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In situ analysis of thioarsenite complexes in neutral to alkaline arsenic sulphide solutions

¹ Dartmouth College, Department of Earth Sciences, Hanover NH 03755, USA
² Stanford University, Department of Geological and Environmental Sciences, Stanford CA 94305-2115, USA
³ Stanford Synchrotron Radiation Laboratory, SLAC, 2575 Sand Hill Road, Menlo Park, CA 94025, USA

^* E-mail: benjamin.c.bostick{at}dartmouth.edu

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)S₂^2– , AsS₃^3– 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 AsS₃S_3+x(SH)_3–x ^–x 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 AsS₃S_3+x(SH)_3–x ^–x) 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.

KEYWORDS: thioarsenite complexes, arsenic sulphide solution, in situ analysis

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