Antimony(V) Retention by Lepidocrocite: Sorption, Coprecipitation, and Extractability

We examined the mechanisms controlling sorption and coprecipitation of Sb(V) with lepidocrocite and assessed the efficacy of commonly used extraction schemes to recover sorbed and coprecipitated Sb(V) from lepidocrocite. Antimony K-edge EXAFS spectroscopy reveals coprecipitation involved Sb(V) incorporation into the lepidocrocite structure via Sb(V)-for-Fe(III) substitution, whereas sorption involved edge and double-corner sharing between SbO₆ and FeO₆ octahedra at the lepidocrocite surface (with the proportional abundance of these linkages decreasing as Sb(V) sorption increased). Sb(V) sorption to lepidocrocite enabled faster Sb dissolution relative to Fe in 1 M HCl, whereas coprecipitation facilitated congruent dissolution of Sb and Fe. The Wenzel sequential extraction scheme substantially underestimated Sb(V) adsorption to the lepidocrocite surface, yet recovered remaining sorbed Sb(V) and all coprecipitated Sb(V) in extraction steps that target hydrous oxide-bound species. In contrast, very little sorbed or coprecipitated Sb(V) was recovered by the reducible step of the BCR sequential extraction, despite lepidocrocite being a readily reducible Fe(III) oxide. Instead, the BCR scheme incorrectly implied that lepidocrocite-bound Sb(V) was mostly oxidizable or nonreactive (i.e., bound in sulfides, organic matter, or highly stable phases such as tripuhyite). This study provides new insights into Sb(V) retention by lepidocrocite while providing useful guidance on the utility of common Sb extraction schemes.