Toxic metal speciation models

Toxic Metal Speciation Models for Soil and Sediment.51... 

Twiss, M., Errecalde, O., Fortin, C., Campbell, P., Jumarie, C., Denizeau, F., Berkelaar, E., Hale, B., and van Rees, K., Coupling the use of computer chemical speciation models and culture techniques in laboratory investigations of trace metal toxicity, Chem Spec Bioavailab, 13 (1), 9-24, 2001. 

An evaluation of the fate of trace metals in surface and sub-surface waters requires more detailed consideration of complexation, adsorption, coagulation, oxidation-reduction, and biological interactions. These processes can affect metals, solubility, toxicity, availability, physical transport, and corrosion potential. As a result of a need to describe the complex interactions involved in these situations, various models have been developed to address a number of specific situations. These are called equilibrium or speciation models because the user is provided (model output) with the distribution of various species. 

Literally hundreds of complex equilibria like this can be combined to model what happens to metals in aqueous systems. Numerous speciation models exist for this application that include all of the necessary equilibrium constants. Several of these models include surface complexation reactions that take place at the particle-water interface. Unlike the partitioning of hydrophobic organic contaminants into organic carbon, metals actually form ionic and covalent bonds with surface ligands such as sulfhydryl groups on metal sulfides and oxide groups on the hydrous oxides of manganese and iron. Metals also can be biotransformed to more toxic species (e.g., conversion of elemental mercury to methyl-mercury by anaerobic bacteria), less toxic species (oxidation of tributyl tin to elemental tin), or temporarily immobilized (e.g., via microbial reduction of sulfate to sulfide, which then precipitates as an insoluble metal sulfide mineral). 

Chemical models of metal speciation have been used to assess the biological availability of different solute metal forms. Pagenkopf and Andrew (l ) used equilibrium models to suggest that the availability of Cu to fishes was controlled by the concentration of the free Cu ion. Equilibrium models were also used to show that the toxicity of Cu to phytoplankton followed the activity of metals rather than total metal concentrations l8) and that the concentration of free Zn ion plus additional factors (e.g. competition from Ca and Mg) may affect the availability of solute Zn to fishes (19>20). 

While Davis and his colleagues illustrated the significance of soil metal speciation in risk assessment, Morrison et al. (1989) pointed out that the toxicity of metals is related to the forms in which they exist in the aqueous phase. This is because the interaction of metals with intracellular compartments is highly dependent on chemical speciation. Some species may be able to bind chemically with extracellular proteins and other biological molecules, some may adsorb onto cell walls, and others may diffuse through cell membranes. Consequently, toxicity is more related to the concentrations of metals in a particular species, than to the total concentrations. Geochemical modeling... 

Examples showing that metal speciation is important to metal toxicity include arsenic, copper, selenium, and chromium. While ionic copper (Cu2+) and CuClj are highly toxic, Q1CO3 and Cu-EDTA have low toxicity (Morrison et al, 1989). Toxicity tests show that As(III) is about 50 times more toxic than As(VI). Trivalent chromium is much less toxic than hexavalent chromium, probably because Cr(VI) is much smaller and the chemical structure of chromate is similar to sulfate. A special channel already exists in biomembranes for sulfate transport. While modeling metal speciation is not always possible, and redox equilibrium is not achieved in all natural waters, geochemical modeling of equilibrium species distribution remains one of the methods of discerning metal speciation. 

Level 2 In the second level (Level 2), physico-chemical speciation models are introduced in order to correct the toxicity data for chemical availability. Indeed, NOEC and/or ECm values that are used in the effects assessment are generally generated in test media with varying physico-chemical characteristics (e.g. pH, hardness, DOC) known to alter metal availability and toxicity. In case metal concentrations are reported and appropriate speciation models (e.g. WHAM, MINTEQA2, etc.) and relevant input data (i.e. main physico-chemical parameters driving the availability of a metal such as pH, DOC, etc.) are available, NOEC and/or ECm values should be expressed on the basis of the metal species of concern in order to reduce uncertainty. Eor regulatory compliance purposes, the dissolved exposure concentrations should also be translated at the same level of availability (expressed in the same units) as the effects assessment. 

Enzymatic extraction carried out using in vitro models of the gastrointestinal tract is cheaper, faster, more reproducible, and ethically easier than the alternative of studies involving people and animals. Estimation of nutrient bioavailability (including trace elements, both essential and toxic) is particularly important for nutritionists, pharmacists, and toxicologists. Application of sequential procedures allows analyte fractionation (metals usually), but enzyme selectivity allows leaching of certain speciation forms of the determined elements. Table 6.9 gives examples of application of enzymatic extraction procedures for trace element analysis and speciation analysis [71, 72]. 

A9.7.2.1.2.2 Where speciation is important, it may be possible to model the concentrations of the different forms of the metal, including those that are likely to cause toxicity. Analysis methods for quantifying exposure concentrations, which are capable of distinguishing between the complexed and uncomplexed fractions of a test substance, may not always be available or economic. 

An area of research where effort is currently being directed is the study of metal complexation in aqueous solutions and on mineral surfaces (see Hochella and White 1990 and references therein). The reason that research is focusing on details of the molecular structure in this type of system is that scientists have begun to realize that speciation can play a dramatic role in the mobility and toxicity of elements in the environment (see Brown et al. 1999 for a review). Molecular modeling has the potential to make an impact on this field in a variety of ways (see Rosso, Rustad or Sherman, this volume), one of which is in modeling vibrational spectra to help interpret observed spectra. When model vibrational frequencies can be combined with model NMR chemical shifts (see Tossell, this volume) and compared with experimental spectra, complex problems may be more easily understood. 

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