Constraints On Precipitation Of The Ferrous Arsenite Solid H7fe4(Aso3)(5)

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 (gamma-FeOOH) and symplesite (Fe(II)(3)(As(V)O-4)(2)]center dot 8H(2)O) 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 K-50 of 34 for the dissolution reaction defined as: H7Fe4(AsO3)(5) +8H(+) = 4Fe(2+) + 5H(3)AsO(3). 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 (AsO3)-O-III 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.