Antimony Isotope Fractionation Revealed from EXAFS during Adsorption on Fe (Oxyhydr)oxides

A lack of knowledge about antimony (Sb) isotope fractionation mechanisms in key geochemical processes has limited its environmental applications as a tracer. Naturally widespread iron (Fe) (oxyhydr)oxides play a key role in Sb migration due to strong adsorption, but the behavior and mechanisms of Sb isotopic fractionation on Fe (oxyhydr)oxides are still unclear. Here, we investigate the adsorption mechanisms of Sb on ferrihydrite (Fh), goethite (Goe), and hematite (Hem) using extended X-ray absorption fine structure (EXAFS) and show that inner-sphere complexation of Sb species with Fe (oxyhydr)oxides occurs independent of pH and surface coverage. Lighter Sb isotopes are preferentially enriched on Fe (oxyhydr)oxides due to isotopic equilibrium fractionation, with neither surface coverage nor pH influencing the degree of fractionation (Δ123Sbaqueous-adsorbed). Limited Fe atoms are present in the second shell of Hem and Goe, resulting in weaker surface complexes and leading to greater Sb isotopic fractionation than with Fh (Δ123Sbaqueous-adsorbed of 0.49 ± 0.004, 1.12 ± 0.006, and 1.14 ± 0.05‰ for Fh, Hem, and Goe, respectively). These results improve the understanding of the mechanism of Sb adsorption by Fe (oxyhydr)oxides and further clarify the Sb isotope fractionation mechanism, providing an essential basis for future application of Sb isotopes in source and process tracing.