The solubility of arsenic in anoxic soil and sediment pore waters is strongly influenced by complexation with dissolved sulphide. Despite their importance in arsenic chemistry, thioarsenite complexes have not been well characterized, and considerable questions remain regarding their structure, protonation state, and relative stabilities. Here we use X-ray absorption spectroscopy to examine the type and structure of aqueous arsenic-sulphur complexes in sulphidic solutions under neutral to alkaline pH. Arsenic formed a variety of thioarsenites, including AsS(SH)(OH) , As(SH)S22− , AsS33− and As(SH)4 complexes. The relative fraction of each species varied strongly with the S:As ratio – with the fraction of AsS(SH)(OH) greatest at low S:As and trithioarsenites dominating As speciation when S:As ratios exceeded 3 in solution. As much as 40% of the total As also was present as AsS3S3+x(SH)3–xx in solutions at S:As ratios of 3 or greater. Sulphide complexation was somewhat dependent on pH, with sulphide complexation generally increasing with pH. The speciation observed in these experiments is similar to, though distinct from, speciation predicted based on As2S3 solubility (inferred to contain AsS2 and AsS3S3+x(SH)3–xx) and chromatographic separation of arsenic species (which does not identify polymeric species). Thus, these data indicate that stability constants for arsenic sulphide complexes must be reappraised.

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