Applications to Bioavailability and Risk Assessment Studies

Davis et al. (1992, 1993) first studied the mineralogy of the mining waste impacted soil in the Butte, Montana, mining district, and then performed speciation-solubility calculations. They reasoned that dissolution of As and Pb under the acidic stomach 

For As solubility calculations, the modeling generated less useful information. Enargite is found to be quite soluble in stomach fluids. Davis et al. (1992) believe that dissolution kinetics or inhibition from reaction with GI fluids by encapsulation with less soluble phases (e.g. jarosite) cause the observed discrepancy between equilibrium calculations and experiments. 

Comparing the modeling results with in vivo experiments using female New Zealand white rabbits shows that the in vivo Pb concentration is lower than the up-bound of soluble Pb calculated from anglesite solubility in stomach fluid. The calculated Pb solubility is 69 mgL-1, whereas the in vivo Pb concentration in the rabbit stomach fluid is 8.5 mgL-1. Ruby et al. (1992) attributed the lower experimental Pb concentration to dissolution kinetics of anglesite and a short transit time in the GI tract (6 hours). 

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. 

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