(NAS Colloquium) Geology, Mineralogy, and Human Welfare

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COLLOQUIUM ON GEOLOGY, MINERALOGY, AND HUMAN WELFARE

NATIONAL ACADEMY OF SCIENCES WASHINGTON, D.C. 1999


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NATIONAL ACADEMY OF SCIENCES

Colloquium Series In 1991, the Nationfal Academy of Sciences inaugurated a series of scientific colloquia, five or six of which are scheduled each year under the guidance of the NAS Council's Committee on Scientific Programs. Each colloquium addresses a scientific topic of broad and topical interest, cutting across two or more of the traditional disciplines. Typically two days long, colloquia are international in scope and bring together leading scientists in the field. Papers from colloquia are published in the Proceedings of the National Academy of Sciences (PNAS).


GEOLOGY, MINERALOGY, AND HUMAN WELFARE

Geology, Mineralogy, and Human Welfare

A Colloquium sponsored by the National Academy of Sciences November 8-9, 1998 PROGRAM Sunday, November 8, 1998 AGRICULTURAL MINERALOGY/SOILS, SURFACES Garrison Sposito, University of California, Berkeley, Surface geochemistry of the clay minerals 9:40am 10:20am Paul M. Bertsch, University of Georgia/SREL, Characterization of complex mineral assemblages: implications for contaminant transport and environmental remediation 11:20am Samuel J. Traina, Stanford University, Contaminant bioavailability in soils, sediments and aquatic environments Gordon E. Brown, Jr., Stanford University, Mineral surfaces and bioavailability of heavy metals: A molecular-scale perspective 12:00pm AEROSOLS AND CLIMATE Joseph M. Prospero, University of Miami RSMAS, Long-range transport of mineral dust in the global atmosphere: Impact of African 2:00pm dust on the environment of the southeastern United States 2:40pm Peter R. Buseck, Arizona State University, Airborne minerals and related aerosol particles: Effects on climate and the environment OCEANS AND BIOMINERALOGY Miriam Kastner, Scripps Institution of Oceanography, Oceanic minerals and rocks, their origin, occurrence, and economic significance 3:40pm 4:20pm Keith A. Kvenvolden, U.S. Geological Survey, Potential effects of gas hydrates on human welfare Jillian F. Banfield, University of Wisconsin, Madison, Biological impact on mineral dissolution - application of the lichen model to 5:00pm understanding mineral weathering in rhizosphere

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Monday, November 9, 1998 RADWASTE, MINING, AND ENVIRONMENTAL ISSUES 9:10am Rodney C. Ewing, University of Michigan, Nuclear waste forms of actinides 9:50am Robert B. Finkelman, U.S. Geological Survey, The health impacts of domestic coal use in China 10:50am Robert P. Nolan, City University of New York, A risk assessment for exposure to grunerite asbestos (amosite) in an iron ore mine 11:30am D. Kirk Nordstrom, U.S. Geological Survey, Negative pH, efflorescent mineralogy, and consequences for environmental restoration at the Iron Mountain Superfund site, California PURE AND APPLIED MINERALOGY 2:00pm David R. Pevear, Exxon Production Research Co., Illite and hydrocarbon exploration 2:40pm Jeffrey E. Post, Smithsonian Institution, National Museum of Natural History, Manganese oxide minerals: crystal structures & economic and environmental significance 3:40pm John D. Sherman, UOP Research Center, Synthetic zeolites and other microporous oxide molecular sieves 4:20pm Frederick A. Mumpton, SUNY - College, Natural zeolites - la rocca magica? J.V. Smith, University of Chicago, Biochemical evolution: III. Polymerization on organophilic silica-rich surfaces; crystal-chemical 5:00pm modeling; formation of first cells; geological clues

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TABLE OF CONTENTS

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PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA

Table of Contents

Geology, mineralogy, and human welfare Joseph V. Smith Characterization of complex mineral assemblages: Implications for contaminant transport and environmental remediation Paul M. Bertsch and John C. Seaman Surface geochemistry of the clay minerals Garrison Sposito, Neal T. Skipper, Rebecca Sutton, Sung-ho Park, Alan K. Soper, and Jeffery A. Greathouse Contaminant bioavailability in soils, sediments, and aquatic environments Samuel J. Traina and Valérie Laperche Airborne minerals and related aerosol particles: Effects on climate and the environment Peter R. Buseck and Mihály Pósfai Oceanic minerals: Their origin, nature of their environment, and significance Miriam Kastner Mineral surfaces and bioavailability of heavy metals: A molecular-scale perspective Gordon E. Brown, Jr., Andrea L. Foster, and John D. Ostergren Long-range transport of mineral dust in the global atmosphere: Impact of African dust on the environment of the southeastern United States Joseph M. Prospero Biological impact on mineral dissolution: Application of the lichen model to understanding mineral weathering in the rhizosphere Jillian F. Banfield, William W. Barker, Susan A. Welch, and Anne Taunton A risk assessment for exposure to grunerite asbestos (amosite) in an iron ore mine R. P. Nolan, A. M. Langer, and Richard Wilson Potential effects of gas hydrate on human welfare Keith A. Kvenvolden Health impacts of domestic coal use in China Robert B. Finkelman, Harvey E. Belkin, and Baoshan Zheng Nuclear waste forms for actinides Rodney C. Ewing Illite and hydrocarbon exploration David R. Pevear Manganese oxide minerals: Crystal structures and economic and environmental significance Jeffrey E. Post Negative pH, efflorescent mineralogy, and consequences for environmental restoration at the Iron Mountain Superfund site, California D. Kirk Nordstrom and Charles N. Alpers La roca magica: Uses of natural zeolites in agriculture and industry Frederick A. Mumpton Synthetic zeolites and other microporous oxide molecular sieves John D. Sherman Biochemical evolution III: Polymerization on organophilic silica-rich surfaces, crystal–chemical modeling, formation of first cells, and geological clues Joseph V. Smith, Frederick P. Arnold, Jr., Ian Parsons, and Martin R. Lee

3348–3349 3350–3357 3358–3364 3365–3371 3372–3379 3380–3387 3388–3395 3396–3403 3404–3411 3412–3419 3420–3426 3427–3431 3432–3439 3440–3446 3447–3454 3455–3462 3463–3470 3471–3478 3479–3485


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LIST OF ATTENDEES

List of Attendees Charles N. Alpers, US Geological Survey Phyl Amadi, Salt River Project David Applegate, Geotimes Ellen Avery Jillian F. Banfield, University of Wisconsin-Madison Paul M. Bertsch, University of Georgia, SREL Gordon E. Brown, Stanford University Peter R. Buseck, Arizona State University Fernando Camara, Arizona State University Winifred Caponigri, Holy Cross College Bill Casey, University of California, Davis Sing-Foong Cheah, University of California, Berkeley Ron Churchill, California Department of Conservation John Clinkenbeard, California Division of Mines and Geology Patricia Colville, James Hardie Building Products Michael L. Cummings, Department of Geology Frank Cynar Tammy Dickinson, National Research Council Katerina Edward W.G. Ernst, Stanford University Rodney C. Ewing, University of Michigan Robert B. Finkelman, U.S. Geological Survey Andrea Foster, Stanford University Julia Gaudinski, Univeristy of California, Irvine Tom Gihring, University of Wisconsin-Madison M. Charles Gilbert, University of Oklahoma Janet Gordon, Pasadena City College Priscilla C. Grew, University of Nebraska-Lincoln Shalini Gupta Noel Heim George R. Helz JoAnn Holloway, Land, Air and Water Resources Robert M. Housely Rick Humphreys, State Water Resouces Control Board Dawn Janney, Arizona State University Miriam Kastner, Scripps Institution of Oceanography Christopher Kim, Stanford University Stephan Kraemer, University of California, Berkeley Konrad B. Krauskopf, Stanford University Keith A. Kvenvolden, U.S. Geological Survey Jia Li, Arizona State University Gwen Loosmore, Lawrence Livermore National Laboratory Elizabeth Magno Juraj Majzlan, University of California, Davis Kevin Mandernack, Colorado School of Mines Carrie Masiello, University of California, Irvine Carleton B. Moore Frederick A. Mumpton, SUNY-College Paulina Mundkowski, University of Chicago Alexandra Navrotsky, University of California, Davis William Nesse, University of Northern Colorado Heino Nitsche, University of California, Berkeley Robert Nolan, City University of New York D. Kirk Nordstrom, U.S. Geological Survey Everest Tan Ong, University of Chicago John Ostergren, Stanford University Sung-Ho Park, University of California, Berkeley Jill D. Pasteris, Washington University Adina Paytan, University of California, San Diego Erich U. Petersen David R. Pevear, Exxon Production Research Co. Bernard Pipkin F.D. Pooley, Cardiff University Jeffrey E. Post, Smithsonian Institution Joseph M. Prospero, University of Miami, RSMAS Jim Ranville, Colorado School of Mines Malcom Ross, U.S. Geological Survey Donald Runnells, Shepherd Miller, Inc. Martin S. Rutstein, State University of New York John D. Sherman, UOP Research Center David K. Shuh, Lawrence Berkeley National Laboratory Fiorella Simoni, George Mason University J.V. Smith, University of Chicago Neville Smith, Lawrence Berkeley National Laboratory Garrison Sposito, University of California, Berkeley John O. Strong Rebecca Sutton, University of California, Berkeley Steve Sutton Alexis Templeton, Stanford University Samuel J. Traina, Stanford University Flavio Vasconcelos, Colorado School of Mines Glenn Waychunas, Lawrence Berkeley Laboratory Potter Wickware, Molecular Applications News Erika Williams, University of Michigan Howard G. Wilshire



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Geology, mineralogy, and human welfare

Proc. Natl. Acad. Sci. USA Vol. 96, pp. 3348–3349, March 1999 Colloquium Paper This paper is the introduction to the following papers, which were presented at the National Academy of Sciences colloquium “Geology, Mineralogy, and Human Welfare,” held November 8–9, 1998 at the Arnold and Mabel Beckman Center in Irvine, CA. JOSEPH V. SMITH* PNAS is available online at www.pnas.org.

Department of Geophysical Sciences and Center for Advanced Radiation Sources, 5734 S. Ellis Avenue, The University of Chicago, Chicago, IL 60637 The complex sciences of geology and mineralogy couple the focused sciences of physics, chemistry, and biology to the “diffuse” disciplines of ecology and the environment. For this colloquium, 18 papers have been selected on matters related to human welfare, particularly health and both physical and mental wellbeing, to demonstrate the importance of new research plans and new instrumentation. Agricultural Mineralogy, Soils, and Surfaces. Emerging “chemical microscopes” using neutrons, synchrotron x-rays, and electrons allow physicochemical characterization of mineral surfaces and adsorbed molecules and ions in soils (1). Plant growth depends on subtle interactions between mineral surfaces, saline fluids, and microbes. Incorporation of “good” trace elements into food depends on the interaction of organic and inorganic components, as does that of toxic ones. One-quarter of the wheat and rice crops are lost to Mn-oxidizing bacteria. Soils become contaminated with mobile toxic elements, including Pb, Cd, Se, and As, which can affect plant growth and food safety (2, 3 and 4). Aerosols and Climate. Mineral dust blown from drying geological basins pervades the atmosphere, and falls to earth with both good results—loess soil in central Europe, China, and North America was important for early agriculture, and still supports large populations—and bad ones—air pollution causes lung problems (5). Chemical microscopes provide physicochemical analysis of tiny particles, particularly useful for distinguishing the natural and industrial components (6). Oceans. Minerals in the oceans range from several dozen types in bioorganisms to various zeolites grown from volcanic ash, sulfides in hot smokers in volcanic ridges, and precipitates in Mn-rich nodules (7). The reactions with sea-water depend on temperature and composition and ultimately can be related to climate and plate-tectonic processes. Highly touted as a major energy source for the future are the methanewater clathrate beds on cold ocean beds; however, current evidence is not promising for successful commercialization (8). Biomineralogy. Chemical microscopes coupled with biochemical techniques are opening up a rapidly expanding field of studies on microbes. Before these tools were developed, mineralogists could only speculate on how microbes concentrated useful elements (including uranium!) into ore bodies, how microbes interact with atmospheric gases to modify the climate, how deep-seated ones relate to the spatial distribution and chemical signatures of natural gas and oil in the continents, and so on. Microbes can make organic acids that accelerate mineral weathering to make soil minerals (good), and eat away outdoor statues (bad) (9). Honeycombed surfaces of weathered feldspars may have been the first home of primitive cells where they were protected from destruction by solar ultraviolet radiation. The internal hydrophobic silica-rich surface in nanometer-wide channels of a zeolite mineral formed from abundant volcanic ash (e.g., mutinaite = synthetic silicalite-ZSM-5) might have scavenged organic species from the proverbial water-rich “soup” and catalyzed assembly into primitive polymers that extruded like spaghetti to become tangled up to form the nucleus of a proto-cell (10). Radwaste, Mining, and Environmental Issues. Perhaps a billion people, including some in developed countries, currently ingest harmful amounts of toxic elements. A prime aim of this colloquium was careful evaluation of selected problems, and establishment of an international plan for coupling scientific and administrative skills for mitigation (11, 12, 13 and 14). Pure and Applied Mineralogy. Improvements in the chemistry of electric storage batteries are related to mineralogy (e.g., long-life Pb; high-energy Li, etc). Over 400 minerals contain Mn; we selected Mn-oxides for presentation because of advances in understanding their chemistry using the new chemical microscopes (15). Perhaps the most spectacular advances have been in the petroleum industries, to the great benefit of the consumer. Three-dimensionalseismic imaging, slant drilling, and other engineering advances are tripling the recovery of petroleum from geologic reservoirs and actually advancing the provable reserves (although most prognostications assert that supply will not meet demand some time before 2030). The value of mineral geochemistry was illustrated by use of subtle argon-age dating of clay minerals across a potential basin to predict its yield of oil (16). An even broader success story has been the invention of zeolite/molecular sieve adsorbent/catalysts and industrial development of myriad applications (17). Almost unknown to all but the zeolite chemists and engineers are everyday applications: the 3-fold increase in yield of gasoline from petroleum from tailored zeolite catalysts, also higher octane number and lower pollutants in automobile exhausts; clearer multipane windows from zeolite adsorbing dirty vapors; safer brakes in trucks and trains from pressure-swing zeolite adsorbent; longer life of refrigerators; and selective adsorbents in nuclear waste. Just moving forward into major use are natural zeolites whose low cost and high exchange capacity are leading to bulk applications in agriculture, gardening, and waste management (18). Geoscientists now have the “chemical microscopes” and other tools to study the scientific characteristics of toxic materials, and we can now study at the atomic level those interactions between the inorganic and organic worlds that have positive aspects for human welfare. However, the available funds are far too small to properly service the growing community of environmental geoscientists. Hence, the colloquium concluded with presentations on a concept for establishment of a new efficient program for instrumentation in the environmental sciences costing only $100 million to be complemented by a similar one for research, teaching, and public

*e-mail: [email protected].



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outreach over the initial five years at universities, colleges, and experimental stations. An additional $100 million is needed to bring together research professionals and students around the world to quantify the dangers to human health of toxic elements, including As, Se, and Pb; to devise plans for dissemination of information; and to evaluate ideas for remediation in the context of diplomatic, social science, and economic planning procedures. To conclude, particularly important for this colloquium is that the boundaries between subdisciplines are falling; that humans are now moving as much material on the Earth's surface as geological processes; that natural climatic changes are being modified willy-nilly by human activities; and that the recent increase in population and use of energy is beginning to slow down as fundamental limits are approached, but they may not slow down fast enough. Doubling of the population might be sustainable, but quadrupling almost certainly would lead to serious problems and possible catastrophes. Biological evolution, as seen in the context of geologic time, indicates that fortune goes with increasingly skilful use of resources of many types, not the maximum use of resources.

1. Sposito, G., Skipper, N. T., Sutton, R., Park, S.-h., Soper, A. K. & Greathouse, J. A. (1999) Proc. Natl. Acad. Sci. USA 96, 3358–3364. 2. Bertsch, P. M. & Seaman, J. C. (1999) Proc. Natl. Acad. Sci. USA 96, 3350–3357. 3. Traina, S. J. & Laperche, V. (1999) Proc. Natl. Acad. Sci. USA 96, 3365–3371. 4. Brown, G. E., Jr., Foster, A. L. & Ostergren, J. D. (1999) Proc. Natl. Acad. Sci. USA 96, 3388–3395. 5. Prospero, J. M. (1999) Proc. Natl. Acad. Sci. USA 96, 3396–3403. 6. Buseck, P. R. & Pósfai, M. (1999) Proc. Natl. Acad. Sci. USA 96, 3372–3379. 7. Kastner, M. (1999) Proc. Natl. Acad. Sci. USA 96, 3380–3387. 8. Kvenvolden, K. A. (1999) Proc. Natl. Acad. Sci. USA 96, 3420–3426. 9. Banfield, J. F., Barker, W. W., Welch, S. A. & Taunton, A. (1999) Proc. Natl. Acad. Sci. USA 96, 3404–3411. 10. Smith, J. V., Arnold, F. P., Jr., Parsons, I. & Lee, M. R. (1999) Proc. Natl. Acad. Sci. USA 96, 3479–3485. 11. Ewing, R. C. (1999) Proc. Natl. Acad. Sci. USA 96, 3432–3439. 12. Finkelman, R. B., Belkin, H. E. & Zheng, B. (1999) Proc. Natl. Acad. Sci. USA 96, 3427–3431. 13. Nolan, R. P., Langer, A. M. & Wilson, R. (1999) Proc. Natl. Acad. Sci. USA 96, 3412–3419. 14. Nordstrom, D. K. & Alpers, C. N. (1999) Proc. Natl. Acad. Sci. USA 96, 3455–3462. 15. Post, J. E. (1999) Proc. Natl. Acad. Sci. USA 96, 3447–3454. 16. Pevear, D. R. (1999) Proc. Natl. Acad. Sci. USA 96, 3440–3446. 17. Sherman, J. D. (1999) Proc. Natl. Acad. Sci. USA 96, 3471–3478. 18. Mumpton, F. A. (1999) Proc. Natl. Acad. Sci. USA 96, 3463–3470.


CHARACTERIZATION OF COMPLEX MINERAL ASSEMBLAGES: IMPLICATIONS FOR CONTAMINANT TRANSPORT AND ENVIRONMENTAL REMEDIATION

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Characterization of complex mineral assemblages: Implications for contaminant transport and environmental remediation

Proc. Natl. Acad. Sci. USA Vol. 96, pp. 3350–3357, March 1999 Colloquium Paper This paper was presented at the National Academy of Sciences colloquium “Geology, Mineralogy, and Human Welfare,” held November 8–9, 1998 at the Arnold and Mabel Beckman Center in Irvine, CA. PAUL M. BERTSCH* AND JOHN C. SEAMAN PNAS is available online at www.pnas.org.

Advanced Analytical Center for Environmental Sciences, Savannah River Ecology Laboratory, The University of Georgia, Drawer E, Aiken, SC 29802 ABSTRACT Surface reactive phases of soils and aquifers, comprised of phyllosilicate and metal oxohydroxide minerals along with humic substances, play a critical role in the regulation of contaminant fate and transport. Much of our knowledge concerning contaminant-mineral interactions at the molecular level, however, is derived from extensive experimentation on model mineral systems. Although these investigations have provided a foundation for understanding reactive surface functional groups on individual mineral phases, the information cannot be readily extrapolated to complex mineral assemblages in natural systems. Recent studies have elucidated the role of less abundant mineral and organic substrates as important surface chemical modifiers and have demonstrated complex coupling of reactivity between permanent-charge phyllosilicates and variable-charge Fe-oxohydroxide phases. Surface chemical modifiers were observed to control colloid generation and transport processes in surface and subsurface environments as well as the transport of solutes and ionic tracers. The surface charging mechanisms operative in the complex mineral assemblages cannot be predicted based on bulk mineralogy or by considering surface reactivity of less abundant mineral phases based on results from model systems. The fragile nature of mineral assemblages isolated from natural systems requires novel techniques and experimental approaches for investigating their surface chemistry and reactivity free of artifacts. A complete understanding of the surface chemistry of complex mineral assemblages is prerequisite to accurately assessing environmental and human health risks of contaminants or in designing environmentally sound, cost-effective chemical and biological remediation strategies. The transport and fate of contaminants in soils and groundwater are highly coupled to the nature and relative abundance of the reactive mineral phases. Clay and oxide minerals, along with humified organic matter, comprise the surface reactive phases that are the primary controllers of sorption processes in soils, thus serving as important regulators of contaminant transport. Major challenges in understanding the processes controlling contaminant behavior in the environment include the complexity of the soil and aquifer matrix and the enormous spatial scales over which these processes occur. Although it is well established that a fundamental understanding of molecular-level interactions is required to explain the underlying mechanisms controlling the fate and transport of solutes and contaminants in soils and subsurface environments, there has been limited success in translating molecular-level information to observations made at the larger scales. Although several explanations for this conundrum can be advanced, a prominent one is that much of our knowledge concerning the surface chemistry of clay and oxide minerals primarily is derived from experiments conducted on model mineral phases. These studies have established boundary conditions defining sorbate/mineral surface interactions and have identified the surface functional groups involved in surface complexation reactions, but they have produced little information that can be readily extrapolated to complex mineral assemblages typically present in heterogeneous soil and aquifer materials (1). Thus, utilization of bulk mineralogical data to represent predominant reactive phases in complex natural systems often has failed to reliably predict solute and contaminant behavior. Reactive Mineral Phases in Soils: An Historical View The pioneering work on reactive mineral phases in soils, which focused primarily on adsorption of group IA and IIA cations, has been comprehensively reviewed (2, 3) as has more recent work on specific sorption of metals and metalloids (1, 4). The concept that surface reactive phases in soils are colloidal and comprised of Al(OH)3, Fe(OH)3, and SiO2 hydrogels was proposed over a century ago (5). By the mid-1920s, a comprehensive understanding of the surface chemistry of Al, Fe, and Si colloids and their role in cation sorption was emerging, largely based on extensive investigations of Mattson (6, 7 and 8). Mattson viewed reactive phases in soils as mixtures of Al2O3, Fe2O3, and SiO2 colloids. Based on the observation that soils with a high SiO2/Al2O3 + Fe2O3 ratio had higher cation exchange capacities (CEC) and that soils with a low SiO2/Al2O3 + Fe2O3 ratios had high anion exchange capacities at low pH and higher CEC at high pH, Mattson concluded that the SiO2 colloids were primarily responsible for the CEC of a soil and that the Al2O3 and Fe2O3 colloids were amphoteric in nature. Mattson's compelling evidence for mixtures of positively and negatively charged colloids, based on careful cation/anion sorption experiments and electrophoresis, was largely disregarded as attention shifted to a new, rapidly emerging paradigm of reactive mineral phases predicated on the notion that soil clays were comprised primarily of crystalline phases. Two classic papers by Pauling (9, 10) figured prominently in this paradigm shift. Shortly thereafter, soil chemists applied x-ray diffraction to soil clays and discovered the existence of, and delineated the structures for, the major classes of phyllosilicate clays commonly found in soils (11, 12). Soon after it was demonstrated that most phyllosilicates in soils had a permanent negative charge resulting from substitution of lower valence cations in both the tetrahedral and octahedral layers (13). For decades the surface chemistry of reactive phases in soils would be interpreted primarily according to this paradigm, i.e., that predominant reactive phases in soil were

Abbreviations: pznc, point of zero net charge; EM, electron microscopy. *To whom reprint requests should be addressed.



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crystalline and comprised of negatively charged minerals of the phyllosilicate class. Three rather fortuitous circumstances solidified this view of reactive mineral phases. First, free Fe oxides and organic macromolecules were removed from soil clay fractions via pretreatment to improve x-ray diffraction patterns by minimizing background scatter and improving preferred orientation of the phyllosilicate clay minerals. Second, most of the active soil mineralogy groups emerging during this period were limited to geographical areas characterized by young circumneutral soils having clay fractions dominated by 2:1 phyllosilicates; albeit, on a worldwide basis these soils were more of an exception. Finally, much of the experimentation during this period continued to involve the adsorption/exchange of class IA and IIA cations, both of which are relatively weakly bound and present at relatively high concentrations (an exception is K+, whose chemistry is controlled by a unique combination of cation size, low hydration energy, and structural properties of micaceous minerals and their weathering products). Thus, much of the data generated under these conditions was consistent with the phyllosilicate model, and the distribution of phyllosilicates within a given soil clay fraction generally could be used to predict observed cation exchange behavior for this limited range of extensively studied soils. There continued to be prominent exceptions to this model that could be better interpreted according to a Mattson-like model of surface reactive phases (14, 15). Evidence for anion adsorption to soil clays having low SiO2/Al2O3 + Fe2O3 ratios and slightly acidic pH continued to appear. Evidence for positively charged regions (edge sites) on phyllosilicate clays in slightly acidic suspensions appeared during this time (16, 17, 18 and 19). This model also was used to interpret anion adsorption and complex flocculation/dispersion behavior of kaolinite suspensions (20). Clearly, the phyllosilicate model of reactive mineral phases based largely on 2:1 minerals in soils of circumneutral pH was limited in its extent of applicability. As mineralogical techniques improved and experimental approaches evolved, another very important body of literature on hybrid phyllosilicate-Al/Fe oxohydroxides emerged. The discovery (21) that 2:1 minerals in soils weathered from parent materials rich in mica schist were interlayered with nonexchangeable, positively charged hydroxo-Al polynuclear components stimulated a significant body of research that continues to this day and includes investigations on an important class of zeolite-like clay catalysts (22). Although this finding explained a number of properties related to the surface chemistry of many 2:1 soil clays, the research emphasis on the hydroxy-interlayered minerals largely focused on explaining the unique adsorption behavior of large weakly hydrated monovalent cations, such as K+, NH4+, and Cs+, with less emphasis on anion sorption. Only many years later would the role of this complex mineral assemblage in the specific sorption of transition metals be considered (22). Concurrent with these exciting developments, a new paradigm of surface reactive mineral phases was emerging. The structural aspects of important functional groups on oxide minerals were being unraveled (4, 23). The notion of surface structural hydroxyl groups having acid/base properties that could quantitatively explain the observed amphoteric behavior of oxides became firmly established (24). Thus, an accurate model of surface functional groups that could explain Mattson's original observations was emerging, and a number of studies on anion adsorption to metal oxide surfaces followed quickly as did spectroscopic evidence for the proposed reactive surface hydroxyls (4). It was now established that solutes could interact with charged metal oxide surfaces via electrostatic (outer sphere) reactions or through specific ligand exchange reactions with the surface functional groups (inner sphere). The conceptual model of surface complexation to describe nonspecific and specific adsorption of anions and cations was advanced shortly thereafter by the classic work of Schindler and Gamsjager (25) and Stumm et al. (26). The surface complexation model has remained the basic framework for research on metal and anion sorption to metal oxide surfaces to the present time (1, 4), and many studies have demonstrated the importance of metal oxides as resident phases for a variety of metals and metalloids (1, 27). Although extensive modeling efforts have demonstrated reasonable success for predicting metal and metalloid sorption to model monomineralic metal oxide phases, applications to natural systems have been less than satisfying (1). A major challenge in extending such results to complex mineral assemblages typically found in nature has been the identification and quantification of the primary reactive phase and associated surface functional groups. High surface area, low abundance metal oxohydroxide phases, and organic materials can be coassociated with more prominent mineral grains as armoring agents or as surface coatings. The term surface coating as used here does not imply the presence of a uniform gel-like phase as is often envisioned. Rather, it is used to describe domains of crystalline or noncrystalline components coassociated with well-defined mineral grains. The complex nature of the electrostatic and van der Waals interactions between finegrained crystalline and poorly ordered phases with mixed surface-charge properties has hampered the development of suitable models to represent surface reactive functional groups in mixed mineral assemblages. Adsorption studies using binary mixtures of model mineral phases have demonstrated remarkable complexity, with adsorption generally being very poorly predicted by considering a weighted sum of individual mineral components (1, 28). Mixed Mineral Assemblages in Natural Systems It is becoming increasingly clear that many natural mineral phases possess different surface chemical properties than their model mineral analogues. Zachara and others (29, 30 and 31) have provided compelling evidence suggesting that the small crystallite size of soil smectites enhances the importance of edge site aluminol (Al-OH) functional groups imparting an oxide-like behavior compared with the widely used Source Clay Repository, SWy-1 montmorillonite. Other studies have suggested that organic or metal oxide minerals may be the primary reactive phases in soils and sediments even at relatively low abundance (32, 33, 34, 35, 36 and 37). A major theme that emerges from these investigations is that surface modifiers in the form of organic/ metal oxohydroxide armoring agents or coatings, rather than bulk mineralogical composition per se, control the surface chemistry of reactive phases in soils and aquifers. For example, it has been demonstrated that organic constituents coassociated with variable charge minerals significantly alter the point of zero net charge (pznc, the pH at which the cation and anion exchange capacities are equal), shifting the pznc to significantly lower pH values (32, 33, 35, 36 and 37). Conversely, Fe and Al oxohydroxide phases coassociated with quartz, and permanent charge phyllosilicate minerals have been found to shift the pznc to higher pH values (38, 39 and 40). A number of recent studies have focused on the surface chemistry of mineral assemblages isolated from natural systems (33, 34 and 35, 37, 38, 40, 41 and 42). Characterizing natural mineral assemblages is challenging, because it has been virtually impossible to isolate them free of artifacts. In fact, the methods used to isolate and concentrate clay minerals involve dispersion of the clay fraction via treatment with harsh reagents designed to significantly alter surface charge properties and destroy complex mineral assemblages present in the original material. Recently, however, collection and examination of complex mineral assemblages has been achieved in a different context: that dealing with the transport of colloidal phases through porous media. The past decade has witnessed great



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interest in the generation and transport of mineral colloidal phases through natural porous media (33, 34, 40, 42, 43, 44, 45, 46 and 47). Interest in this subject has paralleled evidence that colloidal minerals are important vectors for facilitating the transport of contaminants in certain environments (43, 47, 48, 49 and 50). Mobile colloids can be generated by a number of mechanisms, including precipitation of colloidal size phases, dissolution of cementation agents composed of fine-grained crystalline and poorly crystalline secondary minerals, and release from soil and aquifer materials via physicochemically controlled dispersion processes. Transport of the mineral colloids also depends on a number of factors, including fluid flow rate, electrostatic and van der Waals forces between colloids and between colloids and matrix minerals, and physical factors related to the relative size of the colloids and pores and pore throats. Recent evidence has indicated that mobile colloids comprised of minerals and complex mineral assemblages can be generated via dispersion processes and transported through many soils and groundwater systems with relatively minor changes in solute chemistry of the invading fluid, thereby avoiding the serious artifacts typically encountered in the isolation of complex mineral assemblages. Mineralogy and Surface Chemistry of Complex Mineral Assemblages Isolated From Soils and Aquifer Materials Although several studies have demonstrated the enhanced mobility of contaminants in the presence of mobile colloids, far fewer have focused on characterizing the mineralogical composition and surface charge properties of the mineral and organic-mineral assemblages comprising the mobile phase. Over the past decade, our investigations have focused on providing evidence for the facilitated transport of contaminants associated with mobile colloidal phases and in defining the mechanisms leading to the generation and transport of mobile colloidal phases and solutes (33, 34, 40, 46, 49, 51, 52 and 53). These studies have examined surface chemical controls on colloid generation and of colloid and solute migration in surface and subsurface highly weathered oxide-rich systems having similar bulk clay mineralogy. The samples examined are coarse textured (≥85% sand; 20 µmol/m2. Each sample was allowed to “equilibrate” at the final pH for 24–36 hr, then before GFAA analysis the sample was centrifuged and about 95% of the supernatant was removed. Except at the highest metal concentrations, the solutions were undersaturated with respect to the stable precipitate; thus, for example, no Pb(OH)2 or PbO precipitate phase was expected to


MINERAL SURFACES AND BIOAVAILABILITY OF HEAVY METALS: A MOLECULAR-SCALE PERSPECTIVE

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form in the case of the model lead sorption systems. In each case, the sorption sample in the form of a wet paste was loaded into a Teflon sample holder sealed with Mylar windows, and XAFS spectra were collected at the Stanford Synchrotron Radiation Laboratory (SSRL) on wiggler-magnet beamlines within 2 days of sample preparation. XAFS spectra for the model sorption samples were collected in the fluorescence-yield mode by using either a Stern–Heald type gas-filled ionization chamber detector (57) or a 13-element Ge detector under ambient conditions. The XAFS spectra of crystalline model compounds were collected in transmission mode under ambient conditions. In none of these experiments was evidence found for oxidation or reduction of the sorbate ion during or after XAFS data collection, as indicated by the lack of energy shifts in the edge positions, extended x-ray absorption fine structure (EXAFS)-derived metal-oxygen bond lengths, and x-ray photoelectron spectroscopy measurements. XAFS data analysis procedures used for the powdered sorption samples are described in Bargar et al. (38, 39) and O'Day et al. (58). XAFS spectroscopic analysis of the natural soil and mine waste samples containing As, Se, and Pb was carried out by using the experimental and data analysis procedures described in Foster et al. (59, 60), Pickering et al. (61), and Ostergren et al. (62), for As, Se, and Pb, respectively. These natural samples were not significantly modified from their conditions in the field, except for size separation in selected cases, in an attempt to preserve the same speciation of heavy metals present in the original samples. About 100 mg of each sample was placed in a Teflon holder and covered with thin Mylar tape, and XAFS data were collected by using fluorescence-yield detection (either a Stern–Heald detector or a multielement solid state detector) at ambient temperature (298 K) and pressure [1 atm (101.3 kPa)]. In addition to the normal EXAFS spectral fitting, which yields information on the identity and arrangement of first and second neighbors around the central absorber, linear least-squares fitting of the x-ray absorption near edge structure (XANES) spectra was conducted for the As- and Se-contaminated samples, and linear least-squares fitting of the XAFS spectra was conducted for the Pb-contaminated samples. By using this approach (59, 61, 62), the quantitative speciation of the heavy metal was determined by fitting the spectrum of the contaminated sample with spectra of model compounds, including both crystalline phases and model sorption samples with the heavy element sorbed at low surface coverage on different mineral or organic substrates. This approach works well when the spectral signatures of the different models are significantly different, which is often the case. Examples of this type of fitting are given in the sections below. The amount of surface-bound heavy metals in the contaminated soil and mine waste samples was determined in this manner. In addition to XAFS analysis of the contaminated samples, each sample was examined by powder x-ray diffraction, optical microscopy, and electron microprobe to determine the types and amounts of crystalline phases present (59, 62, 63). In addition, selected samples were examined by surface-sensitive x-ray photoelectron spectroscopy (62). RESULTS AND DISCUSSION As in Mine Tailings from the Mother Lode District, California. Almost 150 years of gold mining in the Mother Lode of California has resulted in significant concentrations of arsenic in mine tailings (up to 5,000 ppm), some of which have been used for housing developments in the Sierra Nevada Foothills of Central California. Because of the toxic effects of high concentrations of arsenic on humans and other organisms, there is concern about these tailings, and a number of studies are underway to determine the potential health hazard of these tailings to humans. We have used XAFS spectroscopy to determine the oxidation state, local coordination environment (to a radius of ≈7 Å around As), and the relative proportion of different As species in model compounds and three California mine wastes: a fully oxidized tailings (Ruth Mine), a partially oxidized tailings (Argonaut Mine), and a roasted sulfide ore (Spenceville Mine) (59). Analysis of the XANES spectra of these contaminated samples indicates that As(V) is the predominant oxidation state in the Ruth and Spenceville mine samples, but mixed oxidation states were observed in the Argonaut mine-waste. We obtained qualitative information about As(V) chemical speciation by fitting the XANES spectra of mine samples with a linear combination of component (model compound) spectra (Fig. 3). These analyses suggest the presence of As (V) species similar to those found in scorodite (FeAsO4·2H2O) and As(V) adsorbed on goethite (α-FeOOH) and gibbsite [γ-Al(OH)3]. Nonlinear least-squares fits of mine waste EXAFS spectra indicate variable As speciation in each of the three mine wastes. We conclude that ferric oxyhydroxides and aluminosilicates (probably clay) bind roughly equal portions of As(V) in the Ruth Mine sample. Our analysis suggests that tailings from the Argonaut Mine contain ≈20% reduced As bound in arsenopyrite (FeAsS) and arsenical pyrite (FeS2-xAsx) and ≈80% AS (V) in a ferric arsenate precipitate such as scorodite. Roasted sulfide ore of the Spenceville Mine contains As(V) substituted for sulfate in the crystal structure of jarosite [KFe3(SO4)2(OH)6], and sorbed to hematite surfaces. Determination of solid-phase As speciation by means of EXAFS spectroscopy is a valuable first step in the evaluation of As bioavailability, because the mobility and toxicity of As compounds vary with As oxidation state. As bound in crystalline or x-ray amorphous precipitates is generally considered to be less available for uptake by organisms than when sorbed to mineral surfaces. Se-Contaminated Soils in the Central Valley of California. Selenium occurs naturally in sediments and soils in many parts of the Western U.S. and is assumed to be incorporated in pyrites in marine sedimentary rocks such as shales. When such soils are irrigated for agricultural purposes, this indigenous element becomes soluble and is transported in agricultural drainage waters to ponds and reservoirs where it becomes concentrated in water-borne plants and animals (up to 3,000 ppm). One result of this concentration process was discovered in the early 1980s by government scientists at the Kesterson National Wildlife Refuge in Merced County, California. Wildlife, particularly waterfowl, died or were born deformed from consumption of high levels of selenium (64). Similar problems have since been documented at nine sites in eight Western states comprising some 1.5 million acres of farmland. This problem has major financial and health implications in the San Joaquin Valley of California, a vast area that produces a significant portion of the nation's vegetables, fruits, and other crops. If farmers are prevented from draining irrigation waters in this region, the rapid buildup of salts in the soil will quickly make production of these crops impossible. About 500,000 acres of farmland in the San Joaquin Valley—one quarter of the Valley's agricultural acreage—are at stake, with an annual crop production worth about $500 million (65). To provide molecular-level information on the chemical forms of selenium present in Se-contaminated soils from the Kesterson Reservoir area, we have carried out XAFS spectroscopy studies that showed that selenate and selenite are present in the top few cm of soil adjacent to the drainage ponds but are reduced to elemental selenium at lower soil levels (61, 66). In carefully controlled laboratory studies of soil columns to which selenatecontaining solutions were added, Tokunaga et al. (67, 68) found that selenate is rapidly converted into elemental selenium. The reduction can occur by means of both biotic (4, 69) and abiotic (70) pathways. However, when reoxidized to the selenate form during irrigation, selenium becomes highly mobile and is transported as an aqueous complex in drainage waters. Various solutions to this problem have been proposed, including bacterial reduction, immobilization, and removal of selenium from drainage waters or the use of drainage waters to irrigate land on which salt-resistant plants such as cotton, Eucalyptus trees, and atriplex are grown. None have been adopted and some skeptics doubt that



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a viable solution, which satisfies environmental, financial, and political constraints, will be found. An eventual solution to this problem will require a detailed knowledge of the redox chemistry of selenium, its speciation in soils and groundwaters, and the effect of microbial organisms and inorganic reductants on its speciation. Pb in Mine Tailings from Leadville, CO. Pb is a ubiquitous environmental contaminant in soils because of the intensive use of Pb in batteries, paints, alloys, and solder, ammunition, gasoline additives, and other commercial products and the production of lead by mining and smelting activities. A recent study of the history of atmospheric lead deposition over the past 12,370 years, as measured in a peat bog in the Jura Mountains of Switzerland (71), has shown that the greatest lead flux (15.7 mg/m2/yr) occurred in 1979. This level is 1,570 times the natural background value of 0.01 mg/m2/yr. However, since the elimination of tetraethyl lead as a gasoline additive, beginning in the 1970's in the U.S. and some other countries, lead contamination levels have dropped significantly (25). They still exceed natural background levels by orders of magnitude in soils in many nonurban localities where soils have become polluted as a result of mining and smelting activities and in urban localities where paints and other anthropogenic sources of lead contaminate soils. The bioavailability of Pb is known to vary widely among different Pb species, and this fact is often cited to explain apparently dramatic variations in Pb bioavailability from site to site (e.g., ref. 10). Understanding the detailed relationship between speciation and bioavailability necessarily begins with a complete, accurate, and direct identification of Pb species in environmental media, such as soils and mine waste. Working toward this goal, researchers have recently begun applying synchrotron radiation-based x-ray absorption spectroscopic (XAS) techniques to determine the molecular-scale details of Pb speciation at contaminated sites (62, 63, 72, 73). Here we summarize the results of our work on Pb-bearing mine tailings from Leadville, CO (62). Using the unique advantages of XAS techniques, we emphasize the identification and characterization of poorly crystalline and/or finegrained species, such as sorption complexes and poorly crystalline coprecipitates, which are likely to control Pb bioavailability and mobility in natural systems. Bulk Pb concentrations range from 6,000 to 10,000 ppm in the two tailing piles we sampled at the Leadville site. These concentrations necessarily raise human health and environmental concerns, but bioavailability and chemical lability of Pb in these materials vary dramatically and show little correlation with bulk concentrations (10). Because these samples are heterogeneous multiphase mixtures (Fig. 4), a variety of complementary analytical methods were used, including powder x-ray diffraction, scanning electron microscopy, electron probe microanalysis, x-ray photoelectron spectroscopy, and synchrotron radiation-based x-ray absorption. By using this suite of techniques, and XAS techniques in particular, in conjunction with physical and chemical separation techniques, we were able to identify and characterize a number of species not amenable to detection by conventional microanalytical techniques. In particular, we found direct spectroscopic evidence for Pb adsorbed to mineral surfaces and variations in this surface-bound component with pH as would be predicted on the basis of simplified model system studies of adsorption processes. Least-squares fitting of EXAFS spectra shows that 50% of the total Pb in selected samples of the carbonate-buffered tailings with near-neutral pH occurs as adsorption complexes on iron (hydr)oxides, whereas Pb speciation in sulfide-rich low pH samples is dominated by Pb-bearing jarosites; we find no evidence for adsorbed Pb in these latter samples. Importantly, the dominant Pb species in each of the tailing piles could not be definitively identified without the molecular-scale information provided by EXAFS analysis. Because these species likely control Pb transport and bioavailability in these environments, our results clearly illustrate the need for molecular-scale characterization as basis for understanding the behavior of and health risks posed by Pb in natural environments. SUMMARY AND CONCLUSIONS The heavy metal Pb and the metalloids As and Se are among the most common environmental contaminants resulting from anthropogenic activities and the weathering of natural mineral deposits. These elements occur in a variety of chemical forms or species that can vary widely in solubility, mobility, toxicity, and bioavailability, depending on their speciation. In contaminated soils and mine tailings, for example, they can occur in primary minerals, secondary minerals formed by weathering of primary minerals in situ, solid precipitates formed by reactions of contaminant ions in groundwater with other aqueous ions, and adsorbed species. Although it is relatively easy to determine the types of solid phases present in a contaminated soil or mine tailings sample and the concentration levels of heavy metals and metalloids they contain by using a combination of x-ray diffraction and analytical methods, it is considerably more difficult to assess the importance of adsorbed heavy metal/ metalloid species, particularly at very low surface coverages. Adsorbed species may comprise a significant fraction of the heavy metal or metalloid present, and they are often the most bioavailable fraction. By using a combination of synchrotron-based XAFS spectroscopy and other analytical methods, the molecular-level speciation of As, Se, and Pb, including the types of sorbed species, was determined for selected mine tailings and contaminated soils. Sorbed As and Pb species were found to be major components with potentially high bioavailability in mine tailings from the Mother Lode District of California and Leadville, CO, respectively. Similar studies have shown that the most toxic and potentially bioavailable forms of selenium, Se(VI), are transformed rapidly to environmentally benign forms in contaminated soils through a combination of biotic and abiotic processes. A detailed knowledge of molecular-level speciation of heavy metal/metalloid contaminants and the bioavailability of the different species of a contaminant element is necessary for setting maximum contaminant limits. Knowledge of speciation is also required for efficient and costeffective remediation efforts. This point is well illustrated by cleanup efforts at the former uranium processing plant at Fernald, OH, which serves as the host for the Uranium in Soils Integrated Demonstration. This demonstration project used carbonate soil-washing procedures to remove hexavalent uranium. However, this conventional remediation procedure was not totally effective because of the presence of secondary phases containing U(IV) and U(VI), the latter being in the form of insoluble phosphates. These uranium species were detected by using a combination of XAFS, optical luminescence, Raman spectroscopy, scanning electron microscopy, and powder x-ray diffraction (74) and microXAFS spectroscopy (75). Changes in remediation procedures based on this type of speciation information could result in significant cost savings and more efficient cleanup of environmental contaminants, both of which should be major societal goals. The studies by our group presented in this paper were supported by the Department of Energy, Basic Energy Sciences, and the National Science Foundation. All of the synchrotron-based spectroscopic work was carried out at the Stanford Synchrotron Radiation Laboratory (SSRL), which is supported by the Department of Energy (Basic Energy Sciences and Office of Biology and Environmental Research) and the National Institutes of Health. We are grateful to the staff of SSRL for their technical support of this work. We also acknowledge the collaborations of T. Tokunaga and S. Myneni (Lawrence Berkeley National Laboratory) and I. Pickering (SSRL) in the studies of Secontaminated soils; G. Morin, F. Juillot, and G. Calas (University of Paris 7) in the studies of Pb-contaminated soils from northeastern France, and G. A. Parks and the late T. N. Tingle (Stanford University) in the studies of As- and Pb-contaminated mine tailings.



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LONG-RANGE TRANSPORT OF MINERAL DUST IN THE GLOBAL ATMOSPHERE: IMPACT OF AFRICAN DUST ON THE ENVIRONMENT OF THE SOUTHEASTERN UNITED STATES

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Long-range transport of mineral dust in the global atmosphere: Impact of African dust on the environment of the southeastern United States Proc. Natl. Acad. Sci. USA Vol. 96, pp. 3396–3403, March 1999 Colloquium Paper This paper was presented at the National Academy of Sciences colloquium “Geology, Mineralogy, and Human Welfare,” held November 8–9, 1998 at the Arnold and Mabel Beckman Center in Irvine, CA. JOSEPH M. PROSPERO* PNAS is available online at www.pnas.org.

University of Miami, Rosenstiel School of Marine and Atmospheric Science, 4600 Rickenbacker Causeway, Miami, FL 33149 ABSTRACT Soil dust is a major constituent of airborne particles in the global atmosphere. Dust plumes frequently cover huge areas of the earth; they are one of the most prominent and commonly visible features in satellite imagery. Dust is believed to play a role in many biogeochemical processes, but the importance of dust in these processes is not well understood because of the dearth of information about the global distribution of dust and its physical, chemical, and mineralogical properties. This paper describes some features of the large-scale distribution of dust and identifies some of the geological characteristics of important source areas. The transport of dust from North Africa is presented as an example of possible long-range dust effects, and the impact of African dust on environmental processes in the western North Atlantic and the southeastern United States is assessed. Dust transported over long distances usually has a mass median diameter 20 years since first exposure for all exposure levels were 242 and 279 for the McDonald and Amandus cohorts, respectively. Both cohorts had an SMR of 250 for nonmalignant respiratory disease. Table 4. Deaths from lung cancer asbestosis and mesothelioma* among 17,800 asbestos insulation workers in the United States and Canada (1967– 1986)* Person-years Lung Cancer SMR Asbestosis Mesothelioma Years from onset of exposure E O O/E O No. per 100,000 per yr O No. per 100,000 per yr 50 6,151 25.4 94 370 73 1,186.8 49 796.6 301,593 268.7 1168 435 427 141.6 458 151.9 Total *Best evidence: Causes of death categorized after review of best available information (autopsy, surgical, and clinical). E, expected; O, observed.

Two Cohort of Minnesota Iron Ore Workers. Taconite is a term used particularly in the Lake Superior region of Minnesota for certain iron-containing rocks from the Biwabik Iron Formation. A high-grade ore concentrate is obtained from commercial-grade taconite that contains enough magnetite (Fe3O4) to be economically processed by fine grinding and wet-magnetic separation. Taconite is a hard, dense, fine-grained metamorphic rock that contains substantial quartz (20–50%) and magnetite (10–36%) and various mineral constituents, including hematite, carbonates, amphiboles (principally of the cummingtonite–grunerite series, although actinolite and hornblende also occur), greenalite, chamosite, minnesotaite, and stilpnomelane. Reserve Mining Company. Analysis of mortality data obtained on men who were employed from 1952–1976 has been reported (19). The study was initiated by concerns in the early 1970s that asbestos was released into the air and dumped into lake water during processing of the taconite rock (20, 21). It was inferred that this dust posed a risk to the miners as well as to the general public. Silver Bay and Duluth obtained their drinking water from Lake Superior, into which the pulverized waste rock (or tailings) from the pellet plant was deposited at Silver Bay. The U.S. Department of Justice considered this a potential health hazard. The Department alleged that the amphibole in the waste rock (cummingtonite–grunerite) was asbestos and the exposures would cause gastrointestinal cancer through ingestion and lung cancer from inhalation of the water- and airborne fibers (although they had done no calculation of this). The Reserve cohort consisted of 5,751 men, of which 907 had worked for the company for >20 years and 298 were deceased. The men had been exposed to respirable dust concentrations from 0.02 to 2.75 mg/M3, the modal range being 0.2–0.6 mg/M3. The fibrous particulate content of the dust was occasionally >0.5 fibers per ml (all fibers ≥5 µm), but usually the concentration was much lower. The observed and expected deaths and SMR for all men who had worked one year or longer from 1952–1975 are given in Table 5. There was no relationship between the mortality observed and lifetime exposure to silica dust (that was as high as 1,000 mg/M3 × years). There was no suggestion that deaths from cancer increased after 10 or 20 years of latency. No deaths from mesothelioma or asbestosis were reported.


A RISK ASSESSMENT FOR EXPOSURE TO GRUNERITE ASBESTOS (AMOSITE) IN AN IRON ORE MINE

Table 5. Selected causes of mortality for men who worked one year or longer for the Reserve Mining Company Deaths Cause of death ICD* Expected All causes 000–E999 343.7 Cardiovascular disease 402, 404, 410–429 123.8 Cancers All 140–209 63.4 Respiratory 160–163 17.9 Digestive 150–159 17.6 Urinary 188–189 3.0 Genital 180–187 3.3 Selected nonmalignant respiratory diseases 470–474, 480–486, 490, 491, 493, 510–519. 6.8 All external causes E800–E998 72.8 E810–E823 31.2 Motor vehicle accidents

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Observed 298 112

SMR† 87 90

58 15 20 3 3 4 76 38

92 84 114 101 91 59 104 122

Source: Higgins et al. (1983) *International Classification of Causes of Death, 8th Revision. †Standardized mortality ratio, based on white male mortality in Minnesota, 1952–1976.

Minnesota Taconite Miners. A second epidemiological study of Minnesota taconite workers employed at the Erie and Minntac mines was reported (22). The study cohort, followed from 1947–1988 with a minimum observation period of 30 years for all participants, was made up of 3,341 men, of which 1,058 were deceased. Dusts in the two mines are reported as containing 28–40% and 20% quartz at Erie and Minntac mine, respectively. Concentrations of fibrous particulates were nearly always 1015 m3, but considerable Ba > Sr > Na > Ca > Fe > Al > Mg > Li. This preference for larger cations, including NH4+, was exploited for removing NH4-N from municipal sewage effluent and has been extended to agricultural and aquacultural applications (1, 2). Clinoptilolite and natural chabazite have also been used to extract Cs and Sr from nuclear wastes and fallout. Most zeolites in volcanogenic sedimentary rocks were formed by the dissolution of volcanic glass (ash) and later precipitation of micrometer-size crystals, which mimic the shape and morphology of their basalt counterparts (Fig. 1; ref. 3). Sedimentary zeolitic tuffs are generally soft, friable, and lightweight and commonly contain 50–95% of a single zeolite; however, several zeolites may coexist, along with unreacted volcanic glass, quartz, K-feldspar, montmorillonite, calcite, gypsum, and cristobalite/tridymite. Applications of natural zeolites make use of one or more of the following properties: (i) cation exchange, (ii) adsorption and related molecular-sieving, (iii) catalytic, (iv) dehydration and rehydration, and (v) biological reactivity. Extrinsic properties of the rock (e.g., siliceous composition, color, porosity, attrition resistance, and bulk density) are also important in many applications. Thus, the ideal zeolitic tuff for both cation-exchange and adsorption applications should be mechanically strong to resist abrasion and disintegration, highly porous to allow solutions and gases to diffuse readily in and out of the rock, and soft enough to be easily crushed. Obviously, the greater the content of a desired zeolite, the better a certain tuff will perform, ceteris paribus. (See Table 1 for more information on the properties of zeolites.) Table 1. Representative formulae and selected physical properties of important zeolites* Zeolite Representative unit-cell formula Void volume, % Channel dimensions, Å Analcime Na10(Al16Si32O96)·16H2O 18 2.6 Chabazite (Na2Ca)6(Al12Si24O72)·40H2O 47 3.7 × 4.2 34 3.9 × 5.4 Clinoptilolite (Na3K3)(Al6Si30O72)·24H2O 35 3.6 × 5.2 Erionite (NaCa0.5K)9(Al9Si27O72)·27H2O 47 7.4 Faujasite (Na58)(Al58Si134O384)·240H2O 28 4.3 × 5.5 Ferrierite (Na2Mg2)(Al6Si30O72)·18H2O Heulandite (Ca4)(Al8Si28O72)·24H2O 39 4.0 × 5.5 4.4 × 7.2 4.1 × 4.7 34 4.6 × 6.3 Laumonitte (Ca4)(Al8Si16O48)·16H2O 28 2.9 × 5.7 Mordenite (Na8)(Al8Si40O96)·24H2O 6.7 × 7.0 31 4.2 × 4.4 Phillipsite (NaK)5(Al5Si11O32)·20H2O 2.8 × 4.8 3.3 47 4.2 Linde A (Na12)(Al12Si12O48)·27H2O (Na86)(Al86Si106O384)·264H2O 50 7.4 Linde X

Thermal stability (relative) High High High High High High Low

CEC, meq/g† 4.54 3.84 2.16 3.12 3.39 2.33 2.91

Low High

4.25 2.29

Medium

3.31

High High

5.48 4.73

*Modified from refs. 103 and 104. Void volume determined from water content. †Calculated from unit-cell formula.

APPLICATIONS Construction Dimension Stone. Devitrified volcanic ash and altered tuff have been used for 2,000 years as lightweight dimension stone. Only since the 1950s, however, has their zeolitic nature been recognized. Their low bulk density, high porosity, and homogeneous, close-knit texture have contributed to their being easily sawed or cut into inexpensive building blocks. For example, many Zapotec buildings near Oaxaca, Mexico, were constructed of blocks of massive, clinoptilolite tuff (4), which is still used for public buildings in the region. The easily cut and fabricated chabazite- and phillipsite-rich tuffo giallo napolitano in central Italy has also been used since Roman times in construction, and the entire city of Naples seems to be built out of it (Fig. 2). Numerous cathedrals and public buildings in central Europe were built from zeolitic tuff quarried in the Laacher See area of Germany. Early ranch houses (Fig. 3) in the American West were built with blocks of locally quarried erionite; they were cool and did not crumble in the arid climate. Similar structures made of zeolitic tuff blocks have been noted near almost every zeolitic tuff deposit in Europe and Japan (5). Cement and Concrete. The most important pozzolanic raw material used by the ancient Romans was obtained from the tuffo napolitano giallo near Pozzuoli, Italy (6, 7). Similar materials have been used in cement production throughout Europe. The high silica content of the zeolites neutralizes excess lime produced by setting concrete, much like finely powdered pumice or fly ash. In the U.S., nearly $1 million was saved in 1912 during the construction of the 240-mile-long Los Angeles aqueduct by replacing ≤25% of the required portland cement with an inexpensive clinoptilolite-rich tuff mined near Tehachapi, CA (8, 9).


LA ROCA MAGICA: USES OF NATURAL ZEOLITES IN AGRICULTURE AND INDUSTRY

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FIG. 2. Castel Nuovo (Naples, Italy) constructed of tuffo giallo napolitano [Reproduced with permission from ref. 105 (Copyright 1995, International Committee on Natural Zeolites)]. Lightweight Aggregate. Much like perlite and other volcanic glasses are frothed into low-density pellets for use as lightweight aggregate in concrete, zeolitic tuff can be “popped” by calcining at elevated temperature. Clinoptilolite from Slovenia and Serbia yields excellent aggregates of this type on firing to 1,200–1,400°C. Densities of ≥0.8 g/cm3 and porosities of ≤65% have been reported for expanded clinoptilolite products (10). These temperatures are somewhat higher than those needed to expand perlite, but the products are stronger (11). The Russian Sibeerfoam product is expanded zeolitic tuff and is used as lightweight insulating material (12). In Cuba, mortars for ferrocement boats and lightweight aggregate for hollow prestressed concrete slabs contain indigenous clinoptilolite (13, 14). The mortars have compressive strengths of ≤55.0 MPa; the ferrocement boats can withstand marine environments. Water and Wastewater Treatment Municipal Wastewater. Large-scale cation-exchange processes using natural zeolites were first developed by Ames (1) and Mercer et al. (2), who demonstrated the effectiveness of clinoptilolite for extracting NH4+ from municipal and agricultural waste streams. The clinoptilolite exchange process at the Tahoe–Truckee (Truckee, CA) sewage treatment plant removes >97% of the NH4+ from tertiary effluent (15). Hundreds of papers have dealt with wastewater treatment by natural zeolites. Adding powdered clinoptilolite to sewage before aeration increased O2-consumption and sedimentation, resulting in a sludge that can be more easily dewatered and, hence, used as a fertilizer (16). Nitrification of sludge is accelerated by the use of clinoptilolite, which selectively exchanges NH4+ from wastewater and provides an ideal growth medium for nitrifying bacteria, which then oxidize NH4+ to nitrate (17, 18 and 19). Liberti et al. (19) described a nutrient-removal process called RIM-NUT that uses the selective exchange by clinoptilolite and an organic resin to remove N2 and P from sewage effluent.

FIG. 3. Abandoned ranch house in Jersey Valley, NV, constructed of quarried blocks of erionite-rich tuff [Reproduced with permission from ref. 5 (Copyright 1973, Industrial Minerals)]. FIG. 4. Clinoptilolite-filled columns at a Denver, CO, water-purification plant [Reproduced with permission from ref. 106 (Copyright 1997, AIMAT)]. Drinking Water. In the late 1970s, a 1-MGD (million gallons per day) water-reuse process that used clinoptilolite cation-exchange columns went on stream in Denver, CO, (Fig. 4) and brought the NH4+ content of sewage effluent down to potable standards (63 natural zeolites commonly occur in large beds: analcime (ANA),* chabazite (CHA), clinoptilolite (HEU), erionite (ERI), mordenite (MOR), and phillipsite (PHI) (1); ferrierite (FER) occurs in a few large beds. Each of the seven also has been synthesized, but only mordenite and ferrierite are manufactured in large quantity. Significantly, synthetic mordenite has large pores whereas natural mordenite has small pores (2). Besides mordenite and ferrierite, the principal synthetic (aluminosilicate) zeolites in commercial use are Linde Type A (LTA), Linde Types X and Y (Al-rich and Si-rich FAU), Silicalite-1 and ZSM-5 (MFI), and Linde Type B (zeolite P) (GIS). Other commercially available synthetic zeolites include Beta (BEA), Linde Type F (EDI), Linde Type L (LTL), Linde Type W (MER), and SSZ-32 (MTT). All are aluminosilicates or pure silica analogs. Recently, new nonaluminosilicate, synthetic molecular sieves became available commercially. They include aluminophosphates (family of AlPO4 structures); silicoaluminophosphates (SAPO family); various metal-substituted aluminophosphates [MeAPO family, such as CoAPO-50 (AFY)]; and other microporous framework structures, such as crystalline silicotitanates.† Most current commercial applications use aluminosilicate zeolites or their modified forms. Undoubtedly, commercial uses both for zeolites and other molecular sieves will continue to grow. Development of Synthetic Zeolites and Other Microporous Oxides. The first zeolite mineral (stilbite) was described in Sweden by Baron Cronstedt in 1756 (3, 4 and 5). Highlights of the history of adsorption studies of zeolites were reviewed by Breck (6). By 1926, the adsorption characteristics of chabazite were attributed to tiny pores (97% p-xylene at >99.9% purity from a raffinate containing EB and o- and m-xylenes. Xylene Isomerization. The raffinate from the Parex unit can go to an Isomar unit (29), licensed by UOP, for isomerization to a nearequilibrium mixture of xylenes, which are recycled to the Parex unit. The Isomar unit itself also uses UOP zeolite acid catalysts, such as the Ptbearing I-9 catalyst, which converts EB to xylenes, and the I-100 catalyst, which dealkylates EB to benzene. Both provide efficient EB conversion with excellent xylene retention. Disproportionation of Toluene and Transalkylation of Toluene and Trimethylbenzenes. Recent strong demand for p-xylene has begun to exceed the supply of mixed xylenes. Incorporating the Tatoray process (originated with Toray Industries in Japan and further developed and licensed by UOP) into the aromatics complex in a refinery can more than double the yield of p-xylene from a naphtha feedstock (30). The zeolite-based TA-4 catalyst has two principal functions, disproportionation of toluene into the more-valuable benzene and mixed xylenes, and transalkylation of toluene and trimethylbenzenes to mixed xylenes. The mixed xylenes then are added to the Parex unit to produce more p-xylene. p-Xylene Synthesis from Toluene. Xylenes can be produced by the zeolite-catalyzed disproportionation of toluene alone. Mobil developed the MTPX (Mobil toluene to para-xylene) process for internal use, and the licensed MSTDP (Mobil selective toluene disproportionation) process, based on ZSM-5 (MFI) “product shape-selective” zeolite catalysts (31). The toluene disproportionation generates mixed xylenes inside the catalyst, but the overall relative yield of p-xylene is greater than the thermodynamic equilibrium allows because the pxylene diffuses more rapidly out of the zeolite than do the o- and m-xylenes. UOP's recent PX-Plus process also uses a zeolite catalyst for pxylene synthesis by shape-selective disproportionation of toluene. Aromatics from Light Hydrocarbons. UOP's Cyclar process converts low-value LPG (propane, butanes) or light feedstocks containing olefins and paraffins to high-value, easily transportable, petrochemical-grade liquid aromatic products, particularly BTX (benzene, toluene, and xylenes). It uses a single galliummodified zeolite catalyst developed by BP and UOP in conjunction with UOP's CCR continuous catalytic regeneration system (32, 33). Acidic sites on the zeolite catalyze dehydrogenation, oligomerization, and cyclization. The shape-selectivity of the zeolite cavities helps promote the cyclization reactions and limits the size of the rings (34). M-Forming. Catalytic reforming produces a high-octane liquid reformate product rich in aromatics and hydrogen gas; light hydrocarbon gases, such as LPG; and C6 to C9 paraffins. Mobil's M-Forming process selectively hydrocracks linear and singly branched paraffins in gasoline reformate fractions to LPG by size-selective catalysis by using medium-pore ZSM-5 zeolite (35). Olefins produced from paraffin cracking alkylate the aromatics and also form some aromatics by oligomerization. Other Aromatics Produced by Sorbex Separations. Some other applications of the Sorbex zeolite-based simulated-moving-bed technology (36) are MX the Sorbex process, m-xylene from EB and o- and p-xylenes; the Cymex process, m- and/or p-xylene from a mixture of cymene isomers; and the Cresex process, m- and/or p-cresol from mixtures of cresol and xylenol. Petrochemicals Processing for Olefins Production Light Olefin Production by Methanol-to-Olefins (MTO) Process. Only ≈110 of the ≈2,500 billion cubic meters of natural gas produced annually is wasted (burned in flares). About 103 billion cubic meters per year of natural gas are processed to make liquefied natural gas, but at high cost. Alternatively, natural gas can be converted first to syngas (CO and H2) and then to the more-valuable, easily shipped methanol. However, the methanol market is too small for the available natural gas. The new UOP/HYDRO MTO process provides the means to efficiently convert methanol to even more valuable light olefins (ethylene, propylene, and butenes), which have large, commodity-type petrochemical markets: Ethylene and propylene represent the largest, together accounting for 120 million MTA, and growing (37). This process uses the product-shape-selective UOP MTO-100 catalyst based on a unique molecular sieve. During the late 1980s, Norsk Hydro, assisted by Sintef, started independent work, and UOP and Hydro agreed on joint development of the process, now available from UOP for commercial licensing. Norsk Hydro is running a large (0.75 metric tons/day) UOP/ HYDRO MTO demonstration plant in Porsgrunn, Norway. Olefin Isomerization. The 1990 Clean Air Act increased the demand for blendable ethers in motor fuels and created a demand for isobutene to make methyl tertiary butyl ether and for isopentene to make tertiary amyl methyl ether. In anticipation, the UOP I-500 catalyst, based on a SAPO structure, and two new processes were developed: Butesom for isobutene isomerization and Pentesom for pentene isomerization (38, 39, 40 and 41). In both processes, coke progressively accumulates on the catalyst and is periodically removed by a simple carbon burn-off in the reactor. The Lyondell IsoPlus process (42, 43) uses a ferrierite (FER) zeolite for the isomerization of olefins to isoolefins, and a Mobil patent (44) describes using a medium-pore zeolite catalyst (for example, ZSM-5) for similar applications. Oxygenates Removal Unit. Zeolite adsorbents are used in a UOP oxygenate removal unit down to >1 ppm total of trace oxygenates (e.g., DME, methanol, and methyl tertiary butyl ether) from C4 streams. Depending on the flow scheme, the C4 stream generally goes to a motor fuel alkylation (sulfuric acid or hydrofluoric acid) process or is recycled to a dehydrogenationetherification complex, which has a UOP Oleflex unit and a methyl tertiary butyl ether unit. The advantages of the oxygenate removal unit is that it minimizes the acid consumption otherwise associated with these oxygenates, thus minimizing the acid neutralization wastes, a significant environmental benefit (B. V. Vora, personal communication). In dehydrogenation, the oxygenate removal unit improves catalyst stability and lowers costs of methyl tertiary butyl ether production. Petrochemicals Processing for Detergents Production Linear Paraffins for Biodegradable Detergents. Petroleum derivatives account for most of the total surfactant production and household detergents. During the 1940s and 1950s, sodium dodecylbenzene sulfonate was the most widely used synthetic detergent. However, the dodecyl paraffin side group on the benzene ring is highly branched and not easily biodegraded. In the early 1960s, environmental concerns led to development of linear alkylbenzene sulfonate (LAS) detergents, which are both biodegradable and cost-effective. The key to the manufacture of the linear paraffins required to make linear alkylbenzene (LAB) and, hence, LAS is the use of size-selective synthetic zeolites that adsorb linear paraffins but exclude branched paraffins, naphthenes, and aromatics from mixtures spanning a range of boiling points, as in kerosene (C12 to C18). Although anticipated by the work of McBain (7) and Barrer (8, 9,&10), such a class separation of molecules spanning a range of boiling points was virtually impossible before development of the synthetic molecular sieves by Union Carbide in the 1950s. Two different processes, one vapor phase and the other liquid phase, are used. The vapor-phase IsoSiv process was developed at Union Carbide originally for octane improvement. To produce linear paraffins for detergents, kerosene feed, pretreated to acceptable quality and the desired carbon number range, passes at elevated temperature and just over atmospheric pressure through a bed of zeolite adsorbent that adsorbs just the linear paraffins. Just enough hexane vapor follows the kerosene feed to displace the nonadsorbed feed and isomeric hydrocarbons from the void spaces in the adsorber vessel. The effluent from this step is


SYNTHETIC ZEOLITES AND OTHER MICROPOROUS OXIDE MOLECULAR SIEVES

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combined with the adsorption effluent stream. The linear paraffins adsorbed in the zeolite are desorbed by purging the bed in the opposite direction with hexane. The hexane in the effluent is separated by distillation and is recycled. The remaining linear paraffins comprise the desired product. The liquid-phase Molex process, mentioned earlier, is most often used to produce plasticizers (C6-C10), LABs (C10-C15), and detergent alcohols (C13-C22+, but usually heavier than C16). To make linear paraffins for LAB, increased linearity and low aromatics content are desired. The new high-purity Molex process has improved product purity to 99.7% and reduced aromatics content to 0.05 wt %. In addition, the new OP ADS-34 zeolite adsorbent provides improved long-term separation performance in the Molex process. The linear paraffins made in the Molex process can be sent to UOP's Pacol and DeFine processes (45) for catalytic conversion to monoolefins. These pass to a UOP Detergent Alkylate process (46), which uses a hydrofluoric acid catalyst, or to a Detal process (offered for license in 1995 ), which uses a more environmentally friendly solid, heterogeneous catalyst to produce LAB from the monoolefins plus benzene. Unreacted linear paraffins are recycled to the Pacol and DeFine units and are converted to extinction. These catalytic processes use nonzeolite catalysts. Linear Olefins for Detergent Alcohols. As discussed previously, LAB accounts for half of the detergent intermediate market. Detergent alcohols made from linear olefms are another quarter. Detergent alcohols are made from C10-C15 alpha olefins derived from ethylene or from C10-C15 internal olefins derived from lower-cost kerosene feed. Linear olefins of improved purity are increasingly sought. Linear paraffins from the new high-purity Molex zeolite adsorption process are sent to the Pacol and DeFine processes to convert them to a mixture of monoolefins plus unreacted linear paraffins. The mixture is fed to UOP's Olex process, which uses a zeolite adsorbent in a Sorbex simulated-moving-bed process to separate the linear olefins and the unreacted paraffins, which are recycled back to the Pacol and DeFine units to extinction. The Olex process now uses the new UOP ADS-32 zeolite adsorbent, which provides improved capacity and rates. The linear olefins product has improved product purity (reduced aromatics and diolefins) as a result of improvements in the zeolite adsorbents and in the nonzeolite catalysts and operating conditions in the Molex-PacolDeFine-Olex sequence of processes. Separation and Purification Process Applications Molecular sieve adsorbents are used in many other separation and purification applications: (i) petroleum refining processes, used to remove CO2, chlorides and mercury from a variety of streams; to dry and purify liquids and gases in diverse applications; to treat alkylation unit feed to reduce acid consumption, regenerator use, and corrosion, and to treat refinery hydrogen to prevent corrosion in downstream equipment; to dry and desulfurize refined products; and to dry and purify feed and recycle hydrogen in isomerization units; (ii) petrochemicals, used to dry hydrocarbon liquids, cracked gas, and hydrogen; to dry and purify natural gas liquids, ethane and propane feedstocks in ethylene and polymer plants; and in ethylene, propylene, butadiene, butylenes, amylenes, and various other comonomers and solvents; (iii) natural gas treating, used to dry and desulfurize natural gas to protect transmission pipelines and to remove undesirable impurities from home cooking and heating gas and to desulfurize ethane, propane, and butane and for H2O and CO2 removal before cryogenic processing; (iv) industrial gas production and purification, used to remove H2O and CO2 from air before liquefaction and separation by cryogenic distillation, for pressure swing adsorption (PSA) separation of air, and in PSA purification of hydrogen by using zeolites and other adsorbents, such as activated carbon; (v) specialty and fine chemicals and pharmaceuticals, used for drying; for removal of impurities, including odors; and for other applications in the manufacture of specialty and fine chemicals and pharmaceuticals. Common features of these uses are now summarized. Preprocessing of Gases before Cryogenic Separations. Deep drying and CO2 removal are required before cryogenic liquefaction and subsequent separation processing to prevent formation of ice and dry ice, which would plug up the cryogenic processing equipment. Several synthetic zeolites exhibit great affinity for polar compounds such as H2O and CO2 and have high adsorption capacity at ambient temperature. They are used extensively in processing natural gas to make liquefied natural gas or to recover hydrocarbon liquids or helium; in processing air to make O2, N2, and Ar in cryogenic air-separation plants; and in treating ethylene and other olefins formed in ethylene steam-cracking plants before separation in cryogenic distillation separation units. These pre-treatments work to perfection: Passing ambient air over such zeolite adsorbents at room temperature makes the air drier (−60°C dew point or lower) than in the coldest part of Alaska in the depth of winter. Because the adsorbed impurities are strongly held on the zeolite adsorbents, they are regenerated for subsequent reuse in thermal-swing processes that pass a hot regeneration gas over the spent zeolite to heat it and to carry away the adsorbed compounds. The zeolite then is cooled to ambient temperature and is used to treat more gas. Removal of Impurities from Gases and Liquids Down to Low Levels. Because zeolites bind strongly to polar compounds, including hydrogen sulfide, mercaptans, organic chlorides, CO, and to mercury, they can purify many streams in petroleum refineries, petrochemical plants, natural gas production plants, and chemical plants. In refineries, zeolite adsorbents remove impurities detrimental to downstream processing, including catalyst poisons (e.g., oxygenates and sulfur), corrosive agents, and chloride compound byproducts from processes using chloride catalyst promoters (e.g., catalytic reformers). In natural gas production, zeolite adsorbents are used to dry the gas to prevent freezing and corrosion in pipelines, to remove sulfur compounds from the gas or LPG fractions to prevent corrosion in burners, and to remove compounds that are obnoxious or toxic (such as the odoriferous hydrogen sulfide and mercaptans in natural gas that form sulfur dioxide pollutants when burned for home cooking and heating). Worldwide, > 1,000 units process tens of billions of cubic feet of natural gas daily. Zeolites are used in the preparation of very high-purity fluids for special uses: e.g., gases used in the manufacture of electronics or gases and liquids used in modern analytical laboratory instruments. Air Separation by Pressure Swing Adsorption (PSA) Processes. The following draws primarily from reviews (47, 48, 49, 50, 51,&52) plus the author's personal reflections from direct involvement in zeolite adsorbent development over the last 30 years. Many zeolites adsorb N2 more strongly than O2 [the possible use of zeolites in air separation was indeed the principal impetus for the pioneering work of Milton (11)]. Also, because zeolites adsorb more of both N2 and O2 from air with increasing pressure, air can be separated by using a PSA process. The air is passed at an elevated pressure through a bed of zeolite particles that adsorb the N2 more strongly and hold it on the bed but allow O2 to pass through the bed. Then, the adsorbed N2 is discharged from the feed end of the bed as the pressure in the bed is lowered. Many variations on the process cycle were developed to improve efficiency and capital and operating costs. The PSA and vacuum-swing adsorption (VSA) processes use zeolite adsorbents to produce O2 of 90–94% purity (the balance is primarily argon). The O2 is used, for example, in the manu

Imai, T., Kocal, J. A. & Vora, B. V., Second Tokyo Conference on Advances in Catalytic Science Technology, August 21–26, 1994, Tokyo.



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facture of steel, glass, pulp, paper (in delignification and bleaching), and chemicals and in nonferrous metal recovery, waste incineration, and bioremediation. Zeolites provide benefits in energy efficiency, process efficiency, improved processing rates and product quality, and environmental impact. Over the last 25 years, improvements in the PSA and VSA O2 processes were driven by the development of zeolite adsorbents with improved N2 capacity and selectivity. Zeolites such as NaX and CaA made possible the development of the first economical PSA O2 process at a relatively small scale [up to ≈15 U.S. tons of O2 per day (tpd)] in the early- to mid-1970s. Second-generation adsorbents, such as CaX (53), and third-generation adsorbents, such as LiX (54), LiCaX or LiSrX (55, 56), and MgA (57), together with improved (vacuum PSA or VSA) processes have dramatically reduced both capital and operating costs. Of the third-generation Type X zeolites, only LiX has been used commercially as of 1997.** From the mid-1980s to the mid-1990s, these improvements provided a 5-fold reduction in adsorbent inventory and a nearly 2-fold reduction in power requirements. The commercial viability of a simple two-bed VSA system expanded to well over 100 tpd, and that of an even simpler one-bed system expanded to well over 40 tpd, allowing use of these noncryogenic systems in many applications formerly served by the cryogenic distillation of air. For the delivery of O2 of 90–94% purity, single-bed units are more economical because of lower capital costs (although with higher energy costs) than liquid O2 delivery in the 4–57 tpd range. Two-bed units have lower energy costs (but higher capital costs) and are more economical than either liquid O2 delivery or on-site cryogenic plants in the 57–235 tpd range.†† In 1994, VSA plus PSA O2 production was estimated to be 4–4.5% of the world demand for O2, the fourth largest chemical at 39 billion pounds in 1995 (51). In 1996, PSA/ VSA O2 production was estimated to be >3,500 tpd in the USA and >10,000 tpd (>265,000 Nm3O2 per hour) worldwide (47). Assuming a value of $20 per ton of O2, this production corresponds to a total market value of more than $75 million per year, and growing. Manufacturing Industries and Consumer Products Applications Small Oxygen Concentrators for Medical Use (Medox). In the U.S., a dozen companies manufacture small-scale PSA oxygen concentrators for patients with emphysema and chronic obstructive pulmonary disease. As with the large-scale PSA O2 units, these small PSA concentrators use zeolites to produce 90 and 95% pure oxygen, the balance mainly argon and nitrogen. They can dramatically improve the quality of life. The PSA units are engineered to be small (about the size of a small end table), readily transportable, (weighing ≈40 pounds), quiet, and reliable. They use 3–7 pounds of zeolite adsorbent to produce between 3 and 6 liters per minute of oxygen. Users are freed from needing highpressure cylinders of oxygen delivered or stored in their homes. Use of these concentrators has grown substantially over the last 20 years (S. R. Dunne, personal communication). In 1996, home medical equipment reimbursements (from U.S. Medicare) associated with oxygen concentrators totaled $1.1 billion (58). Most are PSA units, the rest being primarily membrane units, which produce much lower oxygen concentration. Automotive Air-Conditioning and Stationary Refrigerant Drying. Zeolite desiccants remove water and acids formed by breakdown of the refrigerant mixed fluid thus protecting the system from freeze-up and corrosion. UOP supplied zeolite 4AXH-5 desiccant for automotive use and 4A-XH-6 for stationary refrigerant drying. These desiccants dominated the refrigerant industry for use with refrigerants R-12 and R-22 and the associated mineral oil lubricants. However, the 1987 Montreal Protocol heralded their demise because the very long life of fugitive R-12 emissions in the atmosphere became linked to ozone depletion and global warming. The leading contender to replace R-12 was R-134a refrigerant, but R-134a was found to be unstable in the presence of the 4AXH-5 desiccant, leading to acids, sludge, deterioration of the desiccant, and possible failure of the refrigerant system. A UOP team developed the new XH7 zeolite desiccant, which is compatible with the new R134a lubricant systems, to meet the critical legislated deadlines. The deadlines for original automotive equipment manufacture and fleet testing were set at 3 years before system production, an extremely short development time for such a complex application. Another new zeolite desiccant, XH9, was developed a couple of years later; in addition to automobiles, it is widely used in refrigerators (home refrigerators, supermarket freezers, and display cases) and stationary air conditioners. Because XH7 and XH9 desiccants are also compatible with systems using R12 refrigerant and mineral oil lubricants, dryers using the new desiccants can be prefit into R12 systems before total conversion to R134a systems. The XH7 desiccant today holds almost all of the automotive air-conditioning market formerly held by the 4AXH-5 desiccant before the advent of the new refrigerants. Consumers benefited because the availability of current systems was not disrupted and any danger to the environment was alleviated: hence, the American Chemical Society Heroes of Chemistry Award in March 1998 to the UOP team (A. P. Cohen, S. L. Correll, P. K. Coughlin, and J. E. Hurst). Worldwide, millions of pounds of zeolite desiccants are installed in air conditioning units in passenger cars and light trucks. For stationary systems, most refrigerators in the U.S. and many elsewhere use zeolite desiccants to dry and remove acids from the refrigerants. The new desiccants, together with hermetically sealed systems with internal pumps, have extended the service life of refrigeration units by at least two to three times. Air Brake Dryers for Heavy Trucks and Locomotives. Most goods produced worldwide are moved to market by heavy trucks and locomotives whose brake systems are actuated mostly by clean dry air at high pressure. Air brake systems are engineered to be fail-safe: When the air supply system fails, the brakes engage and prevent the trucks or locomotives from moving. A key element of the air supply systems of a truck is a PSA dryer, typically using a single packed bed of ≈3 pounds of molecular sieve to dry the compressed gas; locomotives require more absorbent. The air compressor on a truck runs for 1–3 min at a time. The compressed gas is dried and passed to a reservoir that in turn supplies air pressure to the brakes to prevent the unintended actuation of the brakes while the truck is moving. When the reservoir is full, a signal shuts off the compressor; the dryer is depressurized, and a little dry air is bled back through the dryer to partially regenerate the bed of molecular sieve (S. R. Dunne, personal communication). These dryers have significantly improved the reliability and safety of braking systems for large trucks and locomotives. Insulated Glass Windows. Most insulated glass produced worldwide is manufactured with desiccant contained in channels (or in matrices) that separate the panes of double-, triple-, or quadruple-paned windows. The desiccant scavenges moisture and other trace compounds, such as solvents or plasticizers, that may evolve during manufacturing. Although the sealants used for the manufacture of insulated glass windows are excellent, a finite amount of moisture still leaks into the windows over time. Desiccants, primarily zeolites, prevent fogging, mists, or formation of dew between the windowpanes because they lower the dew point of the gases inside the windows to levels far below the lowest expected surface temperature of the glass. Insulated glass provides aesthetic features, improved human comfort, and energy savings that make them a truly economical and beneficial addition to both commercial buildings and homes (S. R. Dunne, personal communication). Residential and nonresidential dualpane (insulating) windows and patio doors containing zeolite adsorbents have a total window area of ≈46 billion square feet worldwide. The estimated present energy savings in heating during winter and cooling during summer from the use of these insulating windows is equivalent to 450 million barrels of oil per year.

**Notaro, F., Schaub, H. R. & Kingsley, J. P., Second Joint China/U.S. Chemical Engineering Conference, May 22, 1997. ††Notaro, F., Schaub, H. R. & Kingsley, J. P., Second Joint China/U.S. Chemical Engineering Conference, May 22, 1997.



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Environmental Protection Applications In addition to the benefits already listed, many other applications provide environmental benefits. Builders for Phosphate-Free Laundry Detergents. In the 1960s, growing public awareness of eutrophication of natural waters led to efforts to reduce the inflow of plant nutrients, especially phosphate and ammonia or nitrate. Dead algae sinks to the bottom of a pond or lake, where it depletes the oxygen in the water. Too much growth of algae depletes oxygen so much that fish die. As a result, many states, particularly those bordering the Great Lakes, banned phosphate in laundry detergents. The prime function of phosphate “builders” in laundry detergent powders is removal of the hardness ions Ca2+ and Mg2+ in the wash water by complexing. Zeolite ion exchangers in powder form also can provide this service by removing Ca2+ and Mg2+ ions from the solution and replacing them with soft ions such as Na+. Zeolite NaA was known to have high selectivity and capacity for calcium, and its application as a builder in heavy-duty powder detergents was developed in the 1970s, primarily by scientists at Henkel (59, 60, ‡‡) in Germany and Procter and Gamble (61, 62) in the U.S. Round NaA zeolite particles, a few micrometers across, are small enough to pass through the openings in the weave of the fibers in clothing and are not filtered out to form encrustations on the cloth. Recently, zeolite P (GIS), as maximum aluminum P or MAP, was developed by scientists at Unilever (and Crossfield) (63) as an alternative builder for the same applications, and debate on the relative merits of NaA and NaP zeolites continues (64). Today, the conversion of the USA detergent market to zero-phosphate formulas is virtually complete. In Europe, one-third of the powder detergents are zeolite based, and Canada is ≈50% converted. Latin America and many of the Pacific region countries continue to use phosphates (65). In 1987, the Kao Corporation in Japan introduced Attack, a compact powder detergent that has higher bulk density and higher surfactant level and needs lower dosage. Use of compact powders in the Japanese laundry market grew to >90%. In the U.S., from 1990 to 1994, the use of compact powders grew from 2% to >90%. All compact powders in the U.S., Europe, and Japan have no phosphates. Zeolites used as builders in compact powders serve as particle-formation aids. This use of zeolites has facilitated changes in the process of detergent manufacture from spray drying and to alternative processes such as agglomeration. This shift, in turn, has led to increased use of zeolites in laundry detergent powders. Automotive Emissions Control. New stable zeolites have been used successfully in diverse automotive emissions control problems. A very serious challenge in automotive emissions control today is control of nitrogen oxides (NOx) emitted from lean-burn diesel engines. Many catalyst developers and academic researchers are using zeolites with a wide array of added base and precious metals as catalysts to enable hydrocarbon storage, NOx reduction, and oxidation of both hydrocarbons and CO. Today, zeolites are used commercially for enhanced hydrocarbon oxidation in conventional diesel engines and NOx reduction. Four-way catalysts, which provide NOx reduction, HC oxidation, CO oxidation, and particulate control, are being developed. For gasoline-fueled vehicles, the most-serious problem is the cold-start period. Over 90% of the hydrocarbon emission by a car during a cold start occurs within the first 3 min of engine operation. A hydrocarbon trap must contain an adsorbent that captures most of the hydrocarbons during this period. Once captured, most of the hydrocarbons must be held by the adsorber until the catalytic converter has heated up enough to be capable of oxidizing them. Then the adsorber must release the hydrocarbons to be oxidized, rendering them harmless to the atmosphere. The adsorber also must be mechanically, thermally, and hydrothermally durable enough to withstand the harsh environment of the exhaust gas stream. Especially since 1995, major advances have been achieved in the development of hydrocarbon traps. Improved hydrothermal stability of the molecular sieve adsorbent, improved chemistry of the wash coating, and the addition of a noble metal catalyst directly onto the adsorber brick are technical milestones that enable successful implementation of hydrocarbon traps in emissions control (S. R. Dunne, personal communication).§§ Several major automakers have demonstrated excellent emissions reduction and system durability. General Motors achieved 50,000-mileaged converter performance that surpasses Environmental Protection Agency requirements for use on a car designated as a low-emissions vehicle: i.e., a vehicle must emit nonmethane hydrocarbons at a weighted rate

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