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.

Arsenic binding to organic and inorganic sulfur species during microbial sulfate reduction: a sediment flow-through reactor experiment

“…The presence of As V -S is supported by the analysis of the EXAFS data using Fourier transform (see Fig. S1), revealing that the average As-S bond distances (uncorrected for backscattered phase shift) in this sample is equal to the previously reported value for As V -S, [18] which is ,0.10 Å shorter than for As III -S. However, based on our current understanding of the geochemistry of TTAs V , it is doubtful that it is formed in FTR C, because TTAs V is unstable at pH ,10.…”

“…Under the latter conditions, As uptake by Fe monosulfide (FeS m(s) ) occurs through the formation of a sorption complex akin to realgar (AsS (s) ), as shown by Gallegos et al [17] In environments where all the reactive Fe is transformed into Fe sulfides, the remaining free sulfide (S thiolated As species. [18] At the present time, the mechanisms by which aqueous S, including sulfide (S (aq) ÀII ), zero-valent sulfur (S (aq) 0 ) and polysulfides (S n 0 S (aq) ÀII ), all ubiquitous in the aquatic environment, [19] promote thioarsenite and thioarsenate formation are not completely understood. [20] The key influence of Fe and S on the geochemical behaviour of As has led to the classification of subsurface environments as exhibiting either Fe-or S-controlled As sequestration.…”

Pyrobaculum yellowstonensis Strain WP30 Respires on Elemental Sulfur and/or Arsenate in Circumneutral Sulfidic Geothermal Sediments of Yellowstone National Park

“…A commercially available standard (C 2 H 6 S 3 ; Acros Organics) and fractions of a synthesized dimethylpolysulfane mixture were used for quantification (25). The abiotic monothioarsenate control (Na 3 AsO 3 S·7H 2 O; formula weight, 350) was synthesized as previously described (26,27), and the elemental sulfur (10 g liter Ϫ1 ) and 5 mM arsenite (NaAs III O 2 ; SigmaAldrich Chemical Co.) abiotic control was prepared with base medium and incubated at 75°C for 4 days before derivatization. Samples for arsenic speciation analysis were thawed in an anaerobic hood (95% N 2 , 5% H 2 atmosphere) to minimize oxidation.…”

Reassessing the role of sulfur geochemistry on arsenic speciation in reducing environments

“…Because of practical difficulties associated with the experimental characterization of the speciation of trace elements in sulfidic environments (e.g., [17,18]), geochemical equilibrium models remain very popular. These models are powerful tools to help design remediation strategies, predict the evolution of water quality and focus experimental studies [19].…”