Speciation of arsenic in sulfidic waters

Formation constants for thioarsenite species have been determined in dilute solutions at 25 degreesC, SigmaH(2)S from 10(-7.5) to 10(-3.0) M, SigmaAs from 10(-5.6) to 10(-4.8) M, and pH 7 and 10. The principal inorganic arsenic species in anoxic aquatic systems are arsenite, As(OH)(3)(0), and a mononuclear thioarsenite with an S/As ratio of 3 : 1. Thioarsenic species with S/As ratios of 1 : 1, 2 : 1, and 4 : 1 are lesser components in sulfidic solutions that might be encountered in natural aquatic environments. Thioarsenites dominate arsenic speciation at sulfide concentrations > 10(-4.3) M at neutral pH. Conversion from neutral As(OH)(3)(0) to anionic thioarsenite species may regulate the transport and fate of arsenic in sulfate-reducing environments by governing sorption and mineral precipitation reactions.