Toxic Metals in the Environment: The Role of Surfaces Donald L. Sparks1 etals are prevalent in the environment. They are derived from both such as density, 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 metalloids 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 minerals, metal 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 nonmetals (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 densities greater than Civilization was founded upon the metals of antiquity, 5.0 g cm-3 and usually indicates metals and metalloids asso- gold (Au), copper (Cu), silver (Ag), lead (Pb), tin (Sn), ciated with pollution and toxicity; however, the term also iron (Fe), and mercury (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. ores). Anthropogenic sources include agriculture (fertilizers, In some cases, metals have accumulated in terrestrial and animal manures, pesticides), metallurgy (mining, smelting, 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, zinc 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 Native metal (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, mineral 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 group) and demethylation reactions. industry, refineries, paint industry, and carbonates,