Antimony isotopic fractionation during Sb(III) oxidation to Sb(V): Biotic and abiotic processes

Oxidation of antimonite (Sb(III)) to antimonate (Sb(V)) plays an important role in the control of Sb mobility in aquatic systems. Fractionation of Sb isotopes (123Sb/121Sb) during Sb(III) oxidation has been investigated in the present study at concentrations relevant to mining environments. The isotopic composition (δ123Sb) of dissolved Sb(III) and Sb(V) species was analysed during biotic oxidation of Sb(III) at pH 6 and ~ 0.1 mM Sb. Biotic experiments used a aioA gene carrier bacterial strain of the genus Ensifer isolated from Sb-rich river sediments. Chemical oxidation experiments with H2O2 were also conducted either in NaNO3 or HCl medium. During biotic oxidation, the Sb(V) produced was enriched in the light isotope compared to Sb(III), with an apparent fractionation factor Δ123SbSb(V)-Sb(III) of −0.20 ± 0.07 ‰. The Δ123Sb value was independent of the oxidation kinetics within the range 0.03 to 0.05 μmol.L−1.min−1; the fractionation observed was rather attributed to kinetic effect that weakens over time in the experiments, although this hypothesis would require further investigation. During abiotic experiments in NaNO3 medium, the Sb(V) produced was not isotopically fractionated relatively to Sb(III), while in HCl medium, the Sb(V) produced was enriched with the heavier isotope relatively to Sb(III), with a Rayleigh fractionation factor ε123Sb of +0.30 ± 0.05 ‰. These differences were attributed to different reaction pathways involving multi-step reactions and either hydroxy- or chloride-Sb species. Altogether, these results showed a low fractionation of Sb isotopes during Sb(III) oxidation, although significant variability occurred according to experimental conditions. Further experimental research and confrontation with natural systems is necessary before applying Sb isotopes as process tracer in water.