Speciation of arsenic in sulfidic waters

Abstract

Formation constants for thioarsenite species have been determined in dilute solutions at 25 °C, ΣH₂S from 10^−7.5 to 10^−3.0 M, ΣAs 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 > 10^−4.3 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.