Kinetic Fractionation of Antimony Isotopes during Reduction by Sulfide

Stable isotope ratios of antimony (Sb) in the environment can provide valuable information on 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 Sb2S3, while at pH 7 to 8 Sb(III) remained in solution. The Sb(III) product was enriched in the lighter isotope. The isotopic fractionation (ε ≈ δinstantaneous product – δreactant) for the pH 1 experiment was -1.42 ± 0.04‰ while 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).