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Constraints on Precipitation of the Ferrous Arsenite Solid H 7 Fe 4 (AsO 3 ) 5
Author(s) -
MasueSlowey Yoko,
Slowey Aaron J.,
Michel F. Marc,
Webb Samuel M.,
Fendorf Scott
Publication year - 2014
Publication title -
journal of environmental quality
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.888
H-Index - 171
eISSN - 1537-2537
pISSN - 0047-2425
DOI - 10.2134/jeq2013.08.0340
Subject(s) - dissolution , chemistry , precipitation , solubility , arsenite , inorganic chemistry , lepidocrocite , hydrous ferric oxides , ferrous , coprecipitation , solid solution , goethite , arsenic , adsorption , physics , sorption , organic chemistry , meteorology
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)O 4 ) 2 ]·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 H 7 Fe 4 (AsO 3 ) 5 and a log K so of 34 for the dissolution reaction defined as: H 7 Fe 4 (AsO 3 ) 5 + 8H + = 4Fe 2+ + 5H 3 AsO 3 . Extended X‐ray absorption fine structure spectroscopic analysis of H 7 Fe 4 (AsO 3 ) 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 As III O 3 pyramids, which are consistent with previously published structures of As(III)‐rich Fe(II) solids. The H 7 Fe 4 (AsO 3 ) 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 H 7 Fe 4 (AsO 3 ) 5 ; however, a high concentration of Fe(II) is required, which is unusual under alkaline conditions.