title: Discrimination of thioarsenites and thioarsenates by X-ray absorption spectroscopy.
abstract: Soluble arsenic-sulfur compounds play important roles in the biogeochemistry of arsenic in sulfidic waters but conflicting analytical evidence identifies them as either thioarsenates (= As(V)-sulfur species) or thioarsenites (= As(III)-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 As(III)-sulfur species, as well as from arsenite and arsenate. The absorption near edge energy decreases in the order arsenate > thioarsenates > arsenite > As(III)-sulfur species, and individual thioarsenates differ by 1 eV per sulfur atom. Fitted As(V)-S and As(V)-O bond distances in thioarsenates (2.13-2.18 A and 1.70 A, respectively) are significantly shorter than the corresponding As(III)-S and As(III)-O bond distances in As(III)-S species (2.24-2.34 A and 1.78 A, respectively). Finally, we demonstrate that thioarsenates can be identified by principal component analysis in mixtures containing As(III)-sulfur species. This capability is used to study the spontaneous reduction of tetrathioarsenate to As(III)-sulfur species (possibly trithioarsenite) upon acidification from pH 9.5 to 2.8.
authors.affiliation: Environmental Geochemistry, University of Bayreuth, Universitaetsstrasse 30, 95440 Bayreuth, Germany. elke.suess@uni-bayreuth.de
authors.forename: Elke
authors.initials: E
authors.lastname: Suess
authors.forename: Andreas C
authors.initials: AC
authors.lastname: Scheinost
authors.forename: Benjamin C
authors.initials: BC
authors.lastname: Bostick
authors.forename: Broder J
authors.initials: BJ
authors.lastname: Merkel
authors.forename: Dirk
authors.initials: D
authors.lastname: Wallschlaeger
authors.forename: Britta
authors.initials: B
authors.lastname: Planer-Friedrich