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Toxic in the Environment: The Role of Surfaces

Donald L. Sparks1

etals are prevalent in the environment. They are derived from both such as , weight, atomic number, and degree of toxicity natural and anthropogenic sources. Certain metals are essential for (Roberts et al. 2005). Certain met- Mplant growth and for animal and human health. However, if present als and are essential for in excessive concentrations they become toxic. Metals undergo an array of plant growth and for animal and human health. With respect to biogeochemical processes at reactive natural surfaces, including surfaces of plants, these are referred to as clay , oxides and oxyhydroxides, humic substances, plant roots, micronutrients and include B, Cu, and microbes. These processes control the solubility, mobility, bioavailability, Fe, Zn, Mn, and Mo. In addition, and toxicity of metals in the environment. The use of advanced analytical As, Co, Cr, Ni, Se, Sn, and V are essential in animal nutrition. techniques has furthered our understanding of the reactivity and mobility Micronutrients are also referred to of metals in the near-surface environment. as trace elements since they are required in only small quantities, Keywords: critical zone, metals, sorption, surface complexation, biogeochemical processes unlike major nutrients such as N, P, and K. In excess, trace elements INTRODUCTION can be toxic to plants, microbes, animals, and humans. Metals comprise about 75% of the known elements and can Problems also arise when there is a deficiency in essential form alloys with each other and with (Morris elements. 1992). Metals have useful properties such as strength, mal- Important trace elements in the environment are As, Ag, B, leability, and conductivity of heat and electricity. Some Ba, Be, Cd, Co, Cr, Cu, F, Hg, Mn, Mo, Ni, Pb, Sb, Se, Sn, Tl, metals exhibit magnetic properties and some are excellent V, and Zn. Trace elements in natural media are present at conductors of electricity. Metals are important components concentrations of less than 0.1%. In biochemical and bio- of our homes, automobiles, appliances, tools, computers medical research, trace element concentrations in plant and and other electronic devices, and are essential to our infra- animal tissues are normally less than 0.01%. In food nutri- structure including highways, bridges, railroads, airports, tion, a trace element is one that occurs at concentrations electrical utilities, and food production and distribution less than 20 mg kg-1 (0.002% or 20 ppm). The term “heavy (Hudson et al. 1999). metals” refers to elements with greater than Civilization was founded upon the , 5.0 g cm-3 and usually indicates metals and metalloids asso- (Au), (Cu), (Ag), (Pb), (Sn), ciated with pollution and toxicity; however, the term also (Fe), and (Hg). Metals were known to includes elements that are required by organisms at low the Mesopotamians, Greeks, and Romans. Gold was concentrations (Adriano 2001). first discovered about 6000 BC (Aicheson 1960; Thirteen trace metals and metalloids are considered priori- http://neon.mems.cmu.edu/cramb/processing/history.html). ty pollutants (Table 1). They can be derived from both nat- Lead was produced before the rise of the Roman Republic ural (geogenic) and anthropogenic sources. Natural sources and Empire. Since the beginning of the Industrial Age, met- include parent rocks and metallic minerals (metalliferous als have been emitted to and deposited in the environment. ). Anthropogenic sources include agriculture (fertilizers, In some cases, metals have accumulated in terrestrial and animal manures, pesticides), (mining, , aquatic environments in high concentrations and cause metal finishing), energy production (leaded gasoline, harm to animals and humans via ingestion of soil and/or battery manufacture, power plants), microelectronics, and dust, food, and water; inhalation of polluted air; and absorp- sewage sludge and scrap disposal (Adriano 2001). Inputs of tion via the skin from polluted soil, water, and air (Adriano trace metals from various anthropogenic sources are given et al. 2005). As the world’s population and economies con- in Table 1 (Adriano 2001). Atmospheric deposition is a tinue to grow, especially in developing countries, the need major mechanism for metal input to plants and soils. This for metals will increase and so will the potential for soil and is particularly true in forest ecosystems, where metal con- water contamination. This could have serious implications tamination of soils is almost totally due to atmospheric for human health and environmental quality. deposition. Volatile metalloids such as As, Hg, Se, and Sb can be transported over long distances in gaseous forms or Metals have traditionally been classified into categories enriched in particles, while trace metals such as Cu, Pb, and such as light, heavy, semimetal (i.e. metalloids), toxic, and Zn are transported in particulate phases (Adriano 2001; trace, depending on several chemical and physical criteria Adriano et al. 2005). In terrestrial ecosystems, soils are the major recipient of metal contaminants, while in aquatic systems sediments are the major sink for metals. These con- 1 Department of Plant and Soil Sciences University of Delaware, Newark DE 19716-2170, USA taminants can then impact freshwater and groundwater E-mail: [email protected] systems. Freshwater systems are contaminated due to runoff

E LEMENTS, VOL. 1, PP. 193–197 193 SEPTEMBER 2005 and drainage via sediments or disposal, while TABLE 1 groundwater is impacted through leaching or transport via mobile colloids (Adriano 2001; NATURAL AND ANTHROPOGENIC SOURCES AND COMMON FORMS IN WASTES OF TRACE METALS ON THE PRIORITY POLLUTANT LIST Adriano et al. 2005). Element Natural source Anthropogenic sources Common forms BIOGEOCHEMICAL PROCESSES or metallic minerals in wastes CONTROLLING METAL BEHAVIOR Ag Free metal (Ag), chlorargyrite Mining, photographic industry Ag metal, Ag–CN IN THE CRITICAL ZONE (AgCl), acanthite (Ag2S), complexes, Ag halides, copper, lead, ores Ag thiosulfates Metals and metalloids undergo an array of dynamic biogeochemical processes in the criti- As Metal arsenides and arsenates, Pyrometallurgical industry, spoil heaps As oxides (oxyanions), cal zone, “the heterogeneous, near-surface sulfide ores (arsenopyrite), and tailings, smelting, wood preserving, organo-metallic forms, arsenolite (As O ), volcanic fossil fuel combustion, poultry manure, H AsO CH environment in which complex interactions 2 3 2 3 3 gases, geothermal springs pesticides, landfills (methylarsinic acid), involving rock, soil, water, air, and living (CH3)2-AsO2H organisms regulate the natural habitat and (dimethylarsinic acid) determine the availability of life sustaining Be Beryl (Be Al Si O ), Nuclear industry, electronic industry Be alloys, Be metal, resources” (National Research Council 2001). 3 2 6 18 phenakite (Be2SiO4) Be(OH)2 Biogeochemical processes affect the speciation 2+ (form) of the metal, which in turn controls its Cd Zinc carbonate and sulfide Mining and smelting, metal finishing, Cd ions, Cd halides ores, copper carbonate plastic industry, microlectronics, and oxides, Cd–CN solubility, mobility, bioavailability, and toxici- and sulfide battery manufacture, landfills and refuse complexes, Cd(OH)2 ty. Metal ions may enter the soil solution (the disposal, phosphate fertilizer, sewage sludge aqueous liquid phase of the soil and its solutes) sludge, metal scrapheaps and be subject to numerous pathways, all of Cr Chromite (FeCr2O4), Metal finishing, plastic industry, wood Cr metal, Cr oxides which can potentially overlap (Fig. 1). The soil 3+ eskolaite (Cr2O3) treatment refineries, pyrometallurgical (oxyanions), Cr solution can contain metals as free ions or com- industry, landfills, scrapheaps complexes with plexed to inorganic or organic ligands. Both organic/inorganic ligands the free ions and the metal–ligand complexes Cu (Cu), Mining and smelting, metal finishing, Cu metal, Cu oxides, can be (1) taken up by plants, (2) retained on chalcocite (Cu S), microelectronics, wood treatment, refuse Cu humic complexes, surfaces, natural organic matter, and 2 chalcopyrite (CuFeS2), disposal and landfills, pyrometallurgical alloys, Cu ions microbes, (3) transported through the soil pro- industry, swine manure, pesticides, file into groundwater via leaching or by col- scrapheaps, mine drainage loid-facilitated transport, (4) precipitated as Hg Native metal (Hg), Mining and smelting, electrolysis industry, Organo-Hg complexes, solid phases, and (5) diffused in porous media cinnabar (HgS), plastic industry, refuse disposal/landfills, Hg halides and oxides, 2+ 2+ 0 such as soils. Root exudates and microbes affect degassed from Earth's crust paper/pulp industry, fungicides Hg , (Hg2) , Hg the transport and solubility of metals. and oceans Microorganisms can transform metals such as Ni Ferromagnesian minerals, Iron and steel industry, mining and Ni metal, Ni2+ ions, Hg, Se, Sn, As, and Cr by means of oxida- ferrous sulfide ores, smelting, metal finishing, Ni amines, alloys tion–reduction and methylation (the process pentlandite microelectronics, battery manufacture of replacing an atom, usually a H atom, with a Pb Galena (PbS) Mining and smelting, iron and steel Pb metal, Pb oxides methyl ) and demethylation reactions. industry, refineries, paint industry, and carbonates, These processes affect metal toxicity and automobile exhaust, plumbing, battery Pb-metal–oxyanion mobility (Adriano 2001; Sparks 2002; Adriano manufacture, sewage sludge, refuse complexes et al. 2005; Morris 2005). For example, methy- disposal/landfills, pesticides, scrapheaps lated (organic) forms of Hg are more toxic than 3+ Sb Stibnite (Sb2S3), geothermal Microelectronics, pyrometallurgical Sb ions, Sb oxides inorganic forms of the element, and they springs industry, smelting mine drainage and halides bioaccumulate in organisms. Methylation is Se (Se), ferroselite Smelting, fossil fuel combustion, Se oxides (oxyanions), favored in environments characterized by low (FeSe2), uranium deposits, irrigation waters Se-organic complexes oxygen levels, low pH, and high soil organic black shales, chalcopyrite- matter (SOM) contents. pentlandite-pyrrhotite deposits Metal Sorption Tl Copper, lead, silver residues Pyrometallurgical industry, Tl halides, microelectronics, cement industry Tl–CN complexes The sorption (retention) of metals, including alkali (e.g. K), alkaline earth (e.g. Ca), and tran- Zn Sphalerite (ZnS), Mining and smelting, metal finishing, Zn metal, Zn2+ ions, sition (e.g. Cd and Ni) metals, on inorganic willemite (Zn2SiO4), textile, microelectronics, refuse disposal Zn oxides and minerals (clay minerals, metal oxides and oxy- smithsonite (ZnCO3) and landfills, pyrometallurgical industry, carbonates, alloys sewage sludge, pesticides, scrapheaps hydroxides) and organic humic substances is one of the most important geochemical Source: Adriano (2001); mineral nomenclature after Mandarino and Back (2004). processes controlling the fate, transport, and bioavailability of metals in soil and water envi- ative and invariant with respect to pH in the and Fe-oxides are considered variable-charge ronments. Various metal sorption mechanisms case of constant-charge clay minerals such as minerals (Sparks 2002). can occur at natural surfaces, involving both montmorillonite and vermiculite. The con- physical and chemical processes (Fig. 2). (See Both physical and chemical forces are involved stant charge results from ionic substitution in also glossary on p. 190 for sorption vocabulary). in adsorption of solutes from solution. Physical the clay structure (in the past referred to as iso- forces include van der Waals forces (e.g. parti- The most important adsorbents and sorbents morphic substitution). The surface charge can tioning) and electrostatic outer-sphere com- for metals in natural systems such as soils and also be variable, becoming more negative with plexes (e.g. ion exchange). Chemical forces sediments are clay minerals and metal oxides increased pH as surface functional groups on result from short-range interactions, which and oxyhydroxides (Table 2), and humic sub- clay minerals, metal oxides and oxyhydrox- include inner-sphere complexation involving a stances associated with natural organic matter. ides, and organic matter deprotonate. Surface ligand exchange mechanism, covalent bond- These sorbents exhibit significant surface area charge can also become more positive as pH ing, and hydrogen bonding (Stumm 1992; and surface charge, which play a pivotal role in decreases as surface functional groups proto- Sparks 2002). metal sorption. The surface charge can be neg- nate. Clay minerals such as kaolinite and Al-

E LEMENTS 194 SEPTEMBER 2005 Cu2+, Ni2+, and Pb2+ can form on metal oxides, phyllosili- cates, soil clays, and soils (see references in Brown and Parks 2001; Brown and Sturchio 2002; Sparks 2005). Thus, there is often a continuum between surface complexation (adsorption) and surface precipitation. At low-surface cov- erages, surface complexation (e.g. outer- and inner-sphere adsorption) tends to dominate. As surface loadings increase, nucleation occurs and results in the formation of distinct entities or aggregates on the surface. As surface loadings increase further, surface precipitation becomes the domi- nant mechanism (Sparks 2002).

RESEARCH FRONTIERS Particle surfaces, which range in size from nanometers to micrometers, play a profound role in controlling the FIGURE 1 Biogeochemical pathways and processes of metals in the cycling of toxic metals in the environment. The use of critical zone. Me = metal. Tiny green blobs = metal ion; molecular-scale techniques, particularly those that employ brown blobs = soil particles; orange blob = colloid. synchrotron radiation, have significantly advanced our understanding of metal–surface interactions. Accordingly, MOLECULAR-SCALE ASSESSMENT OF METAL the applications of these cutting-edge tools are prominent- SORPTION ON NATURAL SURFACES ly featured in several of the articles in this issue. Until recently, most studies on metal sorption on natural In their article, Hochella and Madden introduce the impor- sorbents have been macroscopic and have investigated the tance of nanoparticles in the cycling of metals. Most of the role that various environmental effects, such as solution total surface area in the near-surface region of the Earth is pH, metal/anion concentration, background electrolyte, associated with very small particles, and nanoparticles have and time, have on sorption. Over the past decade or more, the largest surface area per unit mass of all particulate mat- in situ (under natural conditions where water is present) ter. The small size and large surface area dramatically affect molecular-scale analytical techniques such as X-ray–absorp- geochemical reactions. For example, Madden and Hochella tion fine–structure spectroscopy (XAFS) and attenuated total (2005) found that Mn2+ oxidation reaction rates (the reac- reflectance–Fourier transform infrared (ATR–FTIR) spec- tion rate was surface-area normalized) are up to two orders troscopy have been employed extensively to determine the of magnitude faster on hematite nanoparticles with grain type of metal surface complexes and products that form on diameters of 7 nm than those with grain diameters of minerals and humic substances (see studies reported in 40 nm. Another important way in which nanoparticles can Brown and Parks 2001; Brown and Sturchio 2002; Sparks affect metal cycling in the environment is through wind- 2005). Environmental factors, such as pH, surface loading, blown dust nanoparticles. Utsunomiya (2004) showed that ionic strength, type of sorbent, and time, affect the type of As, Cr, Pb, and Se are associated with airborne nanoparti- sorption complex or product. Based on macroscopic and cles. This could pose a serious human health risk since the molecular-scale studies, one can predict that alkaline earth solubility and reactivity of the metals, due to the small size cations, Mg2+, Ca2+, Sr2+, and Ba2+, primarily form outer- of the particles, could be enhanced if the particles were sphere complexes, while the divalent first-row transition ingested into lung tissue (Hochella 2002). metal cations, Mn2+, Fe2+, Co2+, Ni2+, Cu2+, and Zn2+, and the divalent heavy metal cations, such as Cd2+, Hg2+, and Kretzschmar and Schäfer discuss the role that mobile col- Pb2+, primarily form inner-sphere complexes (Sparks 2002, loidal particles play in the fate and transport of metals in 2005). However, a number of scientists have shown that, at soils and waters. The particles range in size from 1 nm (i.e. higher metal loadings and pHs, multinuclear metal hydrox- a nanoparticle) to 1 micron and are composed of clay min- ide complexes and surface precipitates involving Co2+, Cr3+, erals and mineral precipitate phases (Fe, Al, Mn, or Si oxides and hydroxides, carbonates, and phosphates), humic and ful- vic acids, and biocolloids (viruses, bacteria, and protozoans). With the advent of third-generation synchrotron light sources, which provide more intense X-rays and greater spa- tial resolution, scientists are able to determine the forms and distributions of metals in heterogeneous systems such as soils and plants and mineral–microbe–metal interactions. McNear et al. discuss the use of synchrotron-based micro- focused XAFS, X-ray fluorescence spectroscopy and micro- tomography to speciate metals in heterogeneous soils and plants that accumulate large quantities of metals (hyperac- cumulators). Kemner et al. employ synchrotron-based tech- niques to study mineral–microbe–metal interactions and effects on metal . Villalobos et al. discuss trace metal sorption on biogenic Mn oxides that form due to FIGURE 2 Various mechanisms of sorption of an ion at the miner- enzymatic catalysis by bacteria and fungi via oxidation of al–water interface: (a) adsorption of an ion via formation Mn2+ (Tebo et al. 1997). of an outer-sphere complex; (b) loss of hydration water and formation of an inner-sphere complex; (c) diffusion and isomorphic substitution within the mineral structure (d); rapid lateral diffusion and formation of ACKNOWLEDGEMENTS a surface polymer or (e) adsorption on a ledge (which maximizes the number of bonds to the atom; (f) upon particle growth, surface poly- I thank the editors for inviting me to assemble this special mers end up embedded in the structure; finally, (g) the adsorbed ion issue of Elements on toxic metals in the environment. I am can diffuse back in solution, either as a result of dynamic equilibrium or grateful to the authors for their first-rate articles and to the as a product of surface reactions. From Charlet and Manceau (1993), with permission. reviewers and editors for their helpful suggestions.

E LEMENTS 195 SEPTEMBER 2005 TABLE 2

PROPERTIES OF IMPORTANT CLAY MINERALS AND METAL OXIDE AND OXYHYDROXIDE SORBENTS IN NATURAL SYSTEMS

Mineral Chemical Formula Specific Surface Area (m2g-1) CLAY MINERALS AND OTHER LAYER SILICATES

Kaolinite Al2Si2O5(OH)4 7-30 Halloysite Al2Si2O5(OH)4 10-45 Pyrophyllite Al2Si4O10(OH)2 65-80 Talc Mg3Si4O10(OH)2 65-80 Montmorillonite (Na,Ca)0.3 (Al,Mg)2Si4O10(OH)2·nH2O 600-800 Dioctahedral vermiculite (K,Na,Ca)0.7(Al,Mg,Fe)2(Si,Al)4O10(OH)2•nH20 50-800 Trioctahedral vermiculite Mg0.7Mg3(SiAl)4O10(OH)2•nH2O 600-800 Muscovite KAl2Al Si3O10(OH)2 60-100 Minerals of the biotite series K(Mg,Fe,Al)3(Si,Al)4O10(OH,F)2 40-100 Minerals of the chlorite group [(Mg,Al,Li)2-3OH6](Al,Mg,Li)2-3(SiAl)4O10(OH,F)2 25-150 Allophane Variable (amorphous hydrous aluminosilicates 100-800 with a molar Si/Al ratio of 1.2 to 1:1) METAL OXIDES AND OXYHYDROXIDES Aluminum oxides and oxyhydroxides 100-220

Bayerite ␣-Al(OH)3 Böhmite ␥-AlO(OH)

Corundum ␣-Al2O3 Diaspore ␣-AlO(OH)

Gibbsite ␥-Al(OH)3 Iron oxides and oxyhydroxides 70-250 3+ Akaganéite ␤-Fe (O,OH,Cl) or Fe8(OH,O,Cl)17 Ferrihydrite ~ Fe-4-5(OH,O)12 Feroxyhyte ␦-FeO(OH) Goethite ␣-FeO(OH)

Hematite ␣-Fe2O3 Lepidocrocite ␥-FeO(OH)

Maghemite ␥-Fe2O3 2+ 3+ Magnetite Fe Fe 2O4 oxides and oxyhydroxides 5-360 4+ 3+ Birnessite ␦-MnO2 or (Na,Ca)(Mn ,Mn )4O8·3H2O Manganite ␥-MnO(OH) Groutite ␣-MnO(OH)

Pyrolusite ␤-MnO2

Source: Sparks (2002).

REFERENCES Hudson TL, Fox FD, Plumlee GS (1999) Metal Mining Sparks DL (2002) Environmental Soil Chemistry. and the Environment, Environmental Awareness Academic Press, San Diego Aicheson L (1960) A History of Metals. Interscience, Series 3, American Geological Institute, Alexandra, Sparks DL (2005) Metal and oxyanion sorption on New York Virginia Adriano DC (2001) Trace Elements in the Terrestrial naturally occurring oxide and clay mineral surfaces. Environment, 2nd edition. Springer-Verlag, New York Madden AS, Hochella MF Jr (2005) A test of In: Grassian V (ed) Environmental Catalysts, Taylor geochemical reactivity as a function of mineral and Francis Books Inc, Boca Raton, FL Adriano DC, Bolan NS, Vangronsveld J, Wenzel WW size: Manganese oxidation promoted by hematite Sposito G (1984) The Surface Chemistry of Soils. (2005) . In: D Hillel (ed) Encyclopedia nanoparticles. Geochimica et Cosmochimica Acta Oxford University Press, New York of Soils in the Environment, Elsevier, Amsterdam, 69: 389-398 pp 175-182 Sposito G (1989) The Chemistry of Soils. Oxford Mandarino JA, Back ME (2004) Fleischer’s Glossary of Brown Jr GE, Parks GA (2001) Sorption of trace University Press, New York Mineral Species 2004. elements on mineral surfaces: Modern perspectives The Mineralogical Record, Tucson, AZ Stevenson FJ (1982) Humus Chemistry. John Wiley from spectroscopic studies, and comments on and Sons, New York, sorption in the marine environment. International Morris, C (1992) ed. Academic Press Dictionary of Review 43: 963-1073 Science and Technology. Academic Press, San Stumm W (1992) Chemistry of the Solid-Water Brown Jr GE, Sturchio NC (2002) Applications of Diego, CA Interface. Wiley, New York synchrotron radiation in low-temperature National Research Council (2001) Basic research Tebo BM, Ghiorse WC, van Waasbergen LG, Siering geochemistry and environmental science. In: opportunities in earth science. National Academy PL, Caspi R (1997) Bacterially mediated mineral Fenter PA, Rivers ML, Sturchio NC, Sutton SR (eds), Press, Washington, DC, pp 154 formation: Insights into manganese (II) oxidation Reviews in Mineralogy and Geochemistry 35, from molecular genetic and biochemical studies. Pearson RG (1963) Hard and soft acids Mineralogical Society of America, Washington, DC, In: Fenter PA, Rivers ML, Sturchio NC, Sutton SR and bases. Journal of American Chemical Society pp 1-115 (eds), Reviews in Mineralogy and Geochemistry 35, 85: 3533-3539 Charlet L, Manceau A (1993) Structure, formation, Mineralogical Society of America, Washington, DC, and reactivity of hydrous oxide particles: Insights Pearson RG (1968) Hard and soft acids pp 225-226 and bases, HSAB. Part 1. Fundamental principles. from x-ray absorption spectroscopy. In: Buffle J, Utsunomiya S, Jensen KA, Keeler GJ, Ewing RC Journal of Chemistry Education 45: 581-587 van Leeuwen HP (eds) Environmental Particles, (2004) Direct identification of trace metals in fine Lewis Publishers, Boca Raton, pp 117-164 Roberts DR, Nachtegaal M, Sparks DL (2005). and ultrafine particles in the Detroit urban Hochella MF Jr (2002) Nanoscience and technology: Speciation of metals in soils. In: Tabatabai MA, atmosphere. Environmental Science & Technology The next revolution in the Earth sciences. Earth Sparks DL (eds) Chemistry of Soil Processes. Soil 38: 2289-2297 and Planetary Science Letters 203: 593-605 Science Society of America, Madison, WI.

E LEMENTS 196 SEPTEMBER 2005 Meet the Authors

John Bargar, research as a postdoctoral researcher. His PhD work Shelly D. Kelly received scientist at the Stanford focused on using synchrotron X-ray tech- her PhD from the Synchrotron Radiation niques to study the molecular binding of University of Washington Laboratory since 1996, dissolved metals to organic ligands, both on in 1999. She is currently an received his BS in geology the surface and in solution. His current assistant physicist in the and mineralogy from the research aims at linking the molecular Molecular Environmental Ohio State University (1990) coordination of ferrous iron to its reactivity, Science Group of the and his PhD in geological which has implications for the biogeochemical Environmental Research and environmental sciences from Stanford cycling of iron and the removal of contami- Division of Argonne National Laboratory. University (1996). Bargar’s main research nants from the environment. Kelly has focused her research on the interests lie in the areas of geomicrobiology, state and chemical speciation of contaminants low-temperature aqueous geochemistry, and Michael F. Hochella Jr. in biogeochemical systems as revealed by mineral–water and membrane–water-interface is professor of geochem- synchrotron X-ray techniques with emphasis geochemistry. His current research activities istry and mineralogy at on extended X-ray absorption fine-structure focus on the structural chemistry and environ- Virginia Tech (USA). He (EXAFS) spectroscopy. Her recent interests mental reactivity of bacteriogenic minerals, received his PhD degree include the interaction of uranyl with calcium with emphasis on elucidating their roles in the from Stanford University in and carbonate in solution, the incorporation biogeochemical cycling of metals in the 1981 under Gordon Brown. mechanism of U6+ into the crystallographic biosphere. He carries out these investigations For the last 20 years, his structure of calcite, and the interaction of U6+ under in situ conditions with synchrotron- research has involved mineral surface physics with microbial exudates. based scattering and spectroscopy techniques. and mineral–water interface geochemistry, Maxim I. Boyanov and more recently, nanoscience applied to Kenneth M. Kemner received a BSc and an MSc geochemistry and mineralogy, as well as obtained his PhD in physics degree in physics from the mineral–microbe interactions. All of this work at the University of Notre University of Sofia and a is generally applied to environmental science. Dame in 1993. From 1993 PhD in physics from the He is a former president of the Geochemical to 1996 he was an NRC University of Notre Dame. Society and is a recipient of the Dana Medal Post-Doctoral Fellow at the In 2003, he joined the of the Mineralogical Society of America, the Naval Research Laboratory Molecular Environmental Alexander von Humboldt Research Award, in Washington, DC. In Science Group at Argonne National Laboratory and a Senior Fulbright Fellowship. 1996 he joined the Environmental Research Glossary of terms related to metals and natural surfaces

Lewis acids and bases Sorption terminology involve electrostatic coulombic inter- (naturally occurring heterogeneous actions. Outer-sphere complexation organic substances of high molecu- Metals can be classified according to Adsorption – the accumulation of a is usually a rapid process that is lar weight and of brown to black their hard and soft characteristics, substance or material at the inter- reversible, and adsorption occurs color, i.e., humic and fulvic acids, based on the principle of hard and soft face between a solid surface and a only on surfaces that are of opposite and humin). The major organic acids and bases (Pearson 1963, 1968). bathing solution. charge to the adsorbate. functional groups are carboxyl Lewis acid – an electron pair acceptor. Inner-sphere surface complex – no Polymerization – the formation of small [R-C=O(-OH), where R is an aliphatic All metal ions or atoms and most water is present between multinuclear inorganic species such backbone] and phenolic (Ar-OH, cations are Lewis acids and capable the ion or molecule and the surface as dimers or trimers on a surface. where Ar is an aromatic ring) of accepting a pair of electrons from functional group to which it is (Stevenson 1982; Sparks 2002). The a Lewis base (anion). A metal that is bound, and the bonding is covalent Sorption – a general term that is used major inorganic surface functional a hard Lewis acid has a high positive or ionic. Inner-sphere complexes when the retention mechanism at a groups in soils are (1) the siloxane , small size, and low can be monodentate (metal is bond- surface is unknown. Adsorption, sur- surface associated with the plane of polarizability (e.g. potassium and ed to only one oxygen) or bidentate face precipitation, and polymeriza- oxygen atoms bound to the silica calcium), while a metal that is a soft (metal is bonded to two oxygens) tion are all examples of sorption. tetrahedral layer of a phyllosilicate Lewis acid has a large size, low posi- and mononuclear or binuclear. Surface complex – the interaction of a (clay mineral) and (2) hydroxyl tive charge, and high electronega- Inner-sphere complexation can surface functional group with an ion groups associated with the edges of tivity (e.g. copper and zinc). Most increase, reduce, neutralize, or or molecule present in the soil solu- inorganic minerals such as kaolinite, metals are soft or transition acids, reverse the charge on the sorptive tion can create a stable molecular amorphous materials, and metal meaning they have low positive regardless of the original charge. entity called a surface complex. The oxides and oxyhydroxides. Surface charge and large size and form cova- Adsorption of ions via inner-sphere overall reaction is referred to as sur- functional groups on metal oxide lent bonds with ligands. complexation can occur on a surface face complexation. and oxyhydroxide, clay mineral, regardless of the original charge. Lewis base – an electron pair donor. A Surface functional group – a “chemical- and organic matter surfaces play a Inner-sphere complexation is usually hard Lewis base is an electron donor ly reactive molecular unit attached significant role in metal sorption slower than outer-sphere complexa- of low polarizability and high elec- at the boundary of a solid with the processes. tion and is often not reversible. tronegativity (e.g. nitrate and sul- reactive groups of the unit exposed Surface precipitation – the formation of Outer-sphere surface complex – a fate), while a soft Lewis base is a to the soil solution” (Sposito 1984). a three-dimensional phase product water molecule is present between donor of large size and high polariz- Surface functional groups can be on a surface. the surface functional group and the ability (e.g. iodide and cyanide) organic or inorganic molecular bound ion or molecule (Sposito (Pearson 1963, 1968; Morris 1992; units. Organic functional groups are 1989). Outer-sphere complexes Sparks 2002). associated with humic substances

E LEMENTS 197 SEPTEMBER 2005 Division at Argonne National Laboratory near occurring at the plant–water–soil interface and postdoctoral researchers. A fellow of AAAS, Chicago, Illinois where he formed the how these processes influence the mobility the Soil Science Society of America, and the Molecular Environmental Science Research and bioavailability of heavy metals from American Society of Agronomy, he is the Group, an integrated multidisciplinary geogenic and anthropogenic sources. His recipient of numerous awards and honors, research group making use of third-generation current research focuses on the ways in situ including the Francis Alison Award, the synchrotron radiation for environmental remediation methods impact soil metal University of Delaware’s most prestigious research. Since 1997 he has been investigating speciation and on the biochemical mecha- faculty honor. mineral–microbe–metal interactions and their nisms of metal acquisition, transport, and role in affecting the mobility of contaminant storage in hyperaccumulating plants. He will Garrison Sposito is a metals and radionuclides. In 1999 he received complete his degree in December 2005. professor in the the Presidential Early Career Scientist Award Department of and the DOE-Office of Science Early Career Edward O’Loughlin is a Environmental Science, Scientist Award. In 2000 he received the staff scientist in the Policy and Management International Union of Crystallography Young Environmental Research and the Department of Scientist Award. Division at Argonne Civil and Environmental National Laboratory. He Engineering of the Ruben Kretzschmar is received his Bachelor’s University of California at Berkeley. His professor of soil chemistry degree in biology from research interests include the surface chem- in the Department of Cleveland State University istry of natural particles, the molecular Environmental Sciences at and his Master’s degree in environmental simulation of clay mineral and metal oxide the Swiss Federal Institute microbiology and his PhD in environmental structures, and the chemistry of humic of Technology (ETH) in chemistry from Ohio State University. His substances. He is a fellow of the American Zurich. He received his research interests are in the area of contami- Geophysical Union, the European Association PhD in soil chemistry from nant fate and transport in aquatic and for Geochemistry, the Geochemical Society, North Carolina State University. He and his terrestrial environments. He focuses his work the International Union of Pure and Applied students are investigating the biogeochemical on biogeochemical processes affecting biotic Chemistry, and the Soil Science Society of processes controlling the behavior of trace and abiotic transformations of a range of America. In 1997 he was elected to the elements in soils and sediments. Colloid and organic and inorganic contaminants, French Academy of Agriculture and in 2004 surface chemistry of clays and oxides and including aromatic N-heterocycles, halogenat- he was awarded the Horton Medal of the colloid-facilitated transport of trace metals ed hydrocarbons, heavy metals, and radionu- American Geophysical Union. have been important research topics of his clides. group. He has published more than 40 papers Ryan Tappero is a PhD in international peer-reviewed journals and Thorsten Schäfer is student in environmental several book chapters and reviews. Currently, Head of the Colloid soil chemistry (University he serves as a division chair for the German Chemistry Group since of Delaware). He studied Soil Science Society, as an editorial board 1999 and Head of the geology at Sierra College member of the Journal of Plant Nutrition and Radionuclide Migration and earned his BS in soil Soil Science, and as associate editor for the Group since 2004 at the science and MS in soil Vadose Zone Journal. Institut für Nukleare chemistry from California Entsorgung (INE), Polytechnic State University. His current Andrew Madden Forschungszentrum Karlsruhe (Germany). He research focuses on metal speciation in bulk received his BS degree in received his PhD in hydrogeology from the and rhizosphere soil and hyperaccumulating geology at Michigan State University of Mainz (Germany) in 1998. The plants. He utilizes multiple analytical tools University and his PhD at research of his group focuses on spectroscopic (e.g. synchrotron-based spectromicroscopy Virginia Tech where he and microscopic techniques to elucidate and X-ray absorption spectroscopy, electron worked with Mike radionuclide mobilization and retention microscopy, and liquid chromatography) Hochella in the mechanisms in geo-engineered barriers and along with geochemical techniques to NanoGeoscience and natural host-rock formations. He has authored understand metal cycling at the plant–miner- Technology Laboratory. He was awarded a or co-authored more than 20 articles in peer- al–water interface. He is the recipient of a National Science Foundation Graduate reviewed journals on the topics of radionu- USDA National Fellowship. Research Fellowship in 2001. His dissertation clide/ sorption and incorporation involved experimental studies of the geo- processes in nanoscopic secondary phases, Mario Villalobos has an chemical reactivity of hematite nanoparticles colloid characterization techniques, and up- undergraduate degree in as a function of size, along with the develop- scaling of colloidal transport from laboratory chemistry from UNAM in ment of Nano2Earth, an environmentally to field scale. Mexico, where he currently relevant curriculum designed to introduce holds a full-time researcher nanotechnology to secondary school students. Donald L. Sparks is S. position; a Master’s degree He is currently a postdoctoral research Hallock du Pont Endowed in soil chemistry from associate at the Environmental Sciences Chair of Soil and Wageningen Agricultural Division of Oak Ridge National Laboratory. Environmental Chemistry University in the Netherlands; and a PhD in at the University of civil and environmental engineering from David McNear received Delaware. His research Stanford University. He worked as a postdoc- his BS and MS degrees focuses on the kinetics of toral researcher at the University of California from the Pennsylvania soil chemical reactions and in Berkeley for two and a half years. His State University and the surface chemistry of soils. He and his research area of specialization is the environ- currently is a graduate group have been leaders in employing mental biogeochemistry of the soil–water fellow with Dr. Donald synchrotron-based techniques to study metal, system, including the environmental molecu- Sparks at the University of , and nutrient reactions in soil lar chemistry and surface chemistry of natural Delaware. His research minerals, soils, and biosolids. He has advised particles. He has published fifteen research interests include the biogeochemical processes and mentored 60 graduate students and articles in the field.

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