Constraints on Precipitation of the Ferrous Arsenite Solid H7Fe4(AsO3)5
- Yoko Masue-Slowey *a,
- Aaron J. Sloweyb,
- F. Marc Michelc,
- Samuel M. Webbd and
- Scott Fendorfa
- a Dep. of Environmental Earth System Science, Stanford Univ., Stanford CA 94305
b Lawrence Berkeley National Lab., 1 Cyclotron Rd., Berkeley, CA 94720
c Dep. of Geosciences, Virginia Polytechnic Institute and State Univ., Blacksburg, VA 24061
d Stanford Synchrotron Radiation Lightsource, Menlo Park, CA 94025
Formation of Fe(II)-As(III) solids is suspected to limit dissolved As concentrations in anaerobic environments. Iron(II) precipitates enriched in As(III) have been observed after microbial reduction of As(V)-loaded lepidocrocite (γ-FeOOH) and symplesite (Fe(II)3(As(V)O4)2]·8H2O) and upon abiotic reaction of Fe(II) with As(III). However, the conditions favorable for Fe(II)-As(III) precipitation and the long-term stability (relative to dissolution) of this phase are unknown. Here we examine the composition, local structure, and solubility of an Fe(II)-As(III) precipitate to determine environments where such a solid may form and persist. We reveal that the Fe(II)-As(III) precipitate has a composition of H7Fe4(AsO3)5 and a log Kso of 34 for the dissolution reaction defined as: H7Fe4(AsO3)5 + 8H+ = 4Fe2+ + 5H3AsO3. Extended X-ray absorption fine structure spectroscopic analysis of H7Fe4(AsO3)5 shows that the molecular environment of Fe is dominated by edge-sharing octahedra within an Fe(OH)2 sheet and that As is dominated by corner-sharing AsIIIO3 pyramids, which are consistent with previously published structures of As(III)-rich Fe(II) solids. The H7Fe4(AsO3)5 solid has a pH-dependent solubility and requires millimolar concentrations of dissolved Fe(II) and As(III) to precipitate at pH <7.5. By contrast, alkaline conditions are more conducive to formation of H7Fe4(AsO3)5; however, a high concentration of Fe(II) is required, which is unusual under alkaline conditions.Please view the pdf by using the Full Text (PDF) link under 'View' to the left.
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