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Chapter 4. METAL-SPECIFIC TOPICS and METHODS

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Chapter 4. METAL-SPECIFIC TOPICS and METHODS 1 4. METAL-SPECIFIC TOPICS AND METHODS 2 3 This chapter discusses metal-specific topics and methods to be used in the assessment of 4 risk to humans and ecological entities from exposures to inorganic metals. It applies information 5 and text from the metals issue papers and reflects contributions by EPA scientists and external 6 experts. The final metals issue papers are available on the EPA Web site at 7 http://cfpub.epa.gov/ncea/raf/recordisplay.cfm?deid=86119. 8 Key topics and tools in this section are presented in subsections on environmental 9 chemistry, exposure pathway analysis, human health effects, and ecological effects. The 10 applications and limitations of the various models and methods for conducting metals 11 assessments are presented to inform the reader. Topics and tools related to bioavailability and 12 bioaccumulation are discussed throughout Chapter 4 because they have far reaching impact that 13 crosses many aspects of metals assessment. 14 15 4.1. ENVIRONMENTAL CHEMISTRY 16 4.1.1. Introduction and Terminology 17 A general review of factors pertaining to the chemistry of metals in sediments, soils, 18 waters, and the atmosphere is presented in this chapter in the context of risk assessment. 19 Because the behavior of metals defies simple generalities, it is necessary to understand the 20 chemistry of the particular metal and the environment of concern. However, we can generalize 21 factors that control metal chemistry and environmental characteristics where this generalization 22 allows us to progress with estimates of metal fate and effects. 23 Metal speciation determines the behavior and toxicity of metals in the environment. 24 Speciation refers to the occurrence of a metal in a variety of chemical forms. These forms may 25 include free metal ions, metal complexes dissolved in solution and sorbed on solid surfaces, and 26 metal species that have been coprecipitated in major metal solids or that occur in their own 27 solids. The speciation of a metal affects not only its toxicity but also its volatilization, 28 photolysis, sorption, atmospheric deposition, acid/base equilibria, polymerization, complexation, 29 electron-transfer reactions, solubility and precipitation equilibria, microbial transformations, and 30 diffusivity (Bodek et al., 1988). 31 The following sections address the application of hard and soft acid and base (HSAB) 32 concepts to metal behavior, including the formation of metal complexes, and the importance of 33 pH and oxidation-reduction reactions to metal mobility and toxicity. The chapter then examines 11/24/2004 Peer Review Draft DISCLAIMER: This information is distributed solely for the purpose of peer review under applicable information quality guidelines. It has not been formally disseminated by the EPA and should not be construed to represent any Agency determination or policy. 4-1 1 the occurrence and interactions of the metals of concern in natural media, including in surface 2 and ground waters, soils and aquatic sediments, and the atmosphere. Important topics 3 considered in these sections are metal sorption behavior, aging in soils, metal dissolution and 4 transformation and transfer to plants, and methods of determining metal speciation in soils and 5 sediments. 6 7 4.1.2. Hard and Soft Acids and Bases: The Stability of Complexes 8 Complexes are formed between metals (acids) and Hard and Soft Acids and Bases 9 ligands (bases), both in solution and at the surfaces Hard acids and hard bases. 10 minerals and organisms. The toxic reaction of organisms Complexes formed between divalent hard acid cations and monovalent or divalent 11 to metals can be directly related to the nature of the metal hard bases are ionic and relatively weak 12 complexes formed in solution and at the surface of the and are often termed “ion pairs.” Complexes formed between Be2+ or 13 organism. trivalent hard acids and hard bases tend to 14 A useful concept that helps to explain the strength be ionic and relatively strong. Soft acids and soft bases. Strong, 15 of metal complexing and metal toxicity is that of hard and relatively covalent bonds are formed in 16 soft acids and bases (HSAB), which was introduced by complexes between soft and borderline soft acid cations and soft bases. Ligand 17 Pearson (1973). In this concept, metal cations are Lewis binding sites on the external or internal 18 acids and ligands are Lewis bases, with the metal cation surfaces of organisms are often of soft base character and thus bond strongly 19 and ligand in a complex acting as electron acceptor and with soft and borderline soft acid cations. 20 donor, respectively. “Soft” implies that the species’ 21 electron cloud is deformable or polarizable and the electrons are mobile and easily moved. Soft 22 species prefer to participate in covalent bonding. Hard species are comparatively rigid and 23 nondeformable, have low polarizability, hold their electrons firmly, and prefer to participate in 24 ionic bonds in complex formation (Langmuir, 1997). Hard acids form strong, chiefly ionic 25 bonds with hard bases, whereas soft acids and soft bases form strong, chiefly covalent bonds 26 when they form complexes. In contrast, the bonds formed 27 between hard-soft or soft-hard acids and bases are weak, Ligands 28 such that their complexes tend to be rare. Table 4-1 Ligands are anions or molecules that form complexes with metal ions. 29 summarizes hard and soft acid and base relationships for Depending on whether a ligand shares 30 the metals of concern. The first text box summarizes the one, two, three or more electron pairs with metals, it is called a mono-, bi-, tri­ 31 applicability of hard and soft concepts to the formation of or multidentate ligand. 32 metal complexes; the second text box defines ligands. 11/24/2004 Peer Review Draft DISCLAIMER: This information is distributed solely for the purpose of peer review under applicable information quality guidelines. It has not been formally disseminated by the EPA and should not be construed to represent any Agency determination or policy. 4-2 1 Hard metals (hard acids), which are the least toxic, preferentially bind with hard bases 2 that contain oxygen, forming weaker bonds with soft nitrogen and sulphur species. The strength 3 of binding between hard metals and hard ligands is usually a function of pH. Many of the hard 4 metals are macronutrients. Soft metals (acids) bind preferentially with soft S and N ligands, 5 forming weaker bonds with hard base species such as hydroxide and sulfate. Soft and 6 borderline metals, and Mn2+, which is hard, form bonds of decreasing strength with soft ligands 7 such as 8 Table 4-1. Hard and soft acids (metal cations) and bases (ligands) Hard acids Al3+, Ba2+, Be2+, Co3+,, Cr3+, Fe3+, Mn2+, Sr2+, U4+, 2+ 2+ UO2 , VO Borderline acids (between hard and soft) Co2+, Cu2+, Fe2+, Ni2+, Pb2+, Zn2+ + 2+ + 2+ + + 3+ + Soft acids Ag , Cd , Cu , Hg , Hg , CH3Hg , Tl , Tl ­ ­ 2- 2- - Hard bases F , H2O, oxyanions: OH , SO4 , CO3 , HCO3 , 2- 2- 2- n-3 n-3 C2O4 , CrO4 , MoO4 HnPO4 , HnAsO4 , 2- - ­ SeO4 , H2VO4 , NH3, RNH2, N2H4, ROH, RO , ­ R2O, CH3COO , etc. ­ ­ - 2- n-3 Borderline bases (between hard and soft) Cl , Br , NO2 , SO3 , HnAsO3 , C6H5NH2, C5H5N, N3-, N2 ­ ­ 2- ­ ­ 2- 2- Soft bases I , HS , S , CN , SCN , Se , S2O3 , -SH, -SCH3, -NH2, R-, C2H4, C6H6, RNC, CO, R3P, (RO)3P, - R3As, R2S, RSH, RS Source: Modified after Huheey et al. (1993) and Langmuir (1997). “R” refers to an organic molecule. 1 sulfide, generally in the following order: Pb2+> Cu2+ > Cd2+ > Co2+ – Fe2+ > Ni2+ > Zn2+ > Mn2+. 2 The tendency of metals to bind to soft ligands or to organic substrates (which are usually soft) is 3 greatest for soft and borderline metals (soft acids), followed by the hard metals (hard acids), 4 typically in the order Pb2+> Cu2+>Cd2+> Zn2+ > Ca2+ > Mg2+ >> Na+ (Pickering, 1986). 5 The tendency of metals to form solid phases, such as sulfides in sediments, is also related 6 to their HSAB qualities. For example, extremely insoluble metal sulfides are formed in anoxic 2+ + 7 sediments by soft acid metal cations, such as Hg (log Ksp = –57.25) or Ag (log Ksp = –49.7), 2+ 2+ 8 whereas borderline hard and hard metal cations such as Mn (log Ksp = –19.25) or Fe (log Ksp 9 = –22.39) form slightly more soluble, although still highly insoluble, metal sulfides.1 These 1 Solubility products for all sulfides except Ag2S are from Di Toro et al. (1990). The product for silver sulfide is from Stumm and Morgan (1970). 11/24/2004 Peer Review Draft DISCLAIMER: This information is distributed solely for the purpose of peer review under applicable information quality guidelines. It has not been formally disseminated by the EPA and should not be construed to represent any Agency determination or policy. 4-3 1 differences in solubilities are fundamental to the method of acid-volatile sulfide (AVS) 2 normalization of sediment-associated metals (see Section 4.1.5, on Sediment Chemistry). 3 Finally, according to the Biotic Ligand Model (BLM) (see Sections 3.1.4.2 and 3.4), effects of 4 metals are related to or correlated with metal interactions with biological ligands, which are 5 generally soft base species. 6 7 4.1.3. Aquatic Chemistry 8 4.1.3.1. Speciation and Complexes 9 Metal species dissolved in water may occur as free ions, or aquo-ions, or as complexes. 10 Free metal cations are generally surrounded by coordinating water molecules and so have been 11 termed aquocations, although by convention the water molecules are ignored when writing 12 chemical reactions involving metal cations. 13 The total analytical concentration of a given metal in water is the sum of the 14 concentrations of its free ion and its complexes and any metal associated with suspended solids, 15 whether organic or mineral.
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