On-site separation of Sb(iii) and Sb(v) and antimony stable isotope analysis by a sulfonated silane strong cation exchange resin

Understanding the stable isotope fractionation mechanism of antimony (Sb) under different oxidation states is crucial for tracking its migration and fate in the environment. Redox-specific Sb isotope characteristics and fractionation behavior in natural environments remain poorly known, largely because Sb species of different oxidation states are difficult to separate. To address this limitation, an on-site separation method for Sb(III) and Sb(V) was developed using a strong cation-exchange resin (SCX, sulfonated silane resin) that does not induce Sb isotope fractionation. Separation conditions were optimized, and method robustness was evaluated systematically as a function of pH, ionic strength, total Sb concentration, the Sb(III)/Sb(V) ratio, and resin reuse cycles. The results indicate that Sb concentration and the Sb(III)/Sb(V) ratio have minimal effects on separation performance. Although pH and ionic strength exert pronounced influences on Sb speciation, recoveries remain stable at 95.0–100% over a pH range of 2–9 and an ionic strength range of 0.001–0.05 mol L⁻¹. Application to natural samples yields an Sb recovery of 98.6 ± 3.0%, with isotopic differences before and after separation of less than 0.1‰. Density functional theory (DFT) calculations indicate that Sb(III) is retained via outer-sphere complexation with sulfonic groups without direct bonding, which accounts for the absence of isotope fractionation; in contrast, Sb(V) interacts less favorably on thermodynamic grounds and is therefore weakly retained. By enabling on-site separation of Sb(III) and Sb(V) without inducing Sb isotope fractionation, this method provides the prerequisite for investigating Sb isotope signatures and fractionation behavior in complex natural matrices.