Goethite as an antimony host-phase: Atomic-scale retention mechanisms and the selectivity of commonly-applied extraction schemes

Goethite (α-FeOOH) is one of the most important host-phases for Sb(V) in soils, sediments and geogenic wastes. This study examines, for the first time, how variability in the atomic-scale mechanisms of Sb(V) retention by goethite impacts the selectivity of commonly-applied Sb extraction schemes. EXAFS spectroscopy shows that Sb(V) retention via coprecipitation involves Sb(V) incorporation into goethite's structure through Sb(V)-for-Fe(III) substitution. In contrast, Sb(V) retention via sorption involves edge and double-corner sharing between SbO₆ and FeO₆ octahedra at the goethite surface. Incorporation of Sb(V) into goethite's structure causes Sb(V) to be largely inaccessible to 1 M HCl, steps 1, 2 and 3 of Wenzel et al.'s sequential extraction scheme and all 3 steps of the BCR extraction scheme. In contrast, Sb(V) sorption to goethite's surface facilitates more substantial Sb(V) extractability, which varies with the relative abundance of SbO₆-FeO₆ linkages. Importantly, Sb(V) sorption to the goethite surface is underestimated by both the Wenzel et al. and BCR schemes. In addition, the BCR scheme misidentifies a significant portion of goethite-sorbed Sb(V) as oxidisable phases (e.g. sulfides or organic matter). Hence, in soils, sediments or geogenic wastes where Sb(V) is sorbed to goethite, the BCR scheme is not appropriate for quantifying Sb(V) fractionation. Overall, our results demonstrate that variability in the atomic-scale mechanisms by which goethite retains Sb(V) translate to substantial complexity in Sb(V) extractability.