Skip to main content.
 

Abstract's Details

Solubility and Structure of Ferrous Arsenite Precipitate: Implications for Arsenic Geochemical Cycling in Anaerobic Environment
Abstract IDENV-01 
PresenterYoko  Masue-Slowey
Presentation TypePoster
Full Author ListY. Masue-Slowey (1), S. Webb (2), S. Fendorf (1)
Affiliations(1) Stanford University
(2) Stanford Synchrotron Radiation Laboratory
CategoryEnvironmental Science
AbstractIn soils and sediments, reductive dissolution and transformation of Fe (hydr)oxides are considered to trigger As release under anaerobic conditions; however, recent study suggests reductive transformation of iron phases could sequester As. Formation of Fe(II)-As(III) solids has been proposed as a potential mechanism of As sequestration upon anaerobiosis, and, in fact, a solid of this type has been observed by abiotic reaction of Fe(II) and As(III). Previous reports suggest a local structure, obtained using X-ray absorption spectroscopy (XAS), of double-corner and edge sharing As(III) pyramids linked to Fe(II)-(OH) octahedral sheets. However, the proposed structure does not satisfy the reported 1:1 As:Fe stoichiometry. Moreover, neighboring As atoms were not identified but would seem necessary to account for the high proportion of As within the solids. In addition, the environmental conditions required for the formation of an Fe(II)-As(III) precipitate are unknown. Consequently, the scope of this study is to characterize the mineralogical properties of Fe(II)-As(III) precipitates and to quantify their solubility. In order to resolve the discrepancy in stoichiometry and identify the precise location of As within Fe(II)-As(III) precipitate, polarized extended X-ray absorption spectroscopy (P-EXAFS) was used. An Fe(II)-As(III) precipitate was deposited and dried on polycarbonate sample holder such that basal plane of the particles was oriented parallel to the sample holder. P-EXAFS indicates that As(III) resides in the vacancy of the Fe(OH)2 sheets as well as in double-corner and edge-sharing environments on the Fe octahedral. The solubility minimum for the Fe(II)-As(III) precipitate occurs at pH 7.5, but still results in millimolar concentrations of dissolved Fe(II) and As(III). Thus, this precipitate is likely only in heavily contaminated sites and is unlikely to control dissolved concentrations of arsenic in most environments.
Footnotes 
Funding Acknowledgement