Structure and reactivity of As(III)- and As(V)-rich schwertmannites and amorphous ferric arsenate sulfate from the Carnoulès acid mine drainage, France: Comparison with biotic and abiotic model compounds and implications for As remediation

Poorly ordered nanocrystalline hydroxysulfate minerals of microbial origin, such as schwertmannite, Fe₈O₈(OH)₆SO₄, are important arsenic scavengers in sulfate-rich acid mine drainage (AMD) environments. However, despite the fact that As(III) and As(V) have been shown to sorb on schwertmannite, little is known about the actual mechanism of arsenic scavenging processes after microbial Fe(II) oxidation in AMD environments. The major focus of the present study is to determine the molecular-level structure of poorly ordered As(III) and As(V) bearing Fe oxyhydroxysulfate minerals from the Carnoulès AMD, France, which exhibits exceptional As(III) concentrations. Powder X-ray diffraction (XRD) and extended X-ray absorption fine structure (EXAFS) spectroscopy were used to compare field samples with a large set of synthetic analogs prepared via biotic or abiotic pathways, with As/Fe ratios typical of minerals and mineraloids ranging from nanocrystalline schwertmannite to amorphous hydroxysulfate compounds. Our results yield further evidence for the poisoning effect of As(V) in limiting the nucleation of schwertmannite. For initial dissolved As(V)/Fe(III) molar ratios ≥0.2, amorphous Fe(III)-As(V) hydroxysulfate forms, with a local structure consistent with that of amorphous ferric arsenate. EXAFS data for this amorphous material are consistent with corner-sharing FeO₆ octahedra to which AsO₄ tetrahedra attach via double-corner ²C linkages. For As(V)/Fe(III) molar ratios lower than 0.2, As(V) binds to schwertmannite via ²C surface complexes. In contrast with the As(V)-containing samples, As(III) has a lower affinity for schwertmannite following its nucleation, as this mineral phase persists up to an initial As(III)/Fe(III) molar ratio of 0.6. EXAFS data indicate that during the precipitation process, As(III) forms dominantly ²C surface complexes on schwertmannite surfaces, likely on the sides of double-chains of Fe(III)(O,OH)₆ octahedra, with a smaller proportion of edge-sharing ²E surface complexes at the apexes of these chains. Importantly, dissolved As(V) concentrations in contact with As(V)-schwertmannite or ferric arsenate were found to be ∼10 times lower than dissolved As(III) concentrations in contact with As(III)-schwertmannite for similar As/Fe ratios in the solid phase. Consequently, remediation of As-rich AMD environments is greatly improved by oxidation of As(III) to As(V).