Oxyanions, redox chemistry

Selenium (masses 74, 76, 77, 78, 80, and 82 Table 1) and chromium (masses 50, 52, 53 54 Table 1) are treated together in this chapter because of their geochemical similarities and similar isotope systematics. Both of these elements are important contaminants in surface and ground water. They are redox-active and their mobility and environmental impact depend strongly on valence state and redox transformations. Isotope ratio shifts occur primarily during oxyanion reduction reactions, and the isotope ratios should serve as indicators of those reactions. In addition to environmental applications, we expect that there will be geological applications for Se and Cr isotope measurements. The redox properties of Se and Cr make them promising candidates as recorders of marine chemistry and paleoredox conditions. 

In natural waters, soils, and sediments, the As species of interest are the arsenate oxyanions, As(V) the arsenite oxyanions, As(III) monomethylarsonic acid, As(III) and dimethylarsinic acid, As(I). Arsenic chemistry is governed by many factors. The solubility of their salts, the complexing ability of solid and soluble ligands, the biological reactions, the pH and redox potential, and the presence of other ions are all reported to control As concentration and speciation. 

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