Adsorption-oxidation controls on Sb(III) isotope fractionation at Mn oxide-water interfaces.

Manganese (Mn) oxide-mediated adsorption-oxidation critically controls antimony (Sb) mobility and transformation in contaminated aquatic and terrestrial environments, yet its Sb(Ⅲ) isotope fractionation mechanism remains unclear. Here, α-MnO2 exposing the (100) and (310) facets was used as a model system to resolve Sb isotope fractionation during coupled Sb(III) transformation at Mn oxide-water interfaces. For the (100) facet, surface-bound Sb(III) was initially oxidized to Sb(V) and partly released into solution, whereas rapid transformation on the (310) facet obscured this early signal. The similar corner-sharing Sb-Mn coordination environments indicate that the contrasting isotope responses were not mainly controlled by Sb complexation. Light Sb isotopes were preferentially enriched in the solid phase during initial adsorption, followed by lighter Sb(V) release that modified the aqueous isotope composition. Facets primarily control the extent of Sb isotope fractionation by regulating the oxidation rate and Sb(V) release, rather than altering the Sb-Mn structure. Different oxidation and Sb release kinetics led to contrasting isotope responses, with slower transformation preserving larger apparent fractionation (Δ¹²³Sbaq-solid ≈ -3.8 ε) and faster transformation approaching negligible fractionation. These findings show that Mn oxide-mediated transformation can rework aqueous Sb isotope signatures beyond adsorption-driven fractionation, providing a basis for interpreting Sb isotopes in Mn oxide-bearing environments.