Kinetic Fractionation of Antimony Isotopes during Reduction by Sulfide

Stable isotope ratios of antimony (Sb) in the environment can provide valuable information about sources and processes such as redox transformations. To investigate the fractionation when Sb(V) is chemically reduced by sulfide to Sb(III), experiments with 0.008 to 0.01 mM Sb(V) and 0.009 to 6 mM sulfide at a pH of 1 to 8 were performed. Experiments at pH 1 to 6 precipitated Sb-sulfide, whereas at pH 7 to 8, Sb(III) remained in the solution. The Sb(III) product was enriched with the lighter isotope. The isotopic fractionation (ε ≈ δ_{instantaneous product} – δ_reactant) for the pH 1 experiment was −1.42 ± 0.04‰, whereas those at the pH 5 to 8 experiments ranged from −0.46 ± 0.04 to −0.62 ± 0.04‰. The small magnitude of fractionation observed in experiments at circumneutral pH may decrease the utility of Sb isotope measurement as reduction indicators in natural systems, as adsorption of Sb has been shown to fractionate isotopes in the same direction and similar magnitude (up to 1.14‰) (Wasserman, 2020; Zhou et al., 2023).