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, αadsorbed-aqueous = 0.99975 ± 0.00003) and subsequent isotopic equilibrium stage due to isotope exchange; however, no significant equilibrium isotope fractionation (Δ123Sbaqueous-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-Al2O3. 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. [Display omitted] •EXAFS results indicate Sb(V) adsorption type on γ-Al2O3 is outer-sphere complexes.•TLM effectively predicts the theoretical Sb adsorption amount under different pH.•Sb isotope fractionation is divided into kinetic and subsequent equilibrium stage.•No significant equilibrium isotope fractionation occurs at the end of experiments.