Reassessing the role of sulfur geochemistry on arsenic speciation in reducing environments |
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Authors: | Couture Raoul-Marie Van Cappellen Philippe |
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Affiliation: | School of Earth and Atmospheric Sciences, Georgia Institute of Technology, Atlanta, GA 30332-0340, USA. raoul.couture@eas.gatech.edu |
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Abstract: | Recent evidence suggests that the oxidation of arsenite by zero-valent sulfur (S(0)) may produce stable aqueous arsenate species under highly reducing conditions. The speciation of arsenic (As) in reducing soils, sediments and aquifers may therefore be far more complex than previously thought. We illustrate this by presenting updated E(h)-pH diagrams of As speciation in sulfidic waters that include the most recently reported formation constants for sulfide complexes of As(III) and As(V). The results show that the stability fields of As(III) and As(V) (oxy)thioanions cover a large pH range, from pH 5 to 10. In particular, As(V)-S(-II) complexes significantly enhance the predicted solubility of As under reducing conditions. Equilibrium calculations further show that, under conditions representative of sulfidic pore waters and in the presence of solid-phase elemental sulfur, the S(0)((aq))/HS(-) couple yields a redox potential (E(h))~ 0.1 V higher than the SO(4)(2-)/HS(-) couple. S(0) may thus help stabilize aqueous As(V) not only by providing an electron acceptor for As(III) but also by contributing to a more oxidizing redox state. |
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Keywords: | Zero-valent sulfur Sulfidic environments Thioarsenic anions Equilibrium speciation Eh–pH diagrams |
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