Discrimination of thioarsenites and thioarsenates by x-ray absorption spectroscopy


Discrimination of thioarsenites and thioarsenates by x-ray absorption spectroscopy

Suess, E.; Scheinost, A. C.; Bostick, B. C.; Merkel, B. J.; Wallschlaeger, D.; Planer-Friedrich, B.

Soluble arsenic−sulfur compounds play important roles in the biogeochemistry of arsenic in sulfidic waters but conflicting analytical evidence identifies them as either thioarsenates (= AsV−sulfur species) or thioarsenites (= AsIII−sulfur species). Here, we present the first characterization of thioarsenates (mono-, di-, and tetrathioarsenate) by X-ray absorption spectroscopy and demonstrate that their spectra are distinctly different from those of AsIII−sulfur species, as well as from arsenite and arsenate. The absorption near edge energy decreases in the order arsenate > thioarsenates > arsenite > AsIII−sulfur species, and individual thioarsenates differ by 1 eV per sulfur atom. Fitted AsV−S and AsV−O bond distances in thioarsenates (2.13−2.18 Å and 1.70 Å, respectively) are significantly shorter than the corresponding AsIII−S and AsIII−O bond distances in AsIII−S species (2.24−2.34 Å and 1.78 Å, respectively). Finally, we demonstrate that thioarsenates can be identified by principal component analysis in mixtures containing AsIII−sulfur species. This capability is used to study the spontaneous reduction of tetrathioarsenate to AsIII−sulfur species (possibly trithioarsenite) upon acidification from pH 9.5 to 2.8.

Keywords: Arsenic; speciation; thioarsenite; thioarsenate; EXAFS; XANES

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Publ.-Id: 13286