Antimony isotope fractionation during adsorption on aluminum oxides

The environmental fate of antimony (Sb) is often strongly affected by adsorption, and the Sb isotope fractionation mechanism during adsorption has not been reported. Four batch experiments (kinetic, isothermal, effect of pH, and effect of coexisting anions) were conducted to evaluate the mechanism of Sb(V) adsorption to γ-Al₂O₃ and the fractionation of Sb isotopes. Extended X-ray absorption fine structure (EXAFS) analyses show Sb(V) adsorption on γ-Al₂O₃ occurs via outer-sphere surface complexation. The triple-layer model (TLM) effectively predicted the theoretical Sb adsorption amount under different pH conditions. The Sb isotope fractionation in the adsorption process can be divided into an initial kinetic stage (Rayleigh model, α_{adsorbed-aqueous} = 0.99975 ± 0.00003) and subsequent isotopic equilibrium stage due to isotope exchange; however, no significant equilibrium isotope fractionation (Δ¹²³Sb_{aqueous-adsorbed} = ~0 ± 0.08‰) was evident by the end of the experiments. We propose the lack of significant equilibrium isotope fractionation in the effect of pH and isothermal experiments is due to Sb forming an outer-sphere complex on γ-Al₂O_3. This study reveals Sb equilibrium isotope fractionation does not occur during Sb(V) adsorption onto γ-Al₂O₃, providing a reference for the future study of Sb isotopes and furthering understanding of the Sb isotope fractionation mechanism.