Constraints on Precipitation of the Ferrous Arsenite Solid HFe(AsO)

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)(As(V)O)]·8HO) 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 HFe(AsO) and a log of 34 for the dissolution reaction defined as: HFe(AsO) + 8H = 4Fe + 5HAsO. Extended X-ray absorption fine structure spectroscopic analysis of HFe(AsO) shows that the molecular environment of Fe is dominated by edge-sharing octahedra within an Fe(OH) sheet and that As is dominated by corner-sharing AsO pyramids, which are consistent with previously published structures of As(III)-rich Fe(II) solids. The HFe(AsO) 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 HFe(AsO); however, a high concentration of Fe(II) is required, which is unusual under alkaline conditions.