.77

3.94

334.1 (MP) 365 (MP) 366.3 (MP)

29931 27397 27300

8.97 8.21 8.18

358.06 327.74 326.58

3.71 3.40 3.38

380

26316

7.89

314.81

3.26

391 (MP)

25575

7.67

305.95

3.17

400

25000

7.49

299.07

3.10

404.6 (MP) 407.8 (MP) 435.8 (MP) 492 (MP) 546.1 (MP) 577 (MP) 579.1 (MP)

24716 24522 22946 20325 18312 17331 17268

7.41 7.35 6.88 6.09 5.49 5.20 5.18

295.67 293.35 274.50 243.14 219.06 207.33 206.57

3.06 3.04 2.84 2.52 2.27 2.15 2.14

t 780

12821

3.84

153.37

1.59

800

12500

3.75

149.53

1.55

UV-C t

UV-B t

UV-A t

VIS t

IRe)

t

3.7 Photon Energies, Bond Dissociation Energies, Threshold Wavelengths and Absorption Onset a) Spectral domains are designated according to German national norm (DIN 5031-7), see Figures 38 and 3-9. All other wavelengths correspond to emission lines of mercury arc lamps, with the exception of: b) emission maximum kmax of a xenon excimer (Xe2*) lamp. c) kmax of a krypton chloride excimer (KrCl*) lamp. d) kmax of a xenon chloride excimer (XeCl*) lamp (Kogelschatz, 1992). Mercury lamp types: LP-S, low-pressure mercury lamp with Suprasil quartz envelope; LP, low-pressure mercury lamp with ordinary quartz envelope; MP, medium-pressure mercury lamp with quartz envelope (cf. Pfoertner, 1998, Braun et al., 1991). e) according to DIN 5031-7 infrared radiation is subdivided into near IR (NIR, IR-A: 780–1400 nm, IR-B: 1400–3000 nm), medium IR (MIR, IR-C: 3000–50 000 nm) and far IR (FIR: 50 000–106 nm).

Example 3-7: Calculate the amount of hydrogen peroxide that is theoretically decomposed by irradiation of its aqueous solution (V = 1 m3) within 1 h by a 10 kW medium-pressure mercury lamp that has a radiant power Plamp of 682 W at an ac = (248 + 253.7 + 265)/3 of 256 nm. The quantum yield of tive average wavelength k H2O2 decomposition UH2O2 equals 0.5 at 253.7 nm (Bolton and Cater, 1994). DH2 O2 =molL

1

8:3593

mol 0:5 3600 s 682 W 256 10 9 m Wms 1000 L

DH2 O2 2:63 10 3 mol L

1

2:63 mmol L

1

Under conditions of total absorbance of the UV radiation by H2O2, a theoretical maximum amount of 2.63 mmol hydrogen peroxide per liter can be decomposed by irradiation with the specified lamp by consuming an energy Eel of 10 kW h. Example 3-7 convincingly demonstrates the relationship of photons to stoichiometric calculations of photochemical reactions (Tab. 3-4, section D). Additional examples of calculations concerning photons and energy of lamps can be found in Wöhrle et al. (1998). More important for further discussions is the fact, that the achievable concentration change D[M] of a photochemically active substrate M under conditions of total absorbance is directly proportional to the quantum yield U of the photoreaction, to the irradiation time t, to the radiant power Plamp, to the wavelength k of the UV radiation applied and reciprocally proportional to the irradiated volume V (see Tab. 3-4, section D).

3.7

Photon Energies, Bond Dissociation Energies, Threshold Wavelengths and Absorption Onset of Molecules

For many applications and for the understanding of the photochemical reactivity of molecules of environmental significance it is necessary to compare the homolytic bond dissociation enthalpies D0298 and the corresponding photon energies kTHW (threshold wavelengths) in relation to the absorption onset of these molecules (Bolton and Cater, 1994). D0298 is identical with the bond strength of a de-

51

52

3 Interaction of UV/VIS Radiation with Matter Tab. 3.4 Simplified, practical and easy to use conversions in photochemistry and photochemical

engineering Description A) Conversion of radiant energy Qk of one mol of photons b) of wavelength k to different energy units

Simplified conversion equations a) Qk

1:1963 10 k=m

2:8592 10 k=m

4 kJ m mol

5 kcal m mol

kJ mol

1

kcal mol

1

1:2399 10 6 eV m eV k=m

m m B) Conversion of energy E 1:1963 10 4 kJ 2:8592 10 5 kcal mol mol kJ kcal with different units to the en- k=m E= mol E= mol ergy equivalent of wavelength k in m 1:2399 10 6 eV m E=eV

C) Conversion of radiant power Plamp in watts emitted by a lamp to the photon flow Up c) in mol photons per hour d), mol photons per second, or photons per second e) at a defined wavelength k

UP d)/mol h–1 = 30094

mol Plamp =W k=m Wmh

UP/mol s–1 = 8.3593 ×

mol Plamp =W k=m Wms

1 Plamp =W k=m UP e)/s–1 = 5.0341 × 1024 Wms

D) Calculation of the variation D[M] = [M]t = 0 – [M]t of the concentration (D[M]) of mol UM t=s –1 Plamp =W k=m a reactant M as a function of D[M]/mol L = 8.3593 Wms V=L f) irradiation time under conditions of total absorption of UV radiation at wavelength k a) Derived by numerical calculation using the physical constants of Table 3-1 and conversion factors of Table 3-2. For generality the wavelength k is used with the unit m. b) sometimes designated as molar photon energy Qk /kJ mol–1 (cf. Table 3-3). c) the photon flow Up is defined as the amount of photons Nk per unit time, Up = dNk /dt, the SI unit is mol s–1, its equivalent is einstein s–1 (Verhoeven, 1996). d) according to Pfoertner (1998), derived from Qk = Nk × NA × h × c0/k with Qk /W s = Plamp × t and solving this equation to UP = dNk/dt = Plamp × k × 3600/(NAhc0). e) Up = Plamp × k/(h c0). f) according to Braun et al. (1991, Chap. 4.2, p. 158), cf. Table 3-8, derived from [M]t=0 – [M]t = EP,abs × UM × t/l, with EP,abs = photon irradiance expressed in mol s–1 cm–2, UM = quantum yield of the photochemical reaction at wavelength k, t = irradiation time in s, l = optical trajectory of solution traversed by the light in cm. According to definition EP,abs = l × UP/NA ×V, with V = volume in cm3 of solution irradiated, and using Up in photons s–1 representing the absorbed photon flow as defined above by C).

3.7 Photon Energies, Bond Dissociation Energies, Threshold Wavelengths and Absorption Onset Tab. 3.5 Relationship of homolytic bond dissociation enthalpy D0298, threshold wavelength kTHW

and absorption onset konset of selected molecules in the gas phase Molecule

Primary products a)

D0298/kJ mol–1

Hydrogen peroxide, H2O2

2 •OH

213 c)

562

310 d)

305c)

392

400 e)

e)

127

126 e)

206.3 c)

580

400 e)

498.4 e)

240

190 d)

e)

1123

330 d)

Nitrogen dioxide, NO2 Nitrogen, N2 Nitrous acid, HNO2

NO + •O•(3P)

•

4

2

N( S) + N( D) NO + •OH

•

945

Threshold wavelength b) kTHW/nm

Absorption onset konset/nm

Molecular oxygen, 3O2

2 •O•(3P)

Ozone, O3

3

O2 + O ( P)

106.5

Ozone, O3

O2(1Dg) + (1D)

387 e, f)

309

330 d)

•

305 c)

392

290 e)

H• + •OH

498 c)

240

190 d)

Trichlorofluoromethane, CCl3F (CFC-11) Water, H2O

•

• 3

CCl2F + •Cl

a) The notations in parentheses are term symbols that denote the electronic energy states of atoms or molecules (cf. Huheey, 1988). b) calculated using kTHW/nm = 1.1963 × 105/D0298/kJ mol–1 (cf. Table 3-4, section B). c) Lide (2000). d) Bolton and Cater (1994). e) vanLoon and Duffy (2000). f) this value takes into account the excitation energies (Eex) to form 1O2 and O(1D) which are roughly 90 kJ mol–1 and 190 kJ mol–1, respectively: D0298 (O3)gas = DH0f (O)gas + Eex (O) + DH0f (O2)gas + Eex (O2) – DH0f (O3)gas = 249.2 + 190 + 0 + 90 – 142.7 (vanLoon and Duffy, 2000).

fined chemical bond in a polyatomic molecule. It is also related to the threshold wavelength kTHW (conversion according to Tab. 3-4) that is defined as “the maximum wavelength for which the photon energy matches the bond energy of the bond that breaks in the photolysis reaction” (Bolton and Cater, 1994). On the other hand, the absorption onset konset is defined as that specific wavelength k where absorption of UV/VIS radiation of a molecule begins. Specifically, it represents the wavelength k where the absorption coefficient a (gas phase) is about 0.1 cm–1 atm–1 or the molar absorption coefficient e (liquid phase) reaches 1 L mol–1 cm–1 (Bolton and Cater, 1994). Several bond dissociation enthalpies D0298 in kJ mol–1 and threshold wavelengths kTHW in nm of environmentally important molecules together with their absorption onset konset are collected in Tab. 3-5. The discussion of these data is made easier via their graphical representation as shown in Fig. 3-10. As can be deduced from the examples presented in Fig. 3-10 the threshold wavelength kTHW calculated from the bond dissociation energy D0298 lies slightly below the absorption onset konset only for the compounds nitrogen dioxide and ozone (see Tab. 3-5). The photodissociation of NO2 leads to the forma-

53

54


Fig. 3.10 Bond dissociation enthalpies D0298/ kJ mol–1 versus threshold wavelengths kTHW/ nm and the absorption onset konset/nm of some environmentally significant molecules,

which is represented by a dot (•), cf. Table 35. kTHW/nm = 1.1963 × 105/D0298/kJ mol–1 (cf. Table 3-4, section B).

tion of oxygen atoms in the ground state (3P). This photoreaction is in part responsible for the initiation of the oxygen atom chemistry of the atmosphere, especially in photochemical smog events (vanLoon and Duffy, 2000). According to the spin conservation theory ground state oxygen atoms (3P) react rapidly with ground state oxygen molecules O2 (3R) to form ozone, finally leading to high atmospheric ozone concentrations in polluted area. For ozone, the threshold wavelength kTHW calculates to 309 nm, this being slightly below the absorption onset (Tab. 3-5). Therefore, radiation of this wavelength or less is absorbed by ozone and initiates the bond dissociation with formation of electronically excited cleavage products, i.e. singlet molecular oxygen (1O2) and singlet oxygen atoms (1D) (Braun et al., 1991). The latter are responsible for the formation of hydroxyl radicals in the atmosphere by spin-allowed reaction with water vapor (vanLoon and Duffy, 2000). Because ozone absorbs strongly in the UV-B and UV-C band of the electromagnetic spectrum it protects the Earth from harmful solar radiation in this wavelength region. In the upper atmosphere of the Earth (above 50 km) ozone is produced by reaction of triplet molecular oxygen (3O2) with ground state oxygen atoms (3P) that are formed by photolysis of 3 O2 via absorption of solar VUV radiation (cf. vanLoon and Duffy, 2000). The situation is completely different for the other molecules depicted in Fig. 310 and Tab. 3-5 with respect to their threshold wavelength and absorption onset. All the substrates depicted in Fig. 3-10 having an absorption onset konset below the D0298/kTHW equivalence curve form photoproducts that have excess energy, which is usually either of vibrational or translational nature. For example, the absorption onset of hydrogen peroxide (konset = 310 nm) is considerably lower than the calculated threshold wavelength kTHW (562 nm), which corresponds to the energy necessary for homolysis of the central O–O bond of the molecule. This means, that irradiation of H2O2 with green light at k of 562 nm has absolutely no effect, because H2O2 is transparent at this wavelength. Another important consequence of this high-energetic absorption onset of H2O2 is the fact that irradiation

3.8 Absorption of UV Radiation by Molecules

of H2O2 at wavelengths k below 310 nm provides excess energy to the molecule. This inevitably leads to the formation of hydroxyl radicals with excess vibrational (or translational) energy. This fact is assumed to be one possible explanation for the formation of “free” hydroxyl radicals in water by escaping the water (solvent) cage (Prousek, 1996). The phenomenon of cage recombination of hydroxyl radicals in water will be discussed in Chapter 6.6.

3.8

Absorption of UV Radiation by Molecules

The principles of photochemistry and photophysics related to the interaction of UV/VIS radiation with matter are discussed extensively in several excellent monographs (refer to Chapter 10). However, some comments that are necessary for the understanding of the processes involved in photochemical advanced oxidation will be discussed briefly. Fundamentally, physical phenomena (reflection, refraction and scattering) have to be distinguished from photophysical (absorption, luminescence, etc.) and photochemical processes (formation of photoproducts) as is demonstrated schematically by Fig. 3-11. The physical phenomena are of considerable interest in sophisticated photoreactor modeling, because many photoreactors used for water treatment possess an air/quartz/water interface that influences the fluence rate distribution within a photoreactor. This is especially important for drinking water applications where the optical transmittance of water is usually very high (Bolton, 2000 a). The absorbed radiation is used to produce electronically excited states of the sample molecules initiating photophysical deactivation processes or the formation of photoproducts. The intensities of the incident and of the transmitted beam are correlated by the Beer–Lambert law (see Eq. 3-4), which quantitatively describes the attenuation of UV/VIS radiation by transmitting any absorbing medium.

Fig. 3.11 Schematic representation of physical, photophysical and photochemical phenomena and processes observed during interaction of UV/VIS radiation with matter.

55

56


3.9

The Beer–Lambert Relationship

To assess the photochemical potential of a reaction system it is essential to know its absorbance spectrum. The strength of the absorption of UV/VIS radiation as a function of the wavelength k is given by the well known Beer–Lambert law of absorbing molecules in solution (Eq. 3-4) or of absorbing gas phase components (Eq. 3-6). The relationship A = f(k) is called the absorbance spectrum. Liquid phase (solution): A10 log

Pk0 1 log Pktrans T

log T ek c l

(3-4)

Where, A10 is the (decadic) absorbance of a beam of collimated monochromatic radiation in a homogeneous isotropic medium (Verhoeven, 1996); P0k is the incident spectral radiant power; Ptrans is the transmitted spectral radiant power; k ek is the molar (decadic) absorption coefficient in L mol–1 cm–1 (SI unit: m2 mol–1, for the conversion factor see Tab. 3-2); c is the concentration of the substrate in mol L–1; l is the thickness of the solution traversed by the UV/VIS radiation (also called pathlength of irradiation). The transmittance T is the ratio of transmitted spectral radiant power Ptrans to k that incident on the sample P0k. Attenuance D is defined as the negative decadic logarithm of the transmittance T, i.e. D = –log T. D reduces to absorbance if reflection and scattering of the incident beam are neglected. Traditionally, many authors use the spectral radiant intensity Ik instead of the spectral radiant power Pk (Verhoeven, 1996). In addition, Eq. 3-4 may be expressed in its exponential form (Eq. 3-5): Pktrans Pk0 10

ecl

and Pkabs Pk0

Pktrans Pk0 1

10

ecl

(3-5)

Where Pabs k is the absorbed spectral radiant power. For the sake of simplicity, in many cases it is adequate to express P0k as 100%. For example, absorption of 99% of an incident beam means that the absorbance Pk0 = –log T = log 100% becomes A10 = log Ptrans 1% = 2. k In photochemistry and photochemical technology, conditions with A10 5 2 are usually designated as total absorbance, or in the past as total absorption (Braun et al., 1991) of UV/VIS radiation. Thus, for A10 5 2 the transmittance T is less than 0.01. In the gas phase the (napierian) absorbance Ae is proportional to the partial pressure p of the absorbing species at a specified reference temperature (usually T = 298 K) and to the pathlength l of the radiation absorbing column (Eq. 3-6).

3.9 The Beer–Lambert Relationship

Gas phase : Ae ln

Pk0 ak p l Pktrans

(3-6)

With ak being the absorption coefficient with the common unit cm–1 atm–1 (Finlayson-Pitts and Pitts, 1986). On the other hand, physicists usually use the number of absorbing molecules N per cm3 in gas phase photochemistry instead of the pressure p of the absorbing gas component leading to Eq. 3-7. Gas phase : Ae ln

Pk0 rN l Pktrans

(3-7)

With r representing the absorption cross section in cm2 per molecule (for the conversion to ek see Tab. 3-2). To avoid any misinterpretations it is very important to state the origin of the absorption coefficients ek and ak with respect to their logarithmic base as decadic or as Napierian (for conversions see Tab. 3-2). Many useful conversion factors for changing absorption coefficients from one set of units to another have been summarized by Finlayson-Pitts and Pitts (1986). To get a feeling for the magnitude of these values, several absorption coefficients and absorption cross sections of some molecules of environmental importance are presented in Tab. 3-6. Especially, for AOPs it is essential to note that the absorbance A is an additive property (cf. Braun et al., 1991), with the consequence that the individual compounds of a wastewater or a gas mixture may compete for the absorption of the incident spectral radiant power. Hence, the concentrations ci of any radiation absorbing species i present in water or air must be considered as well as their individual molar absorption coefficients ek,i. Therefore, the Beer–Lambert law changes to Eq. 3-8, which describes the absorbance A of a multi-component mixture at a specified wavelength k. A10 log

ni X Pk0 ek;n cn trans ek;1 c1 ek;2 c2 :::::ek;i ci l l Pk n1

(3-8)

According to Pfoertner (1998) the absorbed spectral radiant power Pabs k,i of a specific absorbed by all other component i is related to the spectral radiant power Pabs k,n components of a mixture according to the relationships of Eqs. 3-9 and 3-10. abs Pk;i abs Pk;n

ek;i ci l Pn l n1 ek;n cn

0 with Pabs k,n = Pk (1–10

l

(3-9) Pn n1

ek;i ci Pk0 1 n1 ek;n cn

abs Pk;i Pn

ek;n cn

), leads to Eq. 3-10:

10

l

Pn n1

ek;n cn

(3-10)

57

58

3 Interaction of UV/VIS Radiation with Matter Tab. 3.6 Absorption coefficients ek and absorption cross sections rk of some simple molecules of environmental importance (subscript k/nm)

Molecule

Molar absorption coefficient a)/ Reference absorption cross section b)

Br2 (gas phase)

e420 = 200 L mol–1cm–1

Pfoertner ( 1998)

C6H6 (solution)

log(e262) = 2.41

Hesse et al. (1997)

CH2Cl2 (gas phase)

e222 = 0.15 L mol cm

Perkampus (1992)

CH3–CHO (solution)

e293 = 12 L mol cm

Hesse et al. (1997)

CH3OH (neat)

e172 = 162 L mol–1cm–1

Heit et al. (1998)

Cl2 (gas phase)

e330 = 66 L mol cm

Pfoertner ( 1998)

–1

–1

–1

–1

–1

–1

Cl2C=CCl2 (gas phase)

–1

e222 = 5500 L mol cm

Perkampus (1992)

Cl2C=CHCl (gas phase)

e222 = 4000 L mol–1cm–1

Perkampus (1992)

ClNO (gas phase, 298 K)

r340 = 13.7 × 10


–1

–20

trans-ClHC=CClH (gas phase) e222 = 1200 L mol

2

cm –1

–1

cm

Perkampus (1992)

F2 (gas phase)

e284.5 = 6 L mol–1 cm–1

H2C=CH2 (solution)

e165 = 16 000 L mol cm

Hesse et al. (1997)

r290 = 1.13 × 10 cm r350 = 0.03 × 10–20 cm2


H2O2 (gas phase, 298 K)

–1

c)

–20

Pfoertner ( 1998) –1

2

I2 (gas phase)

e520 = 950 L mol–1cm–1

Pfoertner ( 1998)

NH3 (solution)

e194 = 5700 L mol cm

Hesse et al. (1997)

NO2 (gas phase, 298 K)

r215 = 40.2 × 10–20 cm2 r410 = 57.7 × 10–20 cm2


O3 (aqueous solution)

r290 = 3.45 ×10–18 cm2


O3 (gas phase, 298 K)

r254 = 1.41 × 10–17 cm2


cis-ClHC=CClH (gas phase)

e222 = 500 L mol cm

Perkampus (1992)

–1

–1

–1

–1

a) e, logarithmic base 10. b) r, logarithmic base e. c) cf. Table 6-2 for data in aqueous phase.

Pn The exponential term of Eq. 3-10 becomes negligible for 10 l n1 ek;n cn 1, i.e. if Pn the absorbance A l n1 ek;n cn of the additional components n is large compared with the absorbance of the component i. This is true in many cases for AOPs using hydrogen peroxide as an auxiliary oxidant to purify highly contaminated water, such as wastewater from industrial origin or from landfill leachates. The term a/cm–1 = ek,ncn is called the (decadic) absorption coefficient of a compound. This term a determines the thickness l of the photoreaction zone (i.e. the penetration depth l). High values of the molar absorption coefficient e (which is substrate specific and wavelength dependent) and high concentrations of a sub-

3.9 The Beer–Lambert Relationship

strate and hence a high value of a, reduce the thickness of this reaction zone considerably; a–1 corresponds to the penetration depth l if the absorbance A = 1.00. Example 3-8: Calculate the spectral radiant power Ptrans (in %) transmitted by a k wastewater with an absorbance A10 of 3.4 at k of 253.7 nm (A10 was determined by UV spectrophotometry).

Pktrans

Pk0 Pktrans

+

A10 log

Pk0 103:4 with Pk0 100% Pktrans

100% 0:04% 103:4

Example 3-9: The (decadic) absorption coefficient aH2O = eH2O × cH2O of pure water at the wavelength k of 172 nm is ca. 550 cm–1 (Weeks et al., 1963). Calculate the penetration depth of the 172 nm radiation into water under conditions of total absorbance (A10 = 2). A10 ek c l aH2 0 l l

A10 2 aH2 O 550 cm

1

0:00364 cm 36:4 10

6

m 36:4 lm

This calculation demonstrates that 99% of the incident 172 nm radiation (VUV radiation) is absorbed by water within a zone of a thickness l of 36.4 lm. This value represents the reaction zone of VUV-initiated photochemistry in water (see Fig. 7-12). Example 3-10: From the UV/VIS spectrum of a wastewater its absorbance A10 is determined to be 2.4 at the wavelength k of 253.7 nm (pathlength l = 1 cm). Calculate the necessary concentration of hydrogen peroxide in mol L–1 that guarantees an absorption of UV radiation at this wavelength of 90% by H2O2 [e253.7(H2O2) = 19.6 L mol–1 cm–1]. ek;H O cH O abs Pk;H Pn 2 2 2 2 Pk0 1 2 O2 n1 ek;n cn

10

l

Pn n1

en cn

Inserting the numerical values yields the following expression: 90% = to yield:

19:6cH2 O2 2:4

cH2 O2

× 100% × (1–10–2.4), where the exponential term can be neglected

90 2:4 0:110 mol L 100 19:6

1

This simple calculation clearly demonstrates the large concentration of hydrogen peroxide that must be used in highly UV absorbing wastewater to exploit effec-

59

60


tively the incident UV radiation for hydrogen peroxide photolysis. Unfortunately, this is due to the low molar decadic absorption coefficient of hydrogen peroxide at k of 253.7 nm in aqueous solution, which is exceptionally low for a primary absorber in photochemical AOPs (see Tab. 6-2). Example 3-11 (adapted from Bolton et al., 1998): A medium-pressure mercury lamp of 1 kW electric input power was used in the H2O2-UV process. The lamp delivers 4 × 10–4 einstein per second in the active wavelength range between k of 200 nm and 300 nm with a radiant power efficiency g of 25%. Calculate the amount of hydroxyl radicals formed from the photolysis of hydrogen peroxide during 1 h of irradiation under conditions of total absorbance by H2O2. The quantum yield of •OH radical formation is U (•OH) = 0.98 (cf. Legrini et al., 1993). This value implies that only one •OH radical is formed via absorption of one (to be exact 0.98) photon by hydrogen peroxide. Photon flow of the lamp: Up = 4 × 10–4 einstein s–1 × 3600 = 1.440 mol (photons) h–1. Amount of hydroxyl radicals formed: n (•OH) = 1.440 mol h–1 × 0.98 = 1.411 mol h–1. Therefore, per kWh about 1.4 mol of •OH radicals are produced.

3.10

The Nature of Electronically Excited States

The energetic situation of electronic excitation in comparison with common thermal or so-called “dark” reactions (ground state reactions) is shown in Fig. 3-12. By excitation of a ground state molecule M0 higher excited states are usually reached that deactivate very rapidly to the lowest electronically excited state M* of the molecule. In addition to a number of other deactivation mechanisms (luminescence emission or non-radiative deactivation), M* can react via a transition state TS* to the photoproduct P'. As in the ground state reaction M0 ? P an activation barrier must be overcome in the excited state reaction M* ? P' (EA and EA*, respectively). However, the activation energy EA* is much smaller than the EA of the ground state reaction. As a consequence of this scheme the photochemical reaction is in fact the thermal reaction of the excited electronic state M* (Suppan, 1994). The most obvious advantage of photochemical reactions is that photoproducts P' can be reached which are thermodynamically unfavorable in ground state reactions (Kopecky, 1992). Two successive approaches to the understanding of photophysical and photochemical processes and of the nature and properties of electronically excited states in molecules have to be considered (Turro, 1978, 1991): a) Specification of the origin of the electrons in an electronically excited molecule and in its ground state with respect to the theory of chemical bonds, and the electron transitions between molecular orbitals. The electron shell of molecules is constructed by linear combination of atomic orbitals (LCAO method) of the participating atoms to form bonding r and p molecular orbitals (MOs), non-bonding n (lone pairs of electrons) and anti-bonding

3.10 The Nature of Electronically Excited States

Fig. 3.12 Energy (E) diagram of ground state (“dark”) reactions and UV/VIS radiation-induced reactions of a molecule M. The values stated for the activation energies (EA) and the reaction enthalpies (DH0) are rough estimates and vary considerably with the nature of the substrate M (TS: transition state).

r* and p* MOs, respectively (Turro, 1978, 1991). Electrons in bonding orbitals stabilize a molecule, whereas anti-bonding electrons destabilize a chemical bond. Almost all stable ground state molecules have antiparallel spin orientations of their bond electrons designated as the S0 state, i.e. all electrons are “spin-paired”. One of the most important exceptions to this rule is the oxygen molecule that has two electrons in two degenerated antibonding orbitals (p*) which, according to Hund’s rule, have parallel spin orientation (::). Therefore, the ground state of molecular oxygen is a triplet state T0, denoted as 3O2. This results directly from construction of the molecular orbital (MO) diagram by linear combination of oxygen atomic orbitals (AOs) as is demonstrated by Fig. 3-13. The electronic transition of 3O2 to 1 O2 involves spin conversion of the electron from the ppy* orbital and its incorporation into the ppx* orbital. Thus, a spin-paired situation is generated, this being the first excited singlet state of oxygen. This process is spin-forbidden according to the rule of Wigner (Wigner and Witmer, 1928). Nevertheless, several possibilities exist for the generation of singlet molecular oxygen, e.g. triplet–triplet energy transfer by photosensitization. As a major consequence, the reactivity of

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Fig. 3.13 Molecular orbital (MO) diagram of

an oxygen molecule illustrating the electronic configuration of the triplet ground state (3O2) and of the first excited singlet state (singlet

3

molecular oxygen, 1O2), neglecting the 1s electrons (modified according to Braun et al., 1991). The energy difference DE between 3O2 and 1O2 is 94.2 kJ mol–1.

O2 (ground state oxygen) with organic molecules in their singlet ground state (S0) is very slow at ambient temperatures, i.e. it is spin-forbidden, thus enabling oxygen-based life on Earth without oxidative destruction of organic matter. The Pauli exclusion principle demands that the spins of two bonding electrons in each molecular orbital of the oxidation products are paired (:;). This, however, is only the case (without additional energy for spin conversion) in reactions of singlet molecular oxygen (1O2) with organic substrates, which is utilized industrially in photooxidation reactions. For a detailed description of 1O2 production and of its reactivity, see Braun et al. (1991). An electronically excited state of a molecule is formed by absorption of a photon promoting an electron from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO). The HOMO– LUMO promotion of an electron is the lowest possible energy transition that occurs between the frontier orbitals of a molecule. This situation is schematically presented in Fig. 3-14.

3.10 The Nature of Electronically Excited States

Fig. 3.14 Generalized molecular orbital diagram: possible electronic transitions in organic molecules, associated absorption regions and examples of common chromophores (modified according to Böttcher, 1991); n, non-bonding MO; antibonding MOs are indicated by *.

The concept of frontier orbitals (Fleming, 1976) is very helpful and illustrative for the understanding of many photochemical reactions. b) Definition of electronic states of a molecule by description of the electronic configuration with respect to the characteristic electron spin orientation and the characteristic molecular orbitals that are involved. This can be achieved by construction of a generalized state diagram, as is demonstrated by Fig. 3-15. Quantum mechanical theory includes the spin concept of electrons (Gilbert and Bagott, 1991). Each electron in a molecule is represented by a spin quantum number of s = ± 1/2. A multi-electron atom or molecule is characterized by the total spin quantum number S that is the sum of all individual electron spins s. The spin multiplicity M of an electronic state of a molecule is determined by the total spin quantum number S and it can be calculated according to M = 2S + 1. Atoms and molecules with an even number of electrons lead to S = 0, 1, 2, 3, resulting in multiplicities of M = 1 (singlet), 3 (triplet), 5 (quintet) and 7 (septet), whereas an odd number of electrons yields S = 1/2, 3/2, 5/2, 7/2 and leads to M = 2 (doublet), 4 (quartet), 6 (sextet) and 8 (octet). Hence, an electronic configuration with two electrons with antiparallel spins (:;) is called a singlet state (S). In a triplet configuration (T), the electrons are arranged with parallel spins (::). The spin multiplicity of the hydroxyl radical (:•OH equivalent to ;•OH) calculates to M = 2 × 1/2 + 1 = 2, i.e. it is a doublet.

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Fig. 3.15 Electronic states of molecules characterized by the orbitals involved and by the spin configuration of the electrons P (spin multiplicity M = 2S + 1, S = si and si = ±1/2, spin of individual electrons).

3.11

The Jablonski Diagram

The concepts mentioned above are clearly illustrated by the generalized Jablonski diagram (Fig. 3-16). It demonstrates the various electronic states of a molecule that are grouped in multiplicity manifolds, including vibrational and rotational sublevels (Suppan, 1994). The latter can usually be neglected because their energy is very low compared with electronically excited state energies. This famous diagram is used to visualize photophysical processes that allow an electronically excited molecule to dissipate its energy by non-radiative and by radiative deactivation phenomena (Braun et al., 1991). For a comprehensive historical assessment of Jablonski’s work, refer to the reference given in Tab. 2-2. Thus, absorption of a photon of sufficient energy by an organic ground state molecule results in promotion of one bonding or non-bonding electron to a vibrational level of the first (S1) or second (S2) excited singlet state, depending on the

3.11 The Jablonski Diagram

Fig. 3.16 Modified Jablonski diagram visualiz-

ing radiative, non-radiative and photoproduct forming processes (adopted from Braun et al., 1991 and Pfoertner, 1998). The lifetimes of the individual processes are given in parentheses. 1, Absorption (excitation) S0 ? S2 and 2, absorption S0 ? S1 (10–15 s); 3, fluorescence S1 ? S0 + hmFl (10–12–10–6 s); 4, phosphorescence T1 ? S0 + hmPh (10–3–102 s); IC: internal conversion S2 ? S1 + heat (10–13–10–12 s) and

IC S1 ? S0 + heat (10–12–10-6 s); ISC: inter system crossing T1 ? S0 + heat (10–7–10 s); ISC S1 ? T1 + heat (10–11–10–6 s). P1: Reaction product from the singlet state (intramolecular); P2: (intermolecular) reaction product from the triplet state. The reactions from S1 and T1 may also include electron or energy transfer reactions. The arrows in the boxes represent the spin orientation of the electrons in the participating MOs.

photon energy, respectively. Internal conversion (IC) from S2 and/or very fast relaxation (10–13–10–12 s) of vibrationally excited levels of the S1 state leads immediately to the occupation of the S1 state in its lowest vibrational level. According to Kasha’s rule (see Kopecky, 1992), all subsequent processes occur from the S1 excited state of a molecule. Either the S1 state deactivates via internal conversion and by vibrational relaxation to the ground state S0, ultimately transforming electronic excitation energy into vibrational energy (i.e. heat), or the S1 state deactivates by fluorescence emission. The short lifetime of the S1 state (10–12–10–6 s) usually enables only intramolecular photoreactions that may involve rearrangements of the molec-

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ular structure or bond cleavage processes leading to reaction product P1. The change of multiplicity by inter system crossing (ISC) is spin-forbidden according to the rule of Wigner (Wigner and Witmer, 1928), but it may occur through mechanisms that are called spin–orbit interactions. Then the relatively long-lived (10–7– 102 s) first excited triplet state (T1) of the molecule is occupied. Again, vibrational deactivation of the T1 state competes with radiative deactivation, called phosphorescence emission. Typical singlet energies E(S1) are in the range from 180 to 500 kJ mol–1 and triplet energies E(T1) are between 125 and 360 kJ mol–1 (Turro, 1978, 1991). It is obvious from the Jablonski diagram that the energy of fluorescence emission is greater than that of phosphorescence emission (schematically illustrated by the length of the solid arrows S1 ? S0 and T1 ? S0, respectively). This indicates the well-known phenomenon that the wavelength of fluorescence emission is shorter than that of phosphorescence. Hence, phosphorescence is shifted bathochromically (i.e. it is red shifted toward a longer wavelength) relative to fluorescence emission. Nearly all intermolecular reactions of electronically excited molecules occur from the T1 state (Braun et al., 1991), generating different molecular structures (reaction product P2). This is mainly due to the long lifetime of the T1 state compared with other competing deactivation processes enabling bimolecular reactions. The reaction products P1 and P2 are usually referred to as photoproducts. The lifetimes of T1 and of S1 are strongly dependent on the molecular structure and on the experimental conditions (Braun et al., 1991). Further, typical reactions of singlet or triplet excited states of many molecules are electron or energy transfer reactions to ground state molecules. The latter process is utilized in so-called photosensitization reactions where UV/VIS radiation is not absorbed by the substrate itself, but by an added sensitizer (photosensitizer). Subsequently, the electronically excited sensitizer molecule transfers its electronic energy to the substrate molecule (which is called a quencher) with formation of an excited substrate molecule and a ground state of the sensitizer molecule. Usually, the sensitizer is deactivated without chemical alteration during the photosensitization process (Pfoertner, 1998, Braun et al., 1991).

3.12

Absorption of UV/VIS Radiation by Solids

Absorption of UV/VIS radiation in the solid state is different from UV/VIS absorption in the liquid or gaseous phase with respect to photophysical processes taking place in the crystal lattice and to the metallic, semiconductor (SC) or insulator properties of the absorbing solid (Böttcher, 1991). In crystals, multiple atomic or molecular orbitals are combined to form broad energy bands, i.e. a valence band (vb) fully occupied by electrons and a conduction band (cb) unoccupied or only partly occupied by electrons. Conduction bands and valence bands have different energetic positions relative to one another depending on the specific substrate. In a SC cluster, electronic transitions between the valence band and the conduction

3.12 Absorption of UV/VIS Radiation by Solids Fig. 3.17 Schematic representation of some photophysical and photochemical processes in and on a semiconductor (SC) particle (for example TiO2). Ebg: Band gap energy; VB: valence band; CB: conduction band; h+: electron “hole” (“defect electron”) in the valence band; e–: photoelectron in the conduction band; LT: lattice trap; ST: surface trap; Aads, Dads: chemical species adsorbed on the surface of the SC particle with A being an electron acceptor and D an electron donor. Formation of an electron–hole pair (exciton) by irradiation: SC + hm ? e–cb + h+vb (modified according to Serpone, 1996 and Böttcher 1991).

band (in analogy with HOMO–LUMO transitions) require at least UV/VIS radiation with an energy equivalent to the band-gap energy (Ebg) (Fig. 3-17). Band-gap excitation initiates several physical and chemical processes within the semiconductor particle and on its surface, respectively. By absorption of a photon with the energy greater than or equal to Ebg an electron is promoted from the valence band to the empty conduction band of the SC particle, creating an electron deficiency in the valence band. This is called a defect electron or a positive hole (h+vb). The electron/hole pair thus formed is called an exciton (Serpone and Salinaro, 1999). The charge carriers (photoelectron e–cb and hole h+vb) can recombine by different mechanisms and decay channels (Hoffmann et al., 1995). In competition with non-radiative (lattice or surface trapping) and radiative deactivation mechanisms they can migrate to the surface of the SC particle. At the interface to the contacting medium, an adsorbed acceptor substrate Aads is reduced by transfer of the photoelectron e–cb to an unoccupied MO of the acceptor molecule and an adsorbed donor substrate Dads is oxidized by transfer of a donor electron to h+vb of the SC particle. Thus, the illumination of an SC particle with UV/VIS radiation ultimately initiates redox chemistry at its surface (Fig. 3-17). The photochemistry of semiconductors like titanium dioxide (TiO2) plays an important role in research concerning heterogeneous photocatalysis (Fujishima et al., 2000, Serpone, 1996) and solar synthetic chemistry (Kisch and Lindner, 2001). For example, the band-gap energy Ebg of TiO2 (crystal form anatase) is about 3.2 eV (Hoffmann et al., 1995). This energy is equivalent to the wavelength k of 387.5 nm (cf. Tab. 3-4). Therefore, TiO2 is able to absorb parts of the solar UV-A and the solar UV-B radiation, making it to one of the most investigated SC materials for solar driven detoxification and disinfection of aqueous or gaseous systems (Rajeshwar, 1995). However, aqueous heterogeneous photocatalysis involves the use of semiconductor suspensions. Therefore, it is accompanied by light scatter-

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ing, low penetration depth of UV radiation into the suspension, adsorption phenomena and by the mechanical problem of catalyst separation after treatment.

3.13

UV/VIS Radiation as a Specific Reagent: Quantum Yield, Quantum Efficiency, Actinometry and Photoreaction Kinetics

The quantum yield U of a photophysical or photochemical event is a quantitative measure of the overall efficiency of this process (Braun et al., 1991). It is a unitless constant, which usually ranges from zero to one. However, some authors express U in units of mol einstein–1, which in fact is unit-less, because an einstein is defined as one mol of photons. Quantum yields greater than one indicate photo-induced chain reactions, which may involve radical species or photo-generated catalysis. Commonly used definitions of U are collected in Tab. 3-7. These definitions describe quantum yields of photophysical events and of photochemical reactions with regard to the reactant diminution or to the formation of the photoproduct. Quantum yields may be dependent on the wavelength of the absorbed UV/VIS radiation, but many photochemical systems exist that have a constant quantum yield U over a defined wavelength range. Such chemical systems can be

Tab. 3.7 Definitions of the quantum yield a) Uk and of the quantum efficiency b) g of a photo-

physical process c) or of a photochemical reaction of a reactant M to yield a photoproduct P' (modified according to Verhoeven, 1996 and Braun et al., 1991) Mathematical expression

Definition

Uk

dnevent=dt Uabs p

Universally valid: Number n of events per unit time divided by the number of photons absorbed during this period

Uk

dnM=dt Uabs p

Number n of reactant molecules M consumed per unit time divided by the number of photons absorbed during this period d)

Uk

dnP0 =dt Uabs p

Number n of photoproduct molecules P‘ formed per unit time divided by the number of photons absorbed during this period d)

g

dmP0 =dt 1 Uabsk p

k2

6 Uk

Ratio of the number m of photoproduct molecules formed per unit time to the total number of photons absorbed in the spectral region k1–k2 during this period e)

a) Based on the number of absorbed photons Uabs P , i.e. on the absorbed photon flow (not on the incident photons!) at a stated wavelength k. b) based on the number of absorbed photons of polychromatic UV/VIS radiation in a defined spectral region k1–k2. c) for example, quantum yield of fluorescence UFl, phosphorescence UPh, internal conversion UIC, intersystem crossing UISC. d) if no photophysical processes or photochemical side reactions occur, the rate of reactant diminution equals the rate of photoproduct formation: –dn(M)/dt = dn(P')/dt. e) accordingly, this definition holds for the diminution of reactant molecules. If the quantum yield U of a photochemical system is independent of the wavelength range k1–k2, then g = Uk

3.13 UV/VIS Radiation as a Specific Reagent

easily used for the measurement of the absorbed photon flow Uabs p . These photochemical reaction systems, with well-known quantum yields, are called actinometers. Chemical actinometry is suitable for the measurement of the absorbed photon flow Uabs p in photoreactors of specific geometry and in a well-defined spectral domain. A comprehensive list of versatile actinometers and quality marks for solid phase, gas and liquid phase photolysis has been published by Kuhn et al. (1989). Braun et al. (1991) gave further examples of the most common and practical chemical actinometric procedures for monochromatic and polychromatic UV/ VIS photon flow determinations. Recently, several interesting applications of actinometers were published. For example, the fluence rate distribution E0 in mW cm–2 at k of 253.7 nm in UV reactors can be determined by the iodide/iodate system (Stefan et al., 2001, Rahn, 1997), this being important for UV disinfection development. In connection with the development of novel incoherent excimer UV lamps several actinometric procedures have been applied to establish their radiant power efficiency, e.g. using 3,4-dimethoxynitrobenzene at k of 308 nm (Zhang et al., 1999), uridine at 222 nm (Zhang et al., 1997), hydrogen bromide (Pfeifer Roland et al., 1998), oxygen (Laszlo et al., 1998) and methanol (Heit et al., 1998) at k of 172 nm. In photochemical AOP practice, however, polychromatic radiant sources (medium-pressure Hg lamps or sunlight) are usually used. Thus, the quantum efficiency g was defined, covering the wavelength range of interest from k1 to k2 (Tab. 3-7). The quantum efficiency can be determined for a given radiant source, if the chemical actinometer used absorbs all of the photons in the spectral region from k1 to k2 during the irradiation period (Braun et al., 1991). Alternatively, the photon emission weighted average of some quantity (e.g. relative photon flow Up of a lamp, the data are usually provided by the manufacturer) over a spectral range k1 to k2 may be used to calculate the relative spectral photon flow of a radiant source over this wavelength band (Bolton, 1999). However, (photo)chemists do not usually think in quantities such as the number of molecules consumed or formed by a reaction (cf. Tab. 3-7), but in concentrations given in mol L–1. Therefore, the quantum yield definition must be converted into a concentration dependency, as is demonstrated by Eqs. 3-11 to 3-13.

Quantum yield Uk

dnM=dt Uabs p

(3-11)

and the number of molecules nM M=

mol NA =mol L

1

V=L

(3-12)

With [M] the concentration of the reactant M; NA, Avogadro’s constant; and V the by its volume containing n molecules of M. Substitution of the photon flow Uabs p –2 –1 s times irradiated area A equivalent Uabs p = Ep × A (photon irradiance EP in m in m2) leads to Eq. 3-13, implying the concentration of M:

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Uk

dM=dtNA V : Ep;abs A

(3-13)

Eq. 3-13 can be used for the actinometric determination of the absorbed photon irradiance Ep,abs, if Uk of the specific photoreaction system is known. The measurement and control of UV/VIS radiation and/or the quantification of the photon flow Up of an industrial lamp is of vital importance to AOP developments. It can also be achieved by using radiometers or UV sensors (McCluney, 1994). Moreover, it is essential to determine the UV/VIS absorbance spectrum of the water or air to be treated by UV spectroscopy prior to further research and development efforts. On the other hand, Eq. 3-13 may be rearranged to Eq. 3-14, expressing a rate equation, and thus raising the question of photochemical reaction kinetics. dM A cf : Tab: 3-8 Uk EP;abs dt NA V

3-14

It is obvious from Eq. 3-14, that the rate of decrease of M is proportional to the quantum yield Uk , the absorbed photon irradiance Ep,abs and to the area A which is irradiated at constant photon irradiance Ep,0, and it is inversely proportional to the irradiated volume V. In contrast to ground state (dark) reactions that are characterized by a reaction rate constant k, a photochemical reaction does not have a specific reaction order (Logan, 1997). This is demonstrated in Tab. 3-8. The kinetic behavior of a photoreaction is strongly dependent on the absorption conditions. Under total absorbance, i.e. with a substrate exhibiting a high absorption coefficient a(M) = eM k [M] (i.e. concentrated solutions of M), the observed decrease of the concentration of the reactant M by irradiation is linear. In very dilute solutions of M or under conditions with a low a(M) only a small fraction Fk of the incident photon irradiance is absorbed by M leading ultimately to an observed exponential decrease of [M]. Thus, photochemical reactions may be classified within two different regimes, depending on the absorption characteristics of the irradiated mixture and thus on the reaction conditions. There must be a stage of transition between the linear and exponential regimes, where the fraction of the absorbed radiation Fk plays an important role. This stage of change of regimes is dependent on the concentration of the absorber M. Consequently, there is no basis of rate constants in photochemical reactions and hence, this term is meaningless. In sharp contrast to thermal reactions, the only parameter that is important for the quantification of photochemical reactions is the quantum yield U.

k1 s–1

First approximation e): if total absorbance, Ak = eM k [M] l >2, M then 10–ek [M] l is negligible and Ep,abs = Ep,0.

M?P Reaction order: 1

M + hm ? M* ? P' Photochemical reaction c) General rate expression d):

UM EP;0 1

10

eM Ml k

A NA V

k0 mol L–1 s–1

M?P Reaction order: 0

dM A UM EP;abs dt NA V

Rate constant, unit

Reaction of substrate M

UM EP;0 for EP,0 = constant;

Mt Mt0

dM A UM EP;0 dt NA V

[M]t = [M]t = 0 e–k1t

dM k1 [M] dt

[M]t = [M]t = 0 – k0 t

dM k0 dt

A t NA V

Reaction rate, integrated form

Tab. 3.8 Comparison of the reaction rate expressions of simple thermal a) and photochemical reactions b) in solution

Graphical representation

3.13 UV/VIS Radiation as a Specific Reagent 71

Reaction rate, integrated form dM A M UM EP;0 2:303 eM k l dt NA V A t Mt Mt0 exp UM EP;0 2:303 eM k l NA V for Ep,0 = constant.

Rate constant, unit

Second approximation f): If Ak = eM k [M] l 100 mg L–1) and one for low concentrations of M (often [M] 250 kW h m–3 order–1 EEO c) ([C]t=0 ?) = 40–250 kW h m–3 order–1 EEO c) ([C]t=0 ?) = 0.5–5 kW h m–3 order–1

H2O2-UV a,b) H2O2-UV b, f) H2O2-UV b, g) UV (k = 253.7 nm) h) O3-UV h) H2O2-UV h) H2O2-O3 h) O3 h) H2O2-UV H2O2-UV H2O2-UV b, j) H2O2-UV b, j) UV-C b) H2O2-UV b) TiO2-UV b) H2O2-UV k) H2O2-UV H2O2-UV H2O2-UV

1,4-Dioxane

1,4-Dioxane Acetone Atrazine Atrazine Atrazine Atrazine Atrazine Benzene and its derivatives Dichloromethane MTBE MTBE NDMA Phenol Phenol Tetrachloroethene Tetrachloromethane Trichloromethane Unsaturated chlorinated Alkenes

a) d) e) h) i)

[H2O2] = 15 mmol L–1. b) EE values determined under optimized conditions using a batch recirculation photoreactor. c) Costs of H2O2 are not included. A significant change of the reaction kinetics was observed: EEM determination during the first 5–7 min of H2O2-UV treatment and subsequently till 20 min of treatment. f) [H2O2] = 200 mg L–1. g) [H2O2] = 3–16 mmol L–1. Determined under optimized conditions in a flow-through reactor system. Including the “stored electric energy” costs of H2O2 [1 kg of H2O2 (100%) : 10 kW h]. In the case of H2O2-UV: [H2O2] = 17 mg L–1 and for H2O2-O3: [H2O2] = 1.7 mg L–1. j) MTBE: methyl-tert-butyl ether, [H2O2] = 30 mg L–1. k) [H2O2] = 88 mg L–1, field-scale pilot plant for treatment of groundwater contaminated with Cl2C=CCl2.

Bolton (2001) Stefan et al. (1996) Müller et al. (2000) Müller et al. (2000) Müller et al. (2000) Müller et al. (2000) Müller et al. (2000) Bolton (2001) Bolton (2001) Cater et al. (2000) Cater et al. (2000) Bolton et al. (2001 b) Bolton et al. (1998) Bolton et al. (1998) Hirvonen et al. (1998) Bolton (2001) Bolton (2001) Bolton (2001)

References

Design parameter

AOP

Substrate

Tab. 8.5 Several examples of typical EEM and EEO values published in the literature

8.5 Economic Factors and Design Parameters (Figures-of-Merit) of Photo-Initiated AOPs 267

268

8 Process Engineering and Reactor Concepts

continuous maintenance. For a pilot or full-scale system, these costs strongly depend on the nature and concentration of the specific pollutant(s), the water quality, the water matrix, their stability over the treatment time, the flow rate V˙ of the effluent, the rector configuration and on the treatment goal. Further, a thorough cost evaluation requires the preceding optimization of the AOP and AOT, at least on a pilot scale. In the literature few data have been reported for AOP or AOT overall costs (refer to references cited in Andreozzi et al., 1999, to Chemviron Carbon, 1997 and to Hand et al., 1995). However, owing to their complex composition and to dependencies on plant specifications these data are often meaningless. More important, several comparable studies have demonstrated that the costs of AOP treatment of aqueous systems are at least comparable to conventional technologies of pollutant abatement (Hirvonen et al., 1998).

8.6

Selected Industrial Applications 8.6.1

Aqueous Systems

Many industrial applications of photo-initiated AOPs, including UV disinfection technology, have been published all over the technical and scientific primary and secondary literature and in many special periodicals. To obtain an overview, several examples of industrial and environmental significance were reviewed by Rice (1997) (O3, O3-UV, H2O2-UV and O3-H2O2-UV) and by Leitzke et al. (1996) (O3H2O2, H2O2-UV, O3-UV). Chemviron Carbon (1997) collected together further case histories of the H2O2-UV AOT. Trageser and Greunig (2000) presented the treatment of wastewater from metal processing industries that contain AOX, cyanides and organic metal complexes. The photo-initiated oxidation of persistent organic compounds in nickel containing waters of low optical transmittance (l 4 by irradiation with MP Hg lamps at a fluence H0 of 10.4 J m–2 accompanied by a slight formation of nitrite ions in a concentration range well below the European Drinking Water Regulation ([NO–2]max = 0.1 mg L–1) (Cervantes et al., 2001). The latest investigations demonstrate that LP Hg lamps are also effective for inactivation of Cryptosporidium parvum oocysts (Craik et al., 2001). The first demonstration that UV radiation is very effective against Cryptosporidium parvum was presented by Bolton et al. (1998); • : minimal space requirements and if necessary, repeated applications in the course of the water distribution systems are possible; • : UV disinfection is a pure physical process thus, no chemical disinfectant must be added, generated, handled, transported or stored; • : no influence on taste and smell of the treated water; • : minimal residual effects (if any) that could be harmful to humans or aquatic life; • : negligible formation of toxic by-products, if any are formed (Zheng et al., 1999 a, b); • : no mutagenic photoproducts could be isolated (Hoyer, 1998); • : no contribution to regrowth problems in distribution systems (Hengesbach et al., 1993); • : the technology is easily automated by UV monitoring with UV sensors and it is user friendly, safe and reliable to operate;

281

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9 UV Disinfection

Fig. 9.1

Various influences on the efficacy of UV disinfection; refer also to Figure 8-15.

• : very short contact time in the order of several seconds is necessary; • ; marked influence of the water quality (e. g. total suspended solids, UV transmittance T, turbidity) on the required UV fluence for a given reduction rate R of pathogen inactivation; • ; fouling of tubes containing the UV lamps must be controlled; • ; photoreactivation, and in the absence of UV/VIS radiation dark repair may lead to a considerable regrowth or recolonialization of several microorganisms depending strongly on the UV fluence applied; • ; in the case of high nitrate ion (NO–3) concentrations the formation of nitrite ions (NO–2) by extensive UV-C irradiation must be taken into account, especially with MP Hg lamps that emit significant amounts of UV radiation below k of 253.7 nm (Cervantes et al., 2001, Hoyer, 1998). The various influences on the efficacy of UV disinfection are compiled in Fig. 9-1 (cf. Malley Jr, 2000). Primarily, the efficacy of UV disinfection depends on the germicidal fluence H0 = E0 × t which is the product of fluence rate E0 and the duration time t of the irradiation (often called “UV dose”, Chapter 2.1) (see Sommer et al., 1998). Other key factors include the hydraulics and hydrodynamics of the UV reactor (Kreft et al., 1986), its geometry (FIGAWA, 1998, Hoyer, 1996), the number and type of UV lamps required (Loge et al., 1996), their temperature profiles with respect to a maximum fluence rate E0 (in the case of LP Hg lamps, cf. Fig. 4-8), the water quality and its variability such as UV absorbance/transmittance (Bolton et al., 2001, Sommer et al., 1997), the water matrix, e. g. nitrate concentration, its potential for quartz fouling by inorganic constituents particularly iron ions and hardness (cf. Chapter 8-2), the turbidity, the particle content (total sus-

9 UV Disinfection

UV sensitivity of indicator germs E. coli, pathogens, facultative pathogens and viruses with and without photo-reactivation; redrawn from Hoyer (1998).

Fig. 9.2

pended solids), the particle size distribution and the corresponding number of particle-associated bacteria (cf. Emerick et al., 1999), the target organism(s) and the level of inactivation that is required (i. e. the desired reduction factor Rf). Furthermore, the biology of the microorganism(s) to be inactivated is of utmost interest: the reproduction rate is dependent on environmental factors (i. e. nutrient supply, oxygen demand etc.) and the UV sensibility is related to the action spectrum. The action spectrum represents a plot of a relative biological response against wavelength under the same fluence H0. However, the correct way to measure an action spectrum is to select a “standard biological response” (e. g. 2 logs inactivation) and at each wavelength determine the fluence H0 required to achieve that response. A plot of 1/H0 versus wavelength is then the relative action spectrum (Bolton, 2002). Several microorganisms have the ability to photo-reactivate on irradiation with longer wavelengths (in the UV-A range), so that two to four times higher UV-C radiant exposure H is necessary for comparable inactivation in these cases (Tosa and Hirata, 1999, Hoyer, 1998). This is demonstrated by the comparison of the UV sensitivity of indicator germs (E. coli), pathogens and facultative pathogens and viruses with and without photoreactivation (Fig. 9-2). Thus, a germicidal radiant exposure H of at least 400 J m–2 seems to be required for the safe UV disinfection of water containing germs and viruses of hygienic relevance (Hoyer, 1998). Photoreactivation testing can be easily performed by exposure of the microorganisms to daylight under standardized conditions. However, several spores (B. subtilis: H ~ 600 J m–2) and viruses (MS2 phage: H ~ 750 J m–2)

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seem to require much higher UV fluences to reach the same order of inactivation (Bolton, 1999). The design and optimization of UV disinfection processes has to take into account the biology of the microorganism(s), the water quality and concepts of hydraulic and photochemical engineering (see Fig. 9-1). Usually, UV disinfection installations for treatment of drinking water or of effluents from sewage treatment in = 40–100 W) that emit mainly at k of plants use large banks of LP Hg lamps (Pel 253.7 nm. Current UV disinfection systems use open channel horizontal, vertical, parallel-to-flow and perpendicular-to-flow lamp configurations or closed chamber arrangements (FIGAWA, 1998, ATV, 1993) (compare with Fig. 8-3). Cryptosporidium control has become a primary focus of regulatory agencies in the U.S.A. and U.K. Sources of drinking water contamination by Cryptosporidium oocysts are human and animal fecal waste. Cyryptosporidiosis causes gastrointestinal illness, such as watery diarrhea (over 5 to 22 days), vomiting, cramps, and a general feeling of illness with headaches for the duration of up to 6 weeks (Roche Encyclopedia, 1987). Outbreaks of cryptosporidiosis may lead to death in immunocompromized individuals (Clancy, 1999). Thus, over the past few years a great deal of research activity has been invested in investigating the effect of UV-C radiation on these parasites. in = 1–5 kW) seemed Originally, the use of a smaller number of MP Hg lamps (Pel to have several advantages over the LP Hg lamp systems, particularly with respect to inactivation of Cryptosporidium parvum oocysts at a low UV fluence H0 of 19 mJ cm–2 and a reduction factor Rf of 3.9 (Bukhari et al., 1999). However, recent investigations consistently established a 3 log-unit inactivation of C. parvum oocysts at a germicidal UV fluence H0 of 25 mJ cm–2 for both MP and LP Hg lamps. Both lamp types were found to be equally effective for the inactivation of oocysts when compared on a germicidal fluence basis (Craik et al., 2001). In contrast, previous studies with LP Hg lamps indicated that a very high UV fluence H0 of up to 8000 mJ cm–2 is required to inactivate Cryptosporidium. This observation was initially assigned to the much lower irradiance E of LP Hg lamps compared with that of MP Hg lamps or to wavelength effects (Bukhari et al., 1999). LP Hg lamps can be treated as being almost monochromatic UV-C sources (kmax = 253.7 nm) whereas MP Hg lamps emit polychromatic radiation in the germicidal range between 200 and 300 nm (cf. Fig.s 4-5 and 4-6). Another and more convincing explanation for this discrepancy of previous and recent studies related to the inactivation of Cryptosporidium refers to the methods of viability testing. So, it seems that in vitro viability assays (chemical excystation and vital stains) that have been used previously may have significantly underestimated the inactivation efficacy of UV-C irradiation of the parasites compared with in vivo infectivity assays applied in recent studies using neonatal mouse models (Craik et al., 2001). This was also demonstrated by UV inactivation of Giardia muris cysts using MP Hg lamps (Craik et al., 2000). In conclusion, the resent research by Clancy et al. (2000, 1998), Bukhari et al. (1999) and Craik et al. (2001) has convincingly demonstrated that UV-C is highly effective at inactivating Cryptosporidium parvum oocysts (and Giardia muris cysts)

9 UV Disinfection Comparison of the emission spectra of LP and MP Hg lamps in relation to the action spectrum of Escherichia coli photoinactivation; received from Heraeus Noblelight (Kleinostheim, Germany).

Fig. 9.3

by using mouse infectivity as a convenient measure of viability. The study of Craig et al. (2001) recommends that the choice of the lamp type should be based on economic considerations. LP Hg lamps usually have twice the germicidal efficiency compared with MP Hg lamps. Thus, the former will be the choice for small UV disinfection installations. On the other hand, MP Hg lamps have a much greater UV-C intensity and thus fewer lamps must be installed reducing the size of the UV reactor vessel and the expenditure on maintenance. Thus, MP Hg lamps are a convenient alternative to LP Hg lamps in large UV disinfection units. For comparison of both lamp types related to the action spectrum of photo-inactivation of Escherichia coli bacteria refer to Fig. 9-3. Thus, it is necessary to develop appropriate new models for the calculation of the total average fluence rate distribution in a photoreactor in the spectral range of 200 to 300 nm when using MP Hg lamps (Bolton, 2000 b). In UV disinfection technology, the germicidal UV fluence H0 in J m–2 (commonly designated as UV dose) is crucial for the efficacy of a photoreactor system. It can be determined using a collimated beam apparatus for the UV treatment of suspensions of microorganisms under defined irradiation conditions (Bolton, 1999, Leuker, 1999, Leuker, 1998, Sommer et al., 1999). This method is called “biodosimetry” (cf. Cabaj et al., 1996). However, it is more accurate and it would be more convenient to designate this procedure as bioactinometry! The results of bioactinometry (“biodosimetry”) can be related to chemical actinometry (cf. Chapter 3-13), e. g. by applying uridine as an appropriate actinometer for the determination of effective germicidal UV fluence from polychromatic UV sources (Jin et al., 2001, Linden and Darby, 1997). The testing of the disinfection performance of UV systems, especially in drinking water supplies, can be established by UV exposure of a harmless test organism (e. g. Bacillus subtilis spores or bacteriophage MS2) under controlled conditions within testing facilities, followed by the determination of the inactivation efficacy. For example, in Germany drinking water disinfection requires an inactivation efficacy of 99.99% of water borne pathogens. This is achieved by an effective UV fluence H0 of 400 J m–2 (= 40 mJ cm–2) at k of 253.7 nm (Hoyer, 2001). To guarantee a constant efficacy of UV disinfection plants their testing, monitoring and certification is of importance (Snozzi et al., 1999). The mechanism of UV disinfection is based on the absorption of UV radiation by proteins, RNA (ribonucleic acid) and DNA (deoxyribonucleic acid) in a given micro-

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Fig. 9.4

UV absorbance spectrum of DNA; redrawn from von Sonntag

(1987).

The molecular structures of DNA and RNA components: the purine bases adenine and guanine, the pyrimidine bases cytosine, thymine and uracil and the structure of riboso-monophosphates.

Fig. 9.5

organism (von Sonntag, 1987). However, damage to the protein structure of a microorganism requires at least ten times the fluence H0 as does inactivation through absorption by DNA or RNA (Bolton, 2002). The absorption of UV radiation by the DNA of a microorganism induces photochemical reactions of DNA components and damage of proteins by destruction of sulfur combinations. Thus, the ability of the microorganism to replicate is disrupted. Hence, the microorganism can no longer infect.

9 UV Disinfection

Photochemical formation of the cyclobutane-type dimer by irradiation of thymine nucleotides.

Fig. 9.6

Biological effects are achieved in the wavelength range k of 190 to about 350 nm. This is in agreement with the absorbance spectrum of DNA (Fig. 9-4). It exhibits two distinct absorption maxima at 200 nm and around k of 260–265 nm, which extends to k of about 350 nm (von Sonntag, 1987). The absorption of DNA above k of 210 nm is due to the absorption of the nucleotide bases adenine (A), guanine (G), thymine (T), cytosine (C) and in the case of RNA to uracil (U). Their structures are shown in Fig. 9-5 together with the deoxyriboso- and riboso-5-phosphates. The nucleotides (i. e. the base-monophosphates) have their absorption maxima around k of 260 nm. The two DNA strands that are formed by base pairing (A-T, G-C) are held together by hydrogen bonds. Thus, a chiral, right-handed double helix (or spiral) is formed which contains the genetic code of living organisms (the Watson–Crick model of the DNA). By absorption of UV radiation, two thymine bases that are located adjacent to each other in the DNA strands react photochemically to produce a thymine dimer by a [2+2]cycloaddition reaction with formation of a new pair of covalent C–C single bonds (Fig. 9-6). After photochemical formation of sufficient thymine dimers, the replication of the DNA is blocked efficiently so that the cell mitosis is inhibited. In addition to the photochemistry of DNA, photoreactions of various cell components, such as membrane proteins, enzymes and other biomolecules may also be relevant to photo-induced cell killing in some cases (Kalisvaart, 2001). In drinking water UV treatment, usually only the photochemical reactions that are induced by direct irradiation are observed. However, if nitrate ion photochemistry (producing •OH radicals) is involved, the complex radical chemistry of DNA/RNA and of their fragments may prevail (Pogozelski and Tullius, 1998). Some microorganisms, especially bacteria, posses repair mechanisms that substitute or dissociate the thymine dimers. These reactivation mechanisms can take place under both dark and UV-A/VIS conditions. They are designated as either dark repair (cutting-repairing) or photoreactivation. However, the repair mechanisms are not universal to all microorganisms. Microorganisms that are unable to repair include the following species (Kalisvaart, 2001): Heamophilus influenzae, Diplococcus pneumo-

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Two-step reaction mechanism of photoreactivation (modified according to Lindenauer and Darby, 1994).

Fig. 9.7

niae, Bacillus subtilis, Micrococcus radiodurans, viruses, Cryptosporidium parvum, Staphylococcus aureus phage A994, Rotavirus SA-11, Poliovirus, MS2 Phage. Typical organisms that are capable of photoreactivation include (Kalisvaart, 2001): Streptomyces spp., Escherichia coli, Saccharomyces spp., Aerobacter spp., Micrococcus spp., Erwinia spp., Proteus spp., Penicillum spp., Neurospora spp., Enterobacter cloacae, Citrobacter freundii, Enterocolitica faecium, Klebsiella pneumoniae, Mycobacterium smegmatis, Pseudomonas aeruginosa, Salmonella typhi, Salmonella typhimurium, Seratia marcescena, Vibrio cholerea, Yersinia enterocolitica (spp. = plural of sp.: species, denotes two or more unspecified species or all species of the genus). Photoreactivation can influence the performance of UV disinfection and it must be taken into account when designing UV systems. Hence, the repair mechanisms can be inhibited by application of high UV-C fluence (see Fig. 9-2). Repair of virus species has never been reported in the literature because they do not possess an appropriate enzyme system (Baron, 1997). During evolution, the bacteria that are capable of photoreactivation developed a subtle mechanism to defend their cells against lethal DNA damage that may be caused by exposure to solar radiation. Postirradiation with UV-B, blue or white light may increase the survival rate of microorganisms significantly. Thus, the efficacy of UV disinfection decreases substantially when photoreactivation is facilitated. A two-step reaction sequence involving a photoreactivating enzyme (PRE) has previously been proposed by Harm in 1975 (Harm, 1975). It is outlined in Fig. 9-7. The pyrimidine dimers that are produced by an irreversible [2+2]photocycloaddition reaction form a PRE-dimer complex. This complex is able to absorb UV-B/VIS radiation, and the photocycloaddition reaction is reversed with formation of the monomers and PRE. The formation of repaired DNA occurs within a millisecond. All known DNA photolyases contain a flavin adenine dinucleotide entity, which is attributed to be the active chromophore (Kato et al., 1997). The magnitude of photoreactivation strongly depends on the fluence H0 of the UV-B/VIS radiation that is applied to the microorganisms after UV-C induced disinfection (Schoenen and Kolch, 1992). Thus, to evaluate the efficacy of UV disinfection it is essential to assess the degree of recovery of photoreactivating microorganisms following UV-B/VIS irradiation under defined conditions. Several large-scale studies concerning UV disinfection are collected in Tab. 9-1.

Type

UV unit

5200 250

LP Hg lamps MP Hg lamps

a) b) c) d)

Coliforms

Coliforms, virus Heterotrophic plate counts Giardia lamblia, Cryptosporidium parvum

E. coli

Kummer (1990) Chiu et al. (1999) Bolton (2000) Lyn et al. (1999)

Ho et al. (1998) Blomberg and Eriksson (2001) Schneider et al. (2001)

Wist et al. (2002)

Andreadakis et al. (1999) Chandrakanth et al. (2001) Schleypen and Loy (1991)

Whitby et al. (1984)

References

in High efficiency and high intensity LP Hg lamps were used in this pilot study at Pel = 1.620 kW. in Pel = 1300 W per lamp. Immediately after irradiation high inactivation yields of 99.98% were obtained. Depending on the water matrix, effective recolonization was observed. Spectrotherm lamps, Pelin = 170 W and P253.7 nm = 64 W.

Wastewater, secondary effluent Dose distribution function Fluence rate distribution Modeling of flow and disinfection

LP Hg lamps 24 Point-source summation method Significance of reflection/refraction Steady two-dimensional model of flow and disinfection

20 216

LP Hg lamps LP Hg lamps

Secondary effluent Surface water treatment: drinking water Unfiltered drinking water

Photocatalytic disinfection c) 40 W, TiO2 (P-25, Degussa) UV versus chlorination 13.824 kW d) at 253.7 nm UV + chlorination 1.716 MW 5 MW; comparison of reactor systems –

Coliforms

768 W at 253.7 nm

LP Hg lamps 48 MP Hg lamps 6 b) Black light lamp 1

Coliforms Coliforms

Lamp fouling 1.008 kW at 253.7 nm

? 2

Fecal coliforms

Microorganism

1.656 kW at 253.7 nm

Remarks

120

No. of UV lamps

Municipal wastewater effluents

Domestic wastewater, secondary LP Hg lamps effluent Municipal sewage, secondary effluent LP Hg lamps Municipal wastewater effluents LP Hg lamps a)

Description

Tab. 9.1 Selected studies related to UV disinfection and modeling of UV disinfection processes

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References Andreadakis A, Mamais D, Christoulas D, Kabylafka S (1999) Ultraviolet Disinfection of Secondary and Tertiary Effluent in the Mediterranean Region, Wat. Sci. Technol. 40, No. 4-5: 253–260. ATV (1993) Desinfektion von Abwasser, Ein Regelwerk der Abwassertechnischen Vereinigung e.V., (Germany), H 205. Augurt TA, van Asten JAAM (1997) Sterilization Techniques, in: Kirk-Othmer, Encyclopedia of Chemical Technology, John Wiley & Sons, New York, Vol. 22: 832–851. Baron J (1997) Repair of Wastewater Microorganisms after Ultraviolet Disinfection under Seminatural Conditions, Wat. Environ. Res. 69, No. 5: 992–998. Belhacova L, Krysa J, Geryk J, Jirkovsky J (1999) Inactivation of Microorganisms in a Flow-through Photoreactor with an Immobilized TiO2 Layer, J. Chem. Technol. Biotechnol. 74: 149–154. Bersillon J-L (1999) Water Purification and Disinfection Processes, Acta Hydrochim. Hydrobiol. 27, No. 2: 98–100. Blake DM (1999) Bibliography of Work on the Heterogeneous Photocatalytic Removal of Hazardous Compounds from Water and Air, update No. 3, National Renewable Energy Laboratory (available on the web, see Tab. 1-1). Block SS (1993) Disinfectants and Antiseptics, in: Kirk-Othmer Vol. 8: 237–292, John Wiley & Sons, New York. Bloomberg J, Eriksson U (2001) From Pilot Plant Studies to a Full-Scale UV Installation at a Large Surface Water Treatment Plant in Sweden, Conference Proceedings of the First International Congress on Ultraviolet Technologies, International Ultraviolet Assoc. (IUVA), June 14–16, Washington, DC, electronic release. Bolton JR (1999) Ultraviolet Applications Handbook, Bolton Photoscience Inc., ISBN 0-9685432-0-0; also published in: Eur. Photochem. Assoc. Newslett. Nr. 66: 9–36. Bolton JR (2000 a) Terms and Definitions in Ultraviolet Disinfection, in: Proceedings, Disinfection 2000: Disinfection of Wastes in the New Millennium, 15–18 March, New Orleans, LA, Water Environment Federation, 601 Wythe St., Alexandria, VA, 22314.

Bolton JR (2000 b) Calculation of Ultraviolet Fluence Rate Distributions in an Annular Reactor: Significance of Refraction and Reflection, Wat. Res. 34, No. 13: 3315–3324. Bolton JR (2002) personal communication. Bolton JR, Dussert B, Bukhari Z, Hargy T, Clancy JL (1998) Inactivation of Cryptosporidium parvum by Medium-Pressure Ultraviolet Light in Finished Drinking Water, Proc. AWWA 1998 Annual Conference, Dallas TX, Vol. A: 389–403. Bolton JR, Stefan MI, Cushing RS, Mackey E (2001) The Importance of Water Absorbance/Transmittance on the Efficiency of Ultraviolet Disinfection Reactors, Conference Proceedings of the First International Congress on Ultraviolet Technologies, International Ultraviolet Assoc. (IUVA), June 14–16, Washington, DC, electronic release. Bukhari Z, Hargy TM, Bolton JR, Dussert B, Clancy JL (1999) Medium-pressure UV for Oocyst Inactivation, J. Am. Water Works Assoc. 91, No. 3: 86–94. Cabaj A, Sommer R, Schoenen D (1996) Biodosimetry: Model Calculations for U.V. Water Disinfection Devices with Regard to Dose Distributions, Wat. Res. 30, No. 4: 1003–1009. Cervantes P, Vidal A, Luck F (2001) Microorganism Inactivation and By-products Formation by an UV-Medium Pressure System, Conference Proceedings of the First International Congress on Ultraviolet Technologies, International Ultraviolet Assoc. (IUVA), June 14–16, Washington, DC, electronic release. Chandrakanth M, Colak P, Laquidara M, Prevost M, Leparc J (2001) Disinfecting Treated Municipal Wastewater Effluents of Varying Qualities Using Low-Pressure High-Intensity UV Lamps, Conference Proceedings of the First International Congress on Ultraviolet Technologies, International Ultraviolet Assoc. (IUVA), June 14– 16, Washington, DC, electronic release. Chiu K, Lyn DA, Savoye P, Blatchley III ER (1999) Integrated UV Disinfection Model Based on Particle Tracking, J. Environ. Eng. January: 7–16; Effect of UV System Modifications on Disinfection Performance, J. Environ. Eng. May: 459–469.

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Alternative Glossary The specialized language of photochemistry and of photochemical technology is not very common to engineers who are active in the expanding field of water and air treatment processes. Fortunately, three excellent and internationally accepted glossaries covering the terminology in physical chemistry (Mills et al., 1993), photocatalysis (Serpone and Emeline, 2002) and photochemistry (Verhoeven, 1996) have been published. The photochemical and photophysical terms and notations used in this book are adapted to the last, which was proposed by the International Union of Pure and Applied Chemistry (IUPAC). Additionally, the remarkable dictionary prepared by Kurzweil (2000) on units, equations, constants and conversions used in science, technology and medicine enables not only newcomers but also specialists to enter interdisciplinary research activities in photoscience. Furthermore, fundamental physical constants, energy equivalents and conversion factors can be found in the CRC Handbook of Chemistry and Physics edited by Lide (2000). Mattay and Griesbeck (1994) collected a versatile accumulation of introductory and sophisticated textbooks, journals and series for further reading on photochemistry, photophysics and photochemical technology. Of course, this reference list contains the standard photochemistry textbook by Turro (1978, 1991), which covers mechanistic and synthetic aspects of photochemical reactions. A landmark of technical literature in this research area represents the outstanding book presented by Braun et al. (1991). It covers the principles of photochemical technology comprehensively. It was specifically written to meet the sophisticated needs of users and engineers and it is a valuable reference desk book. However, this collection of literature has to be extended to include several important recent contributions to the fields of photochemistry, photochemical AOPs and AOTs. For example, the text book by Suppan (1994) covers the basic principles of various UV and light induced processes in different research areas and demonstrates in an excellent manner the interdisciplinary applications and potentials of photoscience. This is also emphasized by Böttcher et al. (1991) who concentrate on several important technical applications using UV/VIS radiation as a reagent, an information carrier and as an energy carrier. Unfortunately, these authors hardly mention the expanding field of photochemical AOTs for waste treatment. Concepts, methods and experiments of photochemistry are perfectly presented in the book by Wöhrle et al. (1998). McCluney (1994) collected a comprehensive

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system of language, mathematics and instrumentation used for the measurement of electromagnetic radiation in radiometry and photometry. Recently, Balzani (2001) edited a comprehensive five-volume handbook on electron transfer reactions in chemistry. It covers different and interrelated fields of active research and presents a current and broad overview of every aspect of electron transfer. This includes the discussion of AOP related topics, such as photocatalytic oxidations and reductions, radiation-chemical techniques and photocatalysis for waste remediation. Another important contribution by Halmann (1996) gives a comprehensive description of the photoreactions of inorganic and organic water pollutants and of xenobiotics that are recalcitrant to biodegradation. In addition, it includes a brief introduction to the mechanisms of AOPs. In this connection, the short introductory handbook on ultraviolet applications prepared by Bolton (2001, 1999) has to be mentioned as a quick desk reference. It has been written to provide information on UV technology related to AOPs. In addition, Bolton (2000) summarized a series of terms and definitions for use in UV disinfection. System sizing and cost calculations, specific treatment examples and several case histories are discussed synoptically in the AOT handbook prepared by Chemviron Carbon (1997). Gottschalk et al. (2000) presented a practical guide to the ozonation of water and wastewater. It also covers briefly some mechanistic aspects of photochemical AOPs using ozone and hydrogen peroxide. Two excellent reviews describe comprehensively the photochemical processes for water treatment (Legrini et al., 1993) and environmental applications of semiconductor photocatalysis (Hoffmann et al., 1995). In addition, Blake (1999) compiled a comprehensive bibliography of work on photocatalysis. The combined reports include patents and more than 3300 citations to work published between 1970 and 1999. They also include tables of organic and inorganic compounds, bacteria, algae and viruses that have been treated in an aqueous or gaseous phase providing the corresponding reference data. In addition to these different books and reviews on themes related to AOPs and AOTs the increasing importance of research and development of photo-initiated water and air treatment technologies is reflected by the “Journal of Advanced Oxidation Technologies”, which was first launched by Al-Ekabi (1996) and several internationally renowned associate editors. Photochemical and photobiological sciences are growing areas of interdisciplinary importance. Therefore, in 2002 the Royal Society of Chemistry launched a new journal, “Photochemical and Photobiological Sciences”, which covers aspects of the interaction of UV radiation and light with molecules, supramolecular systems or biological matter. Further information about the versatile subject coverage are available on the web via www.rsc.org/pps. Furthermore, in addition to many workshops and symposia around the world the yearly international conference on AOTs for water and air remediation has to be mentioned. It was organized successfully for the first time in 1995 in London (Ontario, Canada). In addition, the proceedings of the first international conference on TiO2 photocatalysis (Ollis and Al-Ekabi, 1993) provide a selection of methods and techniques applied in photocatalyzed water and air purification. Recent developments in the field of UV applications can be found in the conference pro-

References

ceedings of the first international congress on ultraviolet technologies (IUVA, 2001). The abstracts of these conferences present an excellent insight into actual research and development activities of the international “photonic community” related to environmental topics. In addition, the reader should refer to AOTs-6 (2000), or to the proceedings of the second international conference on oxidation technologies for water and wastewater treatment, held in Clausthal-Zellerfeld in Germany (Vogelpohl et al., 2001). Furthermore, the proceedings of the symposia on chemical oxidation edited by Eckenfelder et al. (1996) cover many aspects of AOPs written by leading international experts. In general, the diversified interests of photoscientists are represented by biennial symposia on photochemistry organized by IUPAC. General topics related to modern water research are documented comprehensively in the series “Vom Wasser” edited by the Society of German Chemists (GdCh). It also contains a list of international journals and periodicals that are published in the field of water science (e. g. Vom Wasser, 1998, 2001). vanLoon and Duffy (2000) and Spellman (1999) comprehensively covered many aspects of environmental chemistry and of environmental pollution, respectively. These books are suitable as introductory texts concerning the chemistry, physics and biochemistry of the atmosphere, hydrosphere and of the terrestrial environment. Verschueren (2001) presented the latest research in the field of environmental effects of organic chemicals including reference data on dangerous organic chemicals.

References Al-Ekabi H (ed.) (1996) J. Adv. Oxid. Technol. 1, No. 1. AOTs-6 (2000) Abstracts of the Sixth International Conference on Advanced Oxidation Technologies for Water and Air Remediation, London, Ontario (Canada), June 26– 30. Balzani V (ed.) (2001) Electron Transfer in Chemistry, Wiley-VCH, Weinheim. Blake DM (1999) Bibliography of Work on the Heterogeneous Photocatalytic Removal of Hazardous Compounds from Water and Air, update No. 3, National Renewable Energy Laboratory (available on the web, see Tab. 1–1). Bolton JR (1999) Ultraviolet Applications Handbook, Bolton Photoscience Inc., ISBN 0-9685432-0-0; also published in: Eur.. Photochem. Assoc. Newslett. Nr. 66: 9–36. Bolton JR (2000) Terms and Definitions in Ultraviolet Disinfection, in: Proceedings, Disinfection 2000: Disinfection of Wastes in the New Millennium, 15–18 March, New

Orleans, LA, Water Environment Federation, 601 Wythe St., Alexandria, VA, 22314. Bolton JR (2001) Ultraviolet Applications Handbook, 2nd ed., Bolton Photosciences Inc., 628 Cheriton Cres., NW, Edmonton, AB, Canada T6R 2M5, ISBN 0-9685432-19. Böttcher H, Bendig J, Fox MA, Hopf G, Timpe H-J (1991) Technical Applications of Photochemistry, Deutscher Verlag für Grundstoffindustrie, Leipzig. Braun AM, Maurette M-T, Oliveros E (1991) Photochemical Technology, John Wiley & Sons, Chichester. Chemviron Carbon (1997) AOT Handbook, Advanced Oxidation Technologies, ref. AOT/E-03, European Operating Unit of Calgon Carbon Corporation, Brussels (Belgium). Eckenfelder Jr WW, Bowers AR, Roth JA (1996) Chemical Oxidation: Technologies for the Nineties, Vol. 1–6, Technomic Publishing, Lancaster.

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10 Alternative Glossary Gottschalk C, Libra JA, Saupe A (2000) Ozonation of Water and Waste Water, Wiley-VCH, Weinheim. Halmann MM (1996) Photodegradation of Water Pollutants, CRC Press, Boca Raton. Hoffmann MR, Martin ST, Choi W, Bahnemann DW (1995) Environmental Applications of Semiconductor Photocatalysis, Chem. Rev. 95: 69–96. IUVA (2001) Conference Proceedings of the First International Congress on Ultraviolet Technologies, International Ultraviolet Assoc. (IUVA), June 14–16, Washington, DC, electronic release. Kurzweil P (2000) Das Vieweg EinheitenLexikon, 2nd ed.,Vieweg, Braunschweig. Legrini O, Oliveros E, Braun AM (1993) Photochemical Processes for Water Treatment, Chem. Rev. 93: 671–698. Lide DR (ed.) (2000) CRC Handbook of Chemistry and Physics, CRC Press, Boca Raton. Mattay J, Griesbeck AG (1994) Photochemical Key Steps in Organic Synthesis, An Experimental Course Book, VCH, Weinheim: 1–14. McCluney WR (1994) Introduction to Radiometry and Photometry, Artech House, Boston. Mills I, Cvitas T, Homann K, Kallay N, Kuchitsu K (1993) Quantities, Units and Symbols in Physical Chemistry, 2nd ed., Blackwell Scientific Publications, Oxford. Ollis DF, Al-Ekabi H (eds.) (1993) Photocatalytic Purification and Treatment of Water and Air, Proceedings of the 1st International Conference on TiO2 Photocatalytic Purification and Treatment of Water and Air, London, Ontario, Canada 8–13 November 1992, Elsevier, Amsterdam. Serpone N, Emeline AV (2002) Suggested Terms and Definitions in Photocatalysis and Radiocatalysis, Int. J. Photoenergy 4,

No. 3: 91–131; available free of charge at http://www.photoenergy.org.ijp. Spellman FR (1999) Science of Environmental Pollution, Technomic Publishing, Lancaster. Suppan P (1994) Chemistry and Light, Royal Society of Chemistry, Cambridge. Turro NJ (1978) Modern Molecular Photochemistry, The Benjamin/Cummings Publishing Co., Inc., Menlo Park, CA. Turro NJ (1991) Modern Molecular Photochemistry, University Science Books, MillValley, CA. vanLoon GW, Duffy SJ (2000) Environmental Chemistry, A Global Perspective, Oxford University Press, Oxford. Verhoeven JW (1996) Glossary of Terms Used in Photochemistry, 2nd ed., Pure Appl. Chem. 68, No. 12: 2223–2286 (available on the web, see Tab. 1-1). Verschueren K (2001) Handbook of Environmental Data on Organic Chemicals, Wiley-VCH, Weinheim. Vogelpohl A, Geissen SU, Kragert B, Sievers M (issue eds.) (2001) Selected Proceedings of the 2nd International Conference on Oxidation Technologies for Water and Wastewater Treatment, Clausthal-Zellerfeld, Germany, 28–31 May 2000, Wat. Sci. Technol. 44, issue 5; electronic release. Vom Wasser (1998) Appendix: Abbreviations of Journals according to “International Serials Catalogue”, Fachgruppe Wasserchemie (ed.) in der Gesellschaft Deutscher Chemiker, 91: 361–363, Wiley-VCH, Weinheim. Vom Wasser (2001) Wasserchemische Gesellschaft, Fachgruppe in der GdCh (ed.), Vol. 97. Wöhrle D, Tausch MW, Stohrer W-D (1998) Photochemie – Konzepte, Methoden, Experimente, Wiley-VCH, Weinheim.

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Glossary of Terms Used in Photochemistry International Union of Pure and Applied Chemistry Organic Chemistry Division, Commission on Photochemistry

2nd Edition, March 1996 Prepared for publication by: S. E. Braslavsky, K. N. Houk (first edition: Pure & Appl. Chem. 60 (1988): 1055 foll.) and J.W. Verhoeven (2nd edition: Pure & Appl. Chem. 68 (1996) No. 12: 2223–2286), http://www.iupac.org/reports/1996/6812verhoeven/index.html

Alphabetic List of Terms and Definitions (relevant parts reprinted by permission of IUPAC and J. W. Verhoeven)

Absorbance (A): The logarithm to the base 10 of the ratio Rof the spectral radiant power of incident, essentially monochromatic, radiation (I = k Ik dk) to the radiant power of transmitted radiation (Pk): A logPk0 =Pk

log T

In practice, absorbance is the logarithm to the base 10 of the ratio of the spectral radiant power of light transmitted through the reference sample to that of the light transmitted through the solution, both observed in identical cells. T is the (internal) transmittance. This definition supposes that all the incident light is either transmitted or absorbed, reflection or scattering being negligible. Traditionally (spectral )radiant intensity, I, was used instead of spectral radiant power, Pk, which is now the accepted form. (The terms: absorbancy, extinction, and optical density should no longer be used.) See absorption coefficient, absorptance, attenuance, Beer-Lambert law, depth of penetration, internal transmittance, Lambert law, molar absorption coefficient. Absorptance: The fraction of light absorbed, equal to one minus the transmittance (T). See absorbance.

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Absorption (of electromagnetic radiation): The transfer of energy from an electromagnetic field to a molecular entity. Absorption coefficient (decadic-a or Napierian-a): Absorbance divided by the optical pathlength, l: a A=l 1=l logPk0 =Pk Physicists usually use natural logarithms. In this case: a a ln 10 1=l logPk0 =Pk where a is the Napierian absorption coefficient. Since absorbance is a dimensionless quantity, the coherent SI unit for a and a is m–1. Also cm–1 is often used. See also absorptivity, molar absorption coefficient. Absorption cross section (r): Operationally, it can be calculated as the absorption coefficient divided by the number of molecular entities contained in a unit volume of the absorbing medium along the light path: r a=N 1=Nl lnPk0 =Pk where N is the number of molecular entities per unit volume, l is the optical pathlength, and a is the Napierian absorption coefficient. The relation between the absorption cross section and the molar (decadic) absorption coefficient, e, (units M–1 cm–1) is r e=NA 0:1 ln 10 0:2303e=NA 3:825 10

24

e

where r is in m2 and NA is the Avogadro constant. See attenuance, Beer-Lambert law. Absorptivity: Absorptance divided by the optical pathlength. For very low attenuance it approximates the absorption coefficient (within the approximation (1–e–A) ~ A). The use of this term is not recommended. Actinometer: A chemical system or physical device which determines the number of photons in a beam integrally or per unit time. This name is commonly applied to devices used in the ultraviolet and visible wavelength ranges. For example, solutions of iron(III) oxalate can be used as a chemical actinometer, while bolometers, thermopiles, and photodiodes are physical devices giving a reading that can be correlated to the number of photons detected. Action spectrum: A plot of a relative biological or chemical photoresponse (= Dy) per number of incident photons, against wavelength or energy of radiation under

Alphabetic List of Terms and Definitions

the same radiant power of light. This form of presentation is frequently used in the studies of biological or solid state systems, where the nature of the absorbing species is unknown. The action spectrum is sometimes called spectral responsivity or sensitivity spectrum. The precise action spectrum is a plot of the spectral (photon or quantum) effectiveness. By contrast, a plot of the biological or chemical change or response per absorbed photon (quantum efficiency) versus wavelength is the efficiency spectrum. See also excitation spectrum, efficiency spectrum. Adiabatic electron transfer: Electron transfer process in which the reacting system remains on a single electronic surface in passing from reactants to products. For adiabatic electron transfer the electronic transmission factor is close to unity (see Marcus equation). See also diabatic electron transfer. Adiabatic photoreaction: Within the “Born-Oppenheimer approximation”, a reaction of an excited state species that occurs on a single “potential-energy surface”. Compare diabatic photoreaction. Alpha-cleavage (a-Cleavage): Homolytic cleavage of a bond connecting an atom or group to a specified group. Often applied to a bond connected to a carbonyl group, in which case it is called a Norrish type I photoreaction. This reaction should be distinguished from an alpha-(a-)expulsion. Alpha-expulsion (a-Expulsion): A general reaction by which a group attached to the alpha carbon of an excited chromophore is expelled either as an odd electron species or as an anionic species. This reaction should be distinguished from an alpha-(a-)cleavage. AM (0) sunlight: The solar irradiance in space just above the earth atmosphere (air mass, AM, zero). Also called extraterrestrial global irradiance. AM (1) sunlight: The solar irradiance traversing the atmosphere when the sun is in a position perpendicular to the earth surface. Also called terrestrial global irradiance. See also AM (0) sunlight. Annihilation: Two atoms or molecular entities both in an excited electronic state interact often (usually upon collision) to produce one atom or molecular entity in an excited electronic state and another in its ground electronic state. This phenomenon is sometimes referred to as energy pooling. See singlet-singlet annihilation, spin-conservation rule, triplet-triplet annihilation. Antimony-xenon lamp (Arc): An intense source of ultraviolet, visible, and near infra-red radiation produced by an electrical discharge in a mixture of antimony va-

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por and xenon under high pressure. Its output in the ultraviolet region is higher than that of the mercury-xenon arc. See lamp. Anti-stokes shift: See Stokes shift. Apparent lifetime: See lifetime. Argon ion laser: A CW or pulsed laser emitting lines from 334 to 529 nm from singly ionized argon. Principal emissions are at 488.0 and 514.5 nm. See laser, gas lasers. Attenuance (D): The logarithm to the base 10 of the ratio of the transmittance (T): D

log T logPk0 =Pk

Attenuance reduces to absorbance if the incident beam is only either transmitted or absorbed, but not reflected or scattered. See Beer-Lambert law, depth of penetration. Attenuance filter: An optical device (filter) which reduces the radiant power of a light beam by a constant factor over all wavelengths within its operating range. Sometimes called attenuator or neutral density filter. Auxochrome: An atom or group which, when added to or introduced into a chromophore, causes a bathochromic shift and/or a hyperchromic effect in a given band of the chromophore, usually in that of lowest frequency. This term is obsolete. Back electron transfer: A term often used to indicate thermal reversal of excited state electron transfer restoring the donor and acceptor in their original oxidation level. In using this term one should also specify the resulting electronic state of the donor and acceptor. Bandgap energy (Eg): The energy difference between the bottom of the conduction band and the top of the valence band in a semiconductor or an insulator. See conduction band, Fermi level. Bandpass filter: An optical device which permits the transmission of radiation within a specified wavelength range and does not permit transmission of radiation at higher or lower wavelengths. It can be an interference filter. See also cut-off filter. Bathochromic shift (Effect): Shift of a spectral band to lower frequencies (longer wavelengths) owing to the influence of substitution or a change in environment


(e.g., solvent). It is informally referred to as a red shift and is opposite to hypsochromic shift. Beer-Lambert law (or Beer-Lambert-Bouguer Law): The absorbance of a beam of collimated monochromatic radiation in a homogeneous isotropic medium is proportional to the absorption pathlength, l, and to the concentration, c, or – in the gas phase – to the pressure of the absorbing species. This law holds only under the limitations of the Lambert law and for absorbing species exhibiting no concentration or pressure dependent aggregation. The law can be expressed as A logPk0 =Pk ecl or Pk Pk0 10

ecl

where the proportionality constant, e, is called the molar (decadic) absorption coefficient. For l in cm and c in mol dm–3 or M, e will result in dm3 mol–1 cm–1 or M–1 cm–1, which is a commonly used unit. The SI unit of e is m2 mol–1. Note that spectral radiant power must be used because the Beer-Lambert law holds only if the spectral bandwidth of the light is narrow compared to spectral linewidths in the spectrum. See absorbance, attenuance, extinction coefficient, Lambert law. Bioluminescence: Luminescence produced by living systems. See luminescence. Biphotonic excitation: Also called two-photon excitation. The simultaneous (coherent) absorption of two photons (either same or different wavelength) the energy of excitation being the sum of the energies of the two photons. Biphotonic process: A process resulting from biphotonic excitation. See multiphoton process. Biradical (Synonymous with diradical): An even-electron molecular entity with two (possibly delocalized) radical centres which act nearly independently of each other. Species in which the two radical centres interact significantly are often referred to as biradicaloids. If the two radical centres are located on the same atom, they always interact strongly, and such species are called carbenes, nitrenes, etc. The lowest-energy triplet state of a biradical lies below or at most only a little above its lowest singlet state (usually judged relative to kT, the product of the Boltzmann constant k and the absolute temperature T). The states of those biradicals whose radical centres interact particularly weakly are most easily understood in terms of a pair of local doublets. Theoretical descriptions of low-energy states of a biradical display the presence of two unsaturated valences (biradicals contain one fewer

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bond than permitted by the rules of valence): the dominant valence bond structures have two dots, the low energy molecular orbital configurations have only two electrons in two approximately nonbonding molecular orbitals, two of the natural orbitals have occupancies close to one, etc. Biradicaloid: Biradical-like. Bleaching: In photochemistry this term refers to the loss of absorption or emission intensity. Blue shift: Informal expression for hypsochromic shift. Cadmium-helium laser: See Helium-Cadmium laser. Charge hopping: Electron or hole transport between equivalent sites. Charge recombination: Reverse of charge separation. In using this term it is important to specify the resulting electronic state of the donor and acceptor. Charge separation: A process in which, under the influence of a suitable driving force (e.g. provided by photoexcitation), electronic charge moves in a direction that increases the difference in local charges between donor and acceptor sites. Electron transfer between neutral species is one of the most important examples. Charge shift: A process in which under the influence of a suitable driving force (e.g. provided by photoexcitation) electronic charge moves without changing the difference in local charges between donor and acceptor sites. Electron transfer reversing the charges in a system composed of a neutral donor and a cationic acceptor or of a neutral acceptor and an anionic donor provide prominent examples. Charge-transfer (CT) state: A state related to the ground state by a charge transfer transition. Charge-transfer (CT) transition: An electronic transition in which a large fraction of an electronic charge is transferred from one region of a molecular entity, called the electron donor, to another, called the electron acceptor (intramolecular CT) or from one molecular entity to another (intermolecular CT). Typical for donor-acceptor complexes or multichromophoric molecular entities. In some cases the charge transfer absorption band may be obscured by the absorption of the partners. Charge-transfer (CT) complex: A ground-state complex which exhibits an observable charge transfer absorption band. See charge-transfer transition.


Charge-transfer transition to solvent (CTTS): Electronic transition which is adequately described by single electron transfer between a solute and the solvent, as opposed to excitation followed by electron transfer to solvent. See also charge-transfer (CT) transition. Chemical laser: A CW or pulsed laser in which the excitation and population inversion of the emitting species results from a chemical reaction. Typical examples are HF and DF lasers emitting many lines in the IR region. Chemiexcitation: Generation, by a chemical reaction, of electronically excited molecular entities from reactants in their ground electronic states. Chemiluminescence: Luminescence arising from chemiexcitation. Chromophore: That part of a molecular entity consisting of an atom or group of atoms in which the electronic transition responsible for a given spectral band is approximately localized. CIDEP (Chemically Induced Dynamic Electron Polarization): Non-Boltzmann electron spin state population produced in thermal or photochemical reactions, either from a combination of radical pairs (called radical-pair mechanism), or directly from the triplet state (called triplet mechanism), and detected by ESR spectroscopy. CIEEL (Chemically Initiated Electron Exchange Luminescence): A type of luminescence resulting from a thermal electron-transfer reaction. Also called catalyzed chemiluminescence. Collision complex: An ensemble formed by two reaction partners for which the distance is the sum of their Van der Waals radii. As such it constitutes a subclass of the species indicated as encounter complex. See also encounter complex. CO2 laser: A continuous or pulsed source of coherent radiation normally tunable through the CO2 vibration-rotation band centered near 10.6 lm. See gas lasers, laser. Coherent radiation: A source is said to emit coherent radiation when all the elementary waves emitted have a phase difference constant in space and time. Conduction band: A vacant or only partially occupied set of many closely spaced electronic levels resulting from an array of a large number of atoms forming a system in which the electrons can move freely or nearly so. This term is usually used to describe the properties of metals and semiconductors. See bandgap energy, Fermi level, valence band.

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Configuration (electronic configuration): A distribution of the electrons of an atom or a molecular entity over a set of one-electron wavefunctions called orbitals, according to the Pauli principle. From one configuration several states with different multiplicities may result. For example, the ground electronic configuration of the P oxygen molecule (O2) is 1r2g, 1r2u, 2r2g, 2r2u, 1p4u, 3r2g, 1p2g resulting in the 3 g , P 1 Dg , and 1 g states of different energy. Configuration Interaction (CI): The mixing of wavefunctions representing different electronic configurations to obtain an improved wavefunction for a many-electron state. Contact ion pair: Pair of ions in direct contact and not separated by an intervening solvent or other neutral molecule. One mode of formation for a (geminate) contact ion pair is electron transfer between precursor species in an encounter complex (cf. collision complex). If one of the precursors in the encounter complex is electronically excited the contact ion pair formed by electron transfer is equivalent to a polar exciplex. Conversion spectrum: A plot of a quantity related to the absorption (absorbance, cross section, etc.) multiplied by the quantum yield for the considered process against a suitable measure of photon energy, such as frequency, m, wavenumber, r, or wavelength, k. E.g., the conversion cross section, rU, has the SI unit m2. See also action spectrum, efficiency spectrum, spectral effectiveness. Copper vapor laser: A pulsed source of coherent radiation emitting at 578.2 and 510.5 nm from excited copper atoms. See gas lasers, laser. Correlation diagram: A diagram which shows the relative energies of orbitals, configurations, valence bond structures, or states of reactants and products of a reaction, as a function of the molecular geometry, or another suitable parameter. An example involves the interpolation between the energies obtained for the united atoms and the values for the separated atoms limits. Correlation energy: The difference between the Hartree-Fock energy calculated for a system and the exact nonrelativistic energy of that system. The correlation energy arises from the approximate representation of the electron-electron repulsions in the Hartree-Fock method. Critical quenching radius (r0): See Förster excitation transfer. Crystal field splitting: The removal of a degeneracy of the energy levels of molecular entities or ions due to the lower site symmetry created by a crystalline environment. This term is sometimes incorrectly used synonymously with the term ligand field splitting.


CT: Abbreviation for charge-transfer. Current yield: See photocurrent yield. Cut-off filter: An optical device which only permits the transmission of radiation of wavelengths that are longer than or shorter than a specified wavelength. Usually, the term refers to devices which transmit radiation of wavelengths longer than the specified wavelength. See filter. CW (Continuous Wave): Nonpulsed source of electromagnetic radiation. Dark photochemistry (photochemistry without light): Chemical reactions involving electronically excited molecular entities which are generated thermally rather than by absorption of electromagnetic radiation. The use of this term is discouraged. Deactivation: Any loss of energy by an excited molecular entity. See emission, energy transfer, internal conversion, radiationless deactivation and transition, radiative transition. Delayed fluorescence: See delayed luminescence. Delayed luminescence: Luminescence decaying more slowly than that expected from the rate of decay of the emitting state. The following mechanisms of luminescence provide examples: (1) triplet-triplet annihilation to form one molecular entity in its excited singlet state and another molecular entity in its electronic ground state (sometimes referred to as P type), (2) thermally activated delayed fluorescence involving reversible intersystem crossing (sometimes referred to as E type), and (3) combination of oppositely charged ions or of an electron and a cation. For emission to be referred to in this case as delayed luminescence at least one of the two reaction partners must be generated in a photochemical process. Depth of penetration (of light): The inverse of the absorption coefficient. The SI unit is m. If the decadic absorption coefficient, a, is used, the depth of penetration (1/a) is the distance at which the spectral radiant power, Pk decreases to one tenth of its incident value, Pk0 . If the Napierian absorption coefficient, a, is used, the depth of penetration (1/a = b in this case) is the distance at which the spectral radiant power decreases to 1/e of its incident value. See absorbance, attenuance. Dexter excitation transfer (Electron Exchange Excitation Transfer): Excitation transfer occurring as a result of an electron exchange mechanism. It requires an overlap

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of the wavefunctions of the energy donor and the energy acceptor. It is the dominant mechanism in triplet-triplet energy transfer. The transfer rate constant, kET, is given by kET / h=2pP2 J exp 2r=L; where r is the distance between donor (D) and acceptor (A), L and P are constants not easily related to experimentally determinable quantities, and J is the spectral overlap integral. For this mechanism the spin conservation rules are obeyed. See also radiative energy transfer. Diabatic electron transfer: Electron transfer process in which the reacting system has to cross over between different electronic surfaces in passing from reactants to products. For diabatic electron transfer the electronic transmission factor is 1 (see Marcus equation.) The term non-adiabatic electron transfer has also been used and is in fact more widespread, but should be discouraged because it contains a double negation. See also adiabatic electron transfer. Diabatic photoreaction: Within the Born-Oppenheimer approximation, a reaction beginning on one excited state “potential-energy surface” and ending, as a result of radiationless transition, on another surface, usually that of the ground state. Also called non-adiabatic. Compare with adiabatic photoreaction. Diode lasers: Sources of CW or pulsed coherent radiation in the visible and infrared regions. These lasers are semiconductor devices of small dimensions. Also called semiconductor lasers. Di-p-methane rearrangement: A photochemical reaction of a molecular entity comprising two p-systems, separated by a saturated carbon atom (a 1,4-diene or an allyl-substituted aromatic analog), to form an ene- (or aryl-) substituted cyclopropane. The rearrangement formally amounts to a 1,2 shift of one ene group (in the diene) or the aryl group (in the allyl-aromatic analog) and “bond formation” between the lateral carbons of the nonmigrating moiety.

See also oxa-di-p-methane rearrangement. Dipolar mechanism (of energy transfer): Same as Förster excitation transfer. See also energy transfer.


Dipole-dipole excitation transfer: Same as Förster excitation transfer. See also energy transfer. Diradical: This term, synonymous with biradical, is no longer recommended. Dose: The energy or amount of photons absorbed per unit area or unit volume by an irradiated object during a particular exposure time. In medicine and in some other research areas (e.g. photopolymerization and water handling through irradiation) dose is used in the sense of fluence, i.e. the energy or amount of photons per unit area or unit volume received by an irradiated object during a particular exposure time. The SI units are J m–2 or J m–3 and mol m–2 or mol m–3, respectively. See also UV-dose. Doublet state: A state having a total electron spin quantum number equal to 1/2. See multiplicity. Driving force (for electron transfer): Term widely used to indicate the negative of the standard Gibbs energy change (DG0) for (photoinduced) outer-sphere electron transfer. This quantity can often be calculated rather accurately from independently determined properties of the donor and acceptor species involved. Thus e.g. for photoinduced electron transfer between a neutral acceptor (A) and a neutral donor (D) (either one of them may be the electronically excited molecular entity) to form an ion pair, the driving force in a solvent with static dielectric constant es can be approximated as (see: A. Weller, Z. Phys. Chem. Neue Folge 133, 93–98 (1982)): DG0 eE 0 D=D

E 0 A=A

e2 =4pe0 es q

DE0;0

with E0(D/D+) the standard oxidation potential of the donor, E0(A/A–) the standard reduction potential of the acceptor, e2/(4pe0esq) the change in Gibbs energy for bringing the two radical ions to an encounter distance q, and DE0,0 the electronic excitation energy of the excited partner, all data referring to the same solvent. Dye laser: A CW or pulsed source of coherent radiation in which the active medium is usually a solution of a fluorescent organic molecule (the dye) pumped with a suitable pump laser or with a flash lamp. These lasers can be tuned over a large part of the fluorescence band of the dye. Dynamic quenching: See quenching. Effectiveness: See spectral effectiveness.

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Efficiency (of a step; g): The ratio between the useful energy delivered or bound and the energy supplied, i.e., energy output/energy input. It is also used in the sense of a quantitative measure of the relative rate of a given step involving a species with respect to the sum of the rates of all of the parallel steps which depopulate that species. See also quantum yield. Efficiency spectrum: A plot of the efficiency of a step (g) against wavelength or photon energy. See action spectrum, conversion spectrum. Compare spectral effectiveness. Einstein: One mole of photons. Although widely used, it is not an IUPAC sanctioned unit. It is sometimes defined as the energy of one mole of photons. This use is discouraged. Electrochemiluminescence: See electrogenerated chemiluminescence. Electrochromic effect: See Stark effect. Electrogenerated luminescence (ECL): Luminescence produced by electrode reactions. Also called electroluminescence or electrochemiluminescence. Electroluminescence: See electrogenerated chemiluminescence. Electron correlation: The adjustment of electron motion to the instantaneous (as opposed to time-averaged) positions of all the electrons in a molecular entity. See also correlation energy. Electron exchange excitation transfer: Same as Dexter excitation transfer. See also energy transfer. Electronic configuration: See configuration. Electronic energy migration (or Hopping): The movement of electronic excitation energy from one molecular entity to another of the same species, or from one part of a molecular entity to another of the same kind (e.g. excitation migration between the chromophores of an aromatic polymer). The migration can happen via: radiative or radiationless processes. Electronically excited state: A state of an atom or molecular entity which has greater electronic energy than the ground state of the same entity. Electron transfer: The transfer of an electron from one molecular entity to another or between two localized sites in the same molecular entity.


See also inner-sphere electron transfer, outer-sphere electron transfer, Marcus equation. Electron transfer photosensitization: Photochemical process in which a reaction of a non-absorbing substrate is induced by electron transfer (not energy transfer) with an excited light-absorbing sensitizer. The overall process must be such that the sensitizer is recycled. Depending on the action of the excited sensitizer as electron donor or acceptor the sensitization is called reductive or oxidative. See also photosensitization. Electrophotography: Processes of photoimaging which are based on photo-induced changes of electric fields (photo-conductive or photo-electrostatic effects). El-Sayed rules: The rate of intersystem crossing, e.g. from the lowest singlet state to the triplet manifold, is relatively large if the radiationless transition involves a change of orbital type. E.g. 1p,p* ? 3n,p* is faster than 1p,p* ? 3p,p* and 1n,p* ? 3p,p* is faster than 1n,p* ? 3n,p*. See multiplicity. Emission: Radiative deactivation of an excited state; transfer of energy from a molecular entity to an electromagnetic field. See also fluorescence, luminescence, phosphorescence. Emission spectrum: Plot of the emitted spectral radiant power (spectral radiant exitance) or of the emitted spectral photon irradiance (spectral photon exitance) against a quantity related to photon energy, such as frequency, m, wavenumber, r, or wavelength, k. When corrected for wavelength dependent variations in the equipment response, it is called a corrected emission spectrum. Emittance: See radiant exitance. Encounter complex: An intermolecular ensemble formed by molecular entities in contact or separated by a distance small compared to the diameter of solvent molecules and surrounded by several shells of solvent molecules; the innermost shell is the solvent “cage”. If one of the species is excited, the excitation usually takes place prior to formation of the encounter complex. During the lifetime of the encounter complex the reactants can collide several times to form collision complexes, and then undergo structural and electronic changes. If the interaction between the reactants leads to a minimum in the potential energy and one of the entities is electronically excited, the encounter complex may represent an exciplex or excimer. See also contact ion pair and collision complex. Energy migration: See electronic energy migration.

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Energy pooling: See annihilation. Energy storage efficiency (g): The rate of the Gibbs energy storage in an endothermic photochemical reaction divided by the incident irradiance. See also efficiency. Energy transfer: From a phenomenological point of view, the term is used to describe the process by which a molecular entity absorbs light and a phenomenon originates from the excited state of another molecular entity. In mechanistic photochemistry the term has been reserved for the photophysical process in which an excited state of one molecular entity (the donor) is deactivated to a lower-lying state by transferring energy to a second molecular entity (the acceptor), which is thereby raised to a higher energy state. The excitation may be electronic, vibrational, rotational or translational. The donor and acceptor may be two parts of the same molecular entity, in which case the process is called intramolecular energy transfer. See also Dexter excitation transfer, Förster excitation transfer, radiative energy transfer, and spectral overlap. Energy transfer plot: A plot of the quenching rate constant of an excited molecular entity by a series of quenchers versus the excited state energy of the quenchers. Alternatively, a plot of the rate constant for the sensitization of a reaction versus the excited state energy of different sensitizers. This type of plot is used to estimate the energy of the excited molecular entity quenched (in the former case) or produced (in the latter case). Also known as Hammond-Herkstroeter plot. See also Stern-Volmer kinetic relationships. Enhancer: A fluorescent compound which accepts energy and thus enhances or promotes the emission from a sample containing a chemically or enzymatically generated excited molecular entity. ESCA: See photoelectron spectroscopy. Excimer: An electronically excited dimer, “nonbonding” in the ground state, a complex formed by the interaction of an excited molecular entity with a ground state partner of the same structure. See also exciplex. Excimer laser: A source of pulsed coherent radiation obtained from an exciplex. The proper name should be exciplex laser. Typical lasing species are noble gas halides (XeCl, KrF, etc.) emitting in the UV domain. See laser, gas lasers. Exciplex: An electronically excited complex, of definite stoichiometry, “nonbonding” in the ground state. For example, a complex formed by the interaction of an


excited molecular entity with a ground state partner of a different structure. If the partners have pronounced electron-donor and -acceptor character their exciplex attains ionpair character. The terms compact exciplex and loose exciplex have sometimes been used to indicate that such polar exciplexes may have structures closely related to a contact ion pair or a solvent-separated ion pair. Excitation spectrum: Plot of the spectral radiant exitance or of the spectral photon exitance against the frequency (or wavenumber, or wavelength) of excitation. When corrected for wavelength dependent variations in the excitation radiant power this is called a corrected excitation spectrum. See also emission spectrum. Excitation transfer: Same as energy transfer. Excited state: A state of higher energy than the ground state of a chemical entity. In photochemistry an electronically excited state is usually meant. Exciton: In some applications it is useful to consider electronic excitation as if a quasi-particle capable of migrating, were involved. In organic materials two models are used: the band or wave model (low temperature, high crystalline order) and the hopping model (higher temperature, low crystalline order or amorphous state). Energy transfer in the hopping limit is identical with energy migration. See electronic energy migration. Exitance: See radiant exitance. External heavy atom effect: See heavy atom effect. Exterplex: Termolecular analogue of an exciplex. Use of this term is discouraged. See also: exciplex. Extinction: This term, equivalent to absorbance, is no longer recommended. Extinction coefficient: This term, equivalent to molar (decadic) absorption coefficient, is no longer recommended. See Beer-Lambert law. Fermi level (EF): The chemical potential of electrons in a solid (metals, semiconductors or insulators) or in an electrolyte solution. See bandgap energy, conduction band, valence band. Filter (optical): A device which reduces the spectral range (bandpass, cut-off, and interference filter) or radiant power of incident radiation (neutral density or attenuance filter) upon transmission of radiation.

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Flash photolysis: A technique of transient spectroscopy and transient kinetic studies in which a light pulse is used to produce transient species. Commonly, an intense pulse of short duration is used to produce a sufficient concentration of a transient species suitable for spectroscopic observation. Fluence (H0): When applied to energy, it is the total radiant energy traversing a small transparent imaginary spherical target containing the point under consideration, divided by the cross sectionRof this target. The product of the fluence rate and the duration of the irradiation ( E0 dt, simplified expression: H0 = E0 t when the fluence rate is constant over the time considered). The SI unit is J m–2. Energy fluence is identical to spherical radiant exposure and reduces to radiant exposure (H) for a parallel and normally incident beam, not scattered or reflected by the target or its surroundings. See also dose, photon fluence. Fluence rate (E0): The rate of fluence, H0. Four times the ratio of the radiant power, P, incident on a small transparent imaginary spherical volume element containing R the point under consideration, divided by the surface area of that sphere, SK. ( 4p Ldx, simplified expression: E0 = 4 P/SK when the radiant power is constant over the solid angle considered). For energy fluence rate the SI unit is W m–2. It reduces to irradiance, E, for a parallel and perpendicularly incident beam not scattered or reflected by the target or its surroundings. See intensity, radiance. See also photon fluence rate. Fluorescence: Spontaneous emission of radiation (luminescence) from an excited molecular entity with the formation of a molecular entity of the same spin multiplicity. Flux (energy flux): see radiant energy flux, radiant power. f Number: See oscillator strength. Förster excitation transfer (Dipole-Dipole Excitation Transfer): A mechanism of excitation transfer which can occur between molecular entities separated by distances considerably exceeding the sum of their van der Waals radii. It is described in terms of an interaction between the transition dipole moments, (a dipolar mechanism). The transfer rate constant (kD?A) is given by kD!A

K 2 J8:8 10 n4 s0 r 6

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where K is an orientation factor, n the refractive index of the medium, s0 the radiative lifetime of the donor, r the distance (cm) between donor (D) and acceptor (A), and J the spectral overlap (in coherent units cm6 mol–1) between the absorp-


tion spectrum of the acceptor and the fluorescence spectrum of the donor. The critical quenching radius, r0, is that distance at which kD?A is equal to the inverse of the radiative lifetime. See also Dexter excitation transfer, energy transfer, radiative energy transfer. Förster cycle: Indirect method of determination of excited state equilibria, such as pKa values, based on ground state thermodynamics and electronic transition energies. This cycle considers only the difference in molar enthalpy change (DDH) of reaction of ground and excited states, neglecting the difference in molar entropy change of reaction of those states (DDS). Fourier transform spectrometer: A scanning interferometer, containing no principal dispersive element, which first splits a beam into two or more components, then recombines these with a phase difference. The spectrum is obtained by a Fourier transformation of the output of the interferometer. Franck-condon principle: Classically, the Franck-Condon principle is the approximation that an electronic transition is most likely to occur without changes in the positions of the nuclei in the molecular entity and its environment. The resulting state is called a Franck-Condon state, and the transition involved, a vertical transition. The quantum mechanical formulation of this principle is that the intensity of a vibronic transition is proportional to the square of the overlap integral between the vibrational wavefunctions of the two states that are involved in the transition. Franck-condon state: See Franck-Condon principle. Free electron laser: Source of coherent radiation in which the active medium is an electron beam moving at speeds close to the speed of light in the spatially periodic magnetic field produced by an array of magnets (the wiggler). The emitted wavelength, kL, is approximately given by kx/(4E2 ), with kx being the wiggler period and E the kinetic energy of the electrons in MeV. See laser. Free-running laser: It applies to a pulsed laser and means that the laser emission lasts as long as the pumping process is sufficient to sustain lasing conditions. Typical pulse durations are in the ls-ms range, depending on the pumping source. When the operation mode of a pulsed laser is not specified as Q-switched, mode-locked, or anything else, it must be considered as free-running. Frequency (m or x): The number of waveperiods per unit time. The linear frequency, m, is the number of cycles per unit time. The SI unit is Hz : s–1. For the angular frequency, the symbol x (= 2pm) is used, with rad s–1 as the SI unit. Frequency doubling: See harmonic frequency generation, nonlinear optical effects.

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FWHM (Full Width at Half Maximum): See half-(band)width. Gas lasers: CW or pulsed lasers in which the active medium is a gaseous mixture usually composed of a buffer gas (He for instance) and an active medium consisting of: – neutral atoms (e.g., Ne, Cu, Au, etc.) or molecules (e.g., N2, CO2, CO, I2, etc.), or – ionized atoms (e.g., Ar, Kr, Cd, etc.) These lasers are not tunable but most of them can emit several lines which in many cases may be selected from a single apparatus. Pulsed lasers may be free-running, Q-switched, or mode-locked. Some CW lasers may be mode-locked. See argon ion-, CO2-, excimer, copper vapor-, helium-neon-, krypton-, nitrogenlasers. Geminate ion pair: Ion pair, formed from a precursor that constitutes a single kinetic entity. I.e. by electron transfer or ion transfer in an encounter complex (cf. collision complex) or by ionic dissociation of a single molecular entity. Geminate pair: Pair of molecular (or atomic) species in close proximity in liquid solution with a solvent cage and resulting from reaction (e.g. bond scission, electron transfer, group transfer) of a precursor that constitutes a single kinetic entity. Geminate recombination: Recombination reaction of a geminate pair. The reaction can either be a back electron transfer that restores the donor and acceptor species in their ground-state, from which the pair was created via electron transfer, or a bond formation or bond reorganization. Ground state: The lowest energy state of a chemical entity. In photochemistry ground electronic state is usually meant. Half-width (of a band): The full width of a spectral band at a height equal to half of the height at the band maximum. Also known as full width at half maximum (FWHM). The dimension of band width should be either inverse length (wavenumbers) or inverse time (frequencies) so that the values give an indication of the energies. Note the hyphen in half-width. Half bandwidth has the meaning of halfwidth at half maximum. Hammond-Herkstroeter plot: See energy transfer plot. Harmonic frequency generation: Production of coherent radiation of frequency km (k = 2, 3, . . .) from coherent radiation of frequency m. In general, this effect is obtained through the interaction of laser light with a suitable optical medium with nonlinear polarizability. The case k = 2 is referred to as frequency doubling, k = 3 is frequency tripling, k = 4 is frequency quadrupling. Even higher integer values of k are possible.


Harpoon mechanism: Reaction sequence (thermal or photoinduced) between neutral molecular or atomic entities in which long-range electron transfer is followed by a considerable reduction of the distance between donor and acceptor sites as a result of the electrostatic attraction in the ion pair created. Heavy atom effect: Enhancement of the rate of a spin-forbidden process by the presence of an atom of high atomic number, which is either part of, or external to, the excited molecular entity. Mechanistically, it corresponds to a spin-orbit coupling enhancement produced by a heavy atom. Helium-cadmium laser: A CW laser emitting mainly at 325.0 and 441.6 nm from singly ionized cadmium. See gas lasers. Helium-neon laser: A CW laser emitting mainly at 632.8, 1152.3, and 3391.3 nm from excited neutral Ne atoms. See gas lasers. Herkstroeter plot: See energy transfer plot. Heteroexcimer: Same as exciplex. High-pressure mercury lamp (arc): Radiation source containing mercury at a pressure of ca. 8 MPa (ca. 80 bar) or higher which emits lines over a background continuum between about 200 and 1400 nm. See lamp. Hole burning: The photobleaching of a feature, normally a narrow range, within an inhomogeneous broader absorption or emission band. The holes are produced by the disappearance of resonantly excited molecules as a result of photophysical or photochemical processes. The resulting spectroscopic technique is site-selection spectroscopy. Hole transfer: Charge migration process in which the majority of carriers are positively charged. Hot ground state reaction: A hot state reaction of the ground electronic state. Hot quartz lamp: A term sometimes used to describe a high-pressure mercury lamp. The use of this term is not recommended. Hot state reaction: A reaction proceeding from an ensemble of molecular entities possessing a higher average vibrational, rotational or translational energy than they would at thermal equilibrium with the surrounding medium.

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Hund rules: (1) Of the different multiplets resulting from different configurations of electrons in degenerate orbitals of an atom those with greatest multiplicity have the lowest energy (multiplicity rule). (2) Among multiplets having the same multiplicity, the lowest-energy one is that with the largest total orbital angular momentum (angular momentum rule) (valid if the total orbital angular momentum is a constant of motion). (3) In configurations containing shells less than half full of electrons, the term having the lowest total angular momentum J lies lowest in energy, whereas in those with shells more than half filled, the term having the largest value of J lies lowest (fine structure rule). Hund rules apply if the “Russell-Saunders” coupling scheme is valid. Sometimes the first rule is applied to molecules. Hush model: See: Marcus-Hush relationship. Hyperchromic effect: Increase in the intensity of a spectral band due to substituents or interactions with the molecular environment. See also auxochrome. Hyperfine: Due to nuclear spin. Hypochromic effect: Opposite of hyperchromic effect. Hypsochromic shift: Shift of a spectral band to higher frequency or shorter wavelength upon substitution or change in medium (e.g., solvent). It is informally referred to as a blue shift, and is opposite to bathochromic shift. Imaging (photoimaging): The use of a photosensitive system for the capture, recording, and retrieval of information associated with an object using electromagnetic energy. Incoherent radiation: Not having the properties of the coherent radiation. Inner filter effect: This term is used in two different ways. In an emission experiment, it refers to an apparent decrease in emission quantum yield and/or distortion of bandshape as a result of reabsorption of emitted radiation. During a light irradiation experiment, absorption of incident radiation by a species other than the intended primary absorber is also described as an inner filter effect. Inner-sphere electron transfer: Historically an electron transfer between two metal centers sharing a ligand or atom in their respective coordination shells. The definition has more recently been extended to any situation in which the interaction between the donor and acceptor centers in the transition state is significant (>20 kJ mol–1). Compare outer-sphere electron transfer.


Integrating sphere: A hollow sphere having a highly reflecting inside surface used as a device to collect, with very high efficiency, light scattered or emitted from a sample contained in it or located outside and near one of the ports. Small ports allow the entrance of light and access to a detector. Intensity: Traditional term for photon flux, fluence rate, irradiance or radiant power (radiant flux). In terms of an object exposed to radiation, the term should now be used only for qualitative descriptions. Intensity (I) (of a light source): Same as radiant intensity. Intensity (of a spectral feature): Describes the magnitude of the particular feature in the spectrum. Interference filter: See filter. Interferometer: See Fourier transform spectrometer. Internal conversion: A photophysical process. Isoenergetic radiationless transition between two electronic states of the same multiplicity. When the transition results in a vibrationally excited molecular entity in the lower electronic state, this usually undergoes deactivation to its lowest vibrational level, provided the final state is not unstable to dissociation. Internal transmittance: See transmittance. Intersystem crossing: A photophysical process. Isoenergetic radiationless transition between two electronic states having different multiplicities. It often results in a vibrationally excited molecular entity in the lower electronic state, which then usually deactivates to its lowest vibrational level. Intervalence charge transfer: Electron transfer (thermal or photoinduced) between two metal sites differing only in oxidation state. Quite often such electron transfer reverses the oxidation states of the sites. The term is frequently extended to the case of metal-to-metal charge transfer between non-equivalent metal centers. Intimate ion pair: See contact ion pair. Inverted region (for electron transfer): In plots relating rate constants to changes in standard Gibbs energy (DG0) for electron transfer a region may occur in which the rate constants decrease as the exergonicity of the reaction increases. This region is often referred to as the inverted region and its presence is predicted by the theory developed for outer sphere electron transfer for the case –DG0 > k in the Marcus equation, k being the reorganization energy.

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Note the similarity to the energy gap law for radiationless conversion of an excited state. Compare normal region. Irradiance (E): The radiant flux or radiant power, P, of all wavelengths incident on an infinitesimal element of surface containing the point under consideration divided by the area of the element (dP/dS, simplified expression: E = P/S when the –2 radiant power is R constant over the surface area considered). The SI unit is W m . Note that E = k Ek dk, where Ek is the spectral irradiance at wavelength k. For a parallel and perpendicularly incident beam not scattered or reflected by the target or its surroundings fluence rate (E0) is an equivalent term. See also photon irradiance, spectral irradiance. Isoabsorption point: The use of this term, equivalent to isosbestic point, is not recommended. Isoclinic point: A wavelength, wavenumber, or frequency at which the first derivative of an absorption spectrum of a sample does not change upon a chemical reaction or physical change of the sample. Isoemissive point: Same as isostilbic point. Isooptoacoustic point: A wavelength, wavenumber, or frequency at which the total energy emitted by a sample as heat does not change upon a chemical reaction or physical change of the sample. Its position depends on the experimental conditions. The spectral differences between the isosbestic points and the isooptoacoustic points are the result of the nonlinear relationship between the molar absorption coefficient and the photoacoustic signal. See photoacoustic spectroscopy. Isosbestic point: A wavelength, wavenumber, or frequency at which the total absorbance of a sample does not change during a chemical reaction or a physical change of the sample. The term derives from the Greek word for ‘same attenuance’. A simple example occurs when one molecular entity is converted into another which has the same molar absorption coefficient at a given wavelength. As long as the sum of the concentrations of the two molecular entities in the solution is held constant, there will be no change in absorbance at this wavelength as P the ratio of the two entities is varied. In general, A(k)l–1(= in1 eki ci ) must remain constant during the reaction or physical change in order to observe an isosbestic point. The use of the term isoabsorption point is not recommended. Isostilbic point: The wavelength at which the intensity of emission of a sample does not change during a chemical reaction or physical change. The term derives from the Greek word for ‘same luminescence’. The terms isoemissive and isolampsic are sometimes used. See isosbestic point.


Jablonski diagram: Originally, a diagram showing that the fluorescent state of a molecular entity is the lowest excited state from which the transition to the ground state is allowed, whereas the phosphorescent state is a metastable state below the fluorescent state, which is reached by radiationless transition. In the most typical cases the fluorescent state is the lowest singlet excited state and the phosphorescent state the lowest triplet state, the ground state being a singlet. Presently, modified Jablonski diagrams are frequently used and are actually state diagrams in which molecular electronic states, represented by horizontal lines displaced vertically to indicate relative energies, are grouped according to multiplicity into horizontally displaced columns. Excitation and relaxation processes that interconvert states are indicated in the diagram by arrows. Radiative transitions are generally indicated with straight arrows, while radiationless transitions are generally indicated with wavy arrows. Jahn-Teller effect: For nonlinear molecular entities in a geometry described by a point symmetry group possessing degenerate irreducible representations there always exists at least one nontotally symmetric vibration that makes electronically degenerate states unstable at this geometry. The nuclei are displaced to new equilibrium positions of lower symmetry causing a splitting of the originally degenerate states. This effect is due to the odd terms in the vibronic perturbation expansion. See also Renner-Teller effect. Kasha rule: Polyatomic molecular entities luminesce with appreciable yield only from the lowest excited state of a given multiplicity. There are exceptions to this rule. Kasha-Vavilov rule: The quantum yield of luminescence is independent of the wavelength of exciting radiation. There are exceptions to this rule. Koopmans’ theorem: See photoelectron spectroscopy. Krypton ion laser: A CW or pulsed laser emitting lines from 337 to 859 nm from singly ionized krypton. Principal emissions are at 530.9, 568.2, 647.1, and 752.5 nm. See gas lasers. Lambert law: The fraction of light absorbed by a system is independent of the incident spectral radiant power (Pk0 ). This law holds only if Pk0 is small, scattering is negligible, and multiphoton processes, excited state populations, and photochemical reactions are negligible. See absorbance, Beer-Lambert law. Lamp: A source of incoherent radiation.

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See high-pressure, medium-pressure, and low-pressure mercury lamp (arc), and antimony-xenon, mercury-xenon, quartz-iodine, tungsten-halogen, resonance, and xenon lamp. Laporte rule: For monophotonic radiative transitions in centro-symmetric systems, the only nonvanishing electric-dipole transition moments are those which connect an even term (g) with an odd term (u). Laser: A source of ultraviolet, visible, or infrared radiation which produces light amplification by stimulated emission of radiation from which the acronym is derived. The light emitted is coherent except for superradiance emission. See argon ion, helium-cadmium, chemical, CO2 copper vapor, diode, dye, excimer, free electron, free-running, gas, helium-neon, krypton ion, mode-locked, neodymium, nitrogen, Q-switched, solid state, and ruby laser. See also lasing. Lasing: The process of light amplification by stimulated emission of radiation (laser). Latent image: The primary result of radiation absorption in a photo-imaging system which is susceptible to development. Lifetime (s): The lifetime of a molecular entity which decays in a first-order process is the time needed for a concentration of the entity to decrease to 1/e of its original value. Statistically, it represents the life expectation of the entity. It is equal to the reciprocal of the sum of the (pseudo)unimolecular rate constants of all processes which cause the decay. Lifetime is used sometimes for processes which are not first order. However, in such cases, the lifetime depends on the initial concentration of the entity, or of a quencher and therefore only an initial or a mean lifetime can be defined. In this case it should be called apparent lifetime, instead. Occasionally, the term half-life (s1/2) is used, representing the time needed for the concentration of an entity to decrease to one half of its original value. Ligand field splitting: The removal of a degeneracy of atomic or molecular levels in a molecule or ion with a given symmetry induced by the attachment or removal of ligands to produce reduced symmetries. See crystal field splitting. Ligand to ligand charge transfer (LLCT) transition: An electronic transition of a metal complex that corresponds to excitation populating an electronic state in which considerable electron transfer between two ligands has occurred.


Ligand to metal charge transfer (LMCT) transition: An electronic transition in a metal complex that corresponds to excitation populating an electronic state in which considerable electron transfer from a ligand to a metal center has occurred. See also metal to ligand charge transfer transition. Light polarization: When the end point of the electric vector of a polarized light beam is viewed along the direction of light propagation, it moves along a straight line if the light is linearly polarized, along a circle if it is circularly polarized, and along an ellipse if it is elliptically polarized. Light source: See lamp, laser. Low-pressure mercury lamp (Arc): A type of resonance lamp which contains mercury vapor at pressures of about 0.1 Pa (0.75 × 10–3 Torr; 1 Torr = 133.3 Pa). At 25 8C, such a lamp emits mainly at 253.7 and 184.9 nm. Other terms used for such a lamp are germicidal, cold and hot cathode, Wood lamp. See lamp. Luminescence: Spontaneous emission of radiation from an electronically or vibrationally excited species not in thermal equilibrium with its environment. See also bioluminescence, chemiluminescence, electro-generated chemiluminescence, fluorescence, phosphorescence, photoluminescence, radioluminescence, sono luminescence, thermoluminescence, triboluminescence. Lumiphore (Luminophore): A part of a molecular entity (or atom or group of atoms) in which electronic excitation associated with a given emission band is approximately localized. (Analogous to chromophore for absorption spectra.) Marcus equation (for electron transfer): Equation proposed by R.A. Marcus to relate the rate of outer-sphere electron transfer with the thermodynamics of this process (see: R.A. Marcus, J. Chem. Phys. 24, 966–978 (1956)). Essentially the rate constant within the encounter complex (or the rate constant of intramolecular transfer) is given by the Eyring equation: ket jel kT=h exp DGz =RT where k represents Boltzmann’s constant and jel is the so called electronic transmission factor: (jel ~ 1 for adiabatic and 1 for diabatic electron transfer). It was shown by Marcus that for outer-sphere electron transfer the barrier height can then be expressed as: DGz k DG0 2 =4k where DG0 represents the standard Gibbs energy change accompanying the reaction and k the total reorganization energy.

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It should be noted that whereas the classical Marcus equation has been found to be quite adequate in the normal region, it is now generally accepted that in the inverted region a more elaborate formulation, taking into account explicitly the Franck-Condon factor due to quantum mechanical vibration modes, should be employed. Marcus inverted region (for electron transfer): See inverted region. Marcus-hush relationship: Relationship between the barrier (DGz) to thermal electron transfer, the energy of a corresponding optical charge transfer transition (DEop ), and the overall change in standard Gibbs energy accompanying thermal electron transfer (DG0). Assuming a quadratic relation between the energy of the system and its distortions from equilibrium (harmonic oscillator model) the expression obtained is: 2 =4DEop DGz DEop

DG0

The simplest form of this expression obtains for degenerate electron transfer (DG0) in e.g. symmetrical mixed valence systems: DGz DEop =4 Note that for this situation the Marcus equation reads DGz = k/4. See Marcus equation. Medium-pressure mercury lamp (Arc): Radiation source containing mercury vapor at pressures ranging from 100 to several hundred kPa (1 atm = 101.325 kPa). Emits mostly from 310 to 1000 nm with most intense lines at 300, 303, 313, 334, 366, 405, 436, 546, and 578 nm. See lamp. Mercury-xenon lamp (Arc): An intense source of ultraviolet, visible, and near infrared radiation produced by an electrical discharge in a mixture of mercury vapor and xenon under high pressure. See lamp. Merry-go-round reactor (Turntable Reactor): An apparatus in which several samples are rotated around a radiation source in order to expose each to equal amounts of radiation. Metal to ligand charge transfer (MLCT) Transition: An electronic transition of a metal complex that corresponds to excitation populating an electronic state in which considerable electron transfer from the metal to a ligand has occurred. Compare ligand to metal charge transfer transition.


Metal to metal charge transfer (MMCT) transition: An electronic transition of a bior poly-nuclear metal complex that corresponds to excitation populating an electronic state in which considerable electron transfer between two metal centers has occurred. See also intervalence charge transfer. MLCT: See metal to ligand charge transfer. Mode-locked laser: A laser in which many resonant modes are coupled in phase, to yield a train of very short pulses (e.g. ps pulses). The coupling of the modes is obtained by modulation of the gain in the resonator, and can be active (electro-optic modulation of the losses or of the pump intensity), or passive (with a saturable absorber). See also free-running laser. Molar absorption coefficient, molar decadic absorption coefficient: Absorbance divided by the absorption pathlength, l and the concentration, c: e 1=cl logPk0 =Pk A=cl In common usage for l in cm and c in mol dm–3 or M, e results in dm3 mol–1 cm–1 or M–1 cm–1, which equals 0.1 m2 mol–1 (coherent SI units) = 103 cm2 mol–1 = cm2 mmol–1 = dm3 cm–1 mol–1. The term molar absorptivity for molar absorption coefficient should be avoided. See absorbance, absorption coefficient, Beer-Lambert law. Multiphoton absorption: See multiphoton process. See also biphotonic excitation. Multiphoton process: A process involving interaction of two or more photons with a molecular entity. See biphotonic process, two-photon process. Multiplicity (Spin Multiplicity): The number of possible orientations, calculated as 2S + 1, of the spin angular momentum corresponding to a given total spin quantum number (S), for the same spatial electronic wavefunction. A state of singlet multiplicity has S = 0 and 2S + 1 = 1. A doublet state has S = 1/2, 2S + 1 = 2, etc. Note that when S > L (the total orbital angular momentum quantum number) there are only 2L + 1 orientations of total angular momentum possible. Non-adiabatic electron transfer: See diabatic electron transfer. Normal region (for electron transfer): In plots relating rate constants for electron transfer, or quantities related to it, with the standard Gibbs energy for the reaction (DG0), the region for which the rate constants increase with increasing exergoni-

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city of the reaction is called the normal region. This region is predicted by the Marcus theory for outer-sphere electron transfer for the case of DGo ≤ k in the Marcus equation. n ? p* transition: An electronic transition described approximately as promotion of an electron from a “nonbonding” (lone-pair) n orbital to an “antibonding” p orbital designated as p*. n–p* state: An excited state related to the ground state by a n ? p* transition. n ? r* transition: An electronic transition described approximately as promotion of an electron from a “nonbonding” (lone-pair) n orbital to an “antibonding” r orbital designated as r*. Such transitions generally involve high transition energies and appear close to or mixed with Rydberg transitions. Natural lifetime: Same as radiative lifetime. The use of this term is discouraged. Neodymium laser: A CW or pulsed laser emitting radiation from excited Nd+3 principally occurring around 1.06 lm (the precise position depends on the matrix). The Nd+3 is present as a dopant in suitable crystals (e.g., yttrium-aluminum garnet, YAG) or in suitable glasses (phosphate, silicate, etc.). See solid state lasers. Neutral-density filter: See attenuance filter. Nitrogen laser: A source of pulsed semi-coherent superradiance mainly around 337 nm. The lasing species is molecular nitrogen. See gas lasers. Non-adiabatic photoreaction: Same as diabatic photoreaction. Use of double negative is discouraged. Compare adiabatic photoreaction. Non-linear optical effect: An effect brought about by electromagnetic radiation the magnitude of which is not proportional to the irradiance. Non-linear optical effects of importance to photochemists are harmonic frequency generation, lasers, Raman shifting, upconversion, and others. Non-radiative decay: Disappearance of an excited species due to a radiationless transition. Non-vertical energy transfer: An energy transfer process which has a low FranckCondon factor. See Franck-Condon principle.


Norrish type I photoreaction: a-Cleavage of an excited carbonyl compound leading to an acyl-alkyl radical pair (from an acyclic carbonyl compound) or an acyl-alkyl biradical (from a cyclic carbonyl compound) as a primary photoproduct; e.g.,

Norrish type II photoreaction: Intramolecular abstraction of a c-hydrogen by an excited carbonyl compound to produce a 1,4-biradical as a primary photoproduct; e.g.,

Optical density: Synonymous with absorbance. The use of the term optical density is discouraged. Optoacoustic spectroscopy: Same as photoacoustic spectroscopy. Orbital (Atomic or Molecular): A wavefunction which depends explicitly on the spatial coordinates of only one electron. Oscillator strength (f Number): A measure of the intensity of a spectral band: a classical concept (giving the effective number of electrons taking part in a certain transition) adapted to wave mechanics. For a transition between state i and state j, fij

8p2 mcm GR2ij 3h

where m is the mass of the electron, c the velocity of light, m the frequency, h the Planck constant, G the degeneracy of the final state. Rij is the transition moment calculated omitting e, the electron charge, in the integral; better called the transition length. Experimentally, fij is determined by integration of the absorption band, using the equation Z fij k=n erdr; where e(r) is the molar absorption coefficient at wavenumber r and n the average refractive index of the medium. k = 4.32 × 10–9 L–1 mol cm (for e in the common units L mol–1 cm–1); k = 4.32 × 10–8 mol m–1 (for e in SI unit m2 mol–1). The oscillator strength, fij , is a dimensionless quantity. See also transition (dipole) moment.

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Outer-sphere electron transfer: Electron transfer between redox centers which do not share a common atom or group, i.e., the interaction between the relevant electronic orbitals of the two centers in the transition state is weak (

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