XAS-based characterization of thioarsenates and their transformation to thioarsenites in acidic synthetic solutions


XAS-based characterization of thioarsenates and their transformation to thioarsenites in acidic synthetic solutions

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

Recent ab initio calculations predict that As(III)- and As(V)-thioanions can occur simultaneously in sulfidic environments under commonly encountered pH conditions. However, unambiguous structural characterizations for the two homologue series of thioarsenites or thioarsenates are still missing. In the present study, the molecular structures of synthesized mono-, di-, and tetrathioarsenate were characterized by XRD and XAS in their solid state and dissolved at alkaline or near-neutral pH-conditions. The determined As(V)-S and As(V)-O bond distances of 2.13-2.18 Å and 1.70 Å, respectively, are consistent with the theoretical expectations for thioarsenates and can be clearly differentiated from the respective As(III)-bondings, with 2.24-2.34 Å and 1.78 Å. The XANES edge positions of the individual thioarsenates (mono- 11871 eV, di- 11870, and tetrathioarsenate 11869.5) lie between those of As2O5 (11872.3 eV) and As2O3 (11868.2 eV) and reveal the influence of increasing sulfur substitution by decreasing bond energies and hence absorption edge energy. These thioarsenates can be clearly distinguished from the species formed upon acidification of tetrathioarsenate below pH 6.3. These species were identified as thioarsenites. Their XANES edge energy of ca. 1 eV below that of As2O3 indicates the redox-state change from As(V) to As(III), which is further confirmed by the increased As-S distance of 2.23-2.28 Å corresponding to As(III)-S species. At pH 3, the detection of As-As-bonds indicates the beginning formation of colloids, most likely amorphous As2S3. Our results indicate that in synthetic solutions, thioarsenites can form as transitory species between dissolved thioarsenates at near-neutral pH and AsS-precipitates formed under acidic conditions.

Keywords: thioarsenites; thioarsenates; EXAFS; XANES

  • Lecture (Conference)
    Goldschmidt Conference 2009; Challenges to our Volatile Planet, 21.-26.06.2009, Davos, Switzerland
  • Geochimica et Cosmochimica Acta 73(2009)13, A1288-A1288

Permalink: https://www.hzdr.de/publications/Publ-12521
Publ.-Id: 12521