Facet-dependent Antimony Isotope Fractionation during Sb(V) Adsorption on Hematite

The widely distributed hematite in soil and water environments can affect the environmental fate of antimony (Sb) through adsorption, yet little is known about the role played by various hematite facets in Sb isotopic fractionation. This study investigated the Sb(V) adsorption mechanism on the (001) and (012) facets of hematite and the associated isotope fractionation mechanism, combining batch adsorption experiments, extended X-ray absorption fine structure (EXAFS), and density functional theory (DFT) calculations. The results indicate that the adsorption capacity of the (012) facet is stronger than that of the (001) facet, with Sb existing in an edge-sharing complex form on the (001) facet, while edge and corner-sharing complex forms coexist on the (012) facet. The lighter Sb isotope is preferentially enriched on both facets through equilibrium fractionation, with the fractionation extent independent of pH and initial Sb concentration. Since the (001) facet contains fewer Sb atoms in the Sb–Fe shell, the complexation is weaker, resulting in a Sb isotope fractionation value (Δ 123 Sb aqueous-adsorbed = 14.11 ± 0.03 ‰) that is higher than that of the (012) facet (Δ 123 Sb aqueous-adsorbed = 9.8 ± 0.02 ‰). These findings enhance the understanding of Sb isotope fractionation mechanisms at various mineral facets and provide new insights for predicting the adsorption and isotope fractionation of other metals.