Speciation of arsenic in sulfidic waters

Abstract

Formation constants for thioarsenite species have been determined in dilute solutions at 25 °C, ∑H2S from 10-7.5 to 10-3.0 M, σmAs 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 30, 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 > 104.3 M at neutral pH. Conversion from neutral As(OH)30 to anionic thioarsenite species may regulate the transport and fate of arsenic in sulfate-reducing environments by governing sorption and mineral precipitation reactions. © The Royal Society of Chemistry and the Division of Geochemistry of the American Chemical Society 2003.