Antimony(V) uptake by feroxyhyte (δ′-FeOOH): Insights from X-ray absorption spectroscopy and selective extractions

Feroxyhyte is an Fe(III) oxyhydroxide mineral capable of immobilizing large amounts of Sb(V). However, the mechanisms governing the uptake of Sb(V) by feroxyhyte have not been systematically examined and are poorly understood. This study presents the first investigation of Sb(V) uptake by feroxyhyte through both sorption and coprecipitation processes across an environmentally-relevant range of Sb(V) loadings. Antimony K-edge extended X-ray absorption fine structure (EXAFS) spectroscopy revealed that Sb(V) sorption and coprecipitation (at all loadings) involved the development of edge and corner sharing linkages between Sb^V(O,OH)₆ and multiple Fe^III(O,OH)₆ octahedra. The Sb K-edge EXAFS results indicate that Sb(V) coprecipitation involved incorporation into feroxyhyte's structure via heterovalent Sb(V)-for-Fe(III) substitution, while Sb(V) sorption likely involved occupancy of vacant octahedral sites in feroxyhyte's near-surface structure. As a result of these uptake mechanisms, both sorbed and coprecipitated Sb(V) displayed very strong resistance to desorption via ligand exchange when exposed to SO₄^2-- or PO₄^3--rich solutions (during a commonly-used sequential extraction scheme). Overall, these findings provide new insights into Sb(V) uptake by feroxyhyte and highlight the role that feroxyhyte can potentially play in treating Sb(V)-contaminated water or stabilizing Sb(V) in contaminated soil, sediment and geogenic waste.