Antimony isotopes as indicators of redox reactions in aqueous systems: fractionation during Sb(V) reduction by sulfide and isotope exchange kinetics between dissolved Sb(III) and Sb(V)

Antimony (Sb) has a history of being overlooked as an aqueous contaminant. The widespread commercial use of Sb has caused scientists to realize the importance of studying the fate and transport of Sb. For other contaminant elements, stable isotopic ratio measurements have been useful for source tracing and detection of environmentally critical chemical reactions (e.g., Wiederhold 2015). We seek to develop Sb isotopic ratios as tools for detecting and quantifying Sb in the environment. Currently, processes that generate shifts in 123Sb/121Sb are not well studied, though they can be inferred from theory and the isotopic systematics of other elements. To provide a more precise understanding of the drivers of Sb isotope variation, the magnitude of isotopic fractionation for individual reactions must be determined. In this study, we determine the magnitude of isotopic fractionation in environmentally relevant reactions. We use anion exchange resin and hydride generation MC-ICP-MS methods to obtain precise measurements of 123Sb/121Sb. The anion exchange method is also effective for separating Sb(III) from Sb(V). First, we find that isotopic exchange between aqueous Sb(V) and Sb(III) at higher than natural concentrations (3 mM Sb) is negligible over a timescale of 6 weeks. At the lower concentrations of natural systems, exchange is likely to be much slower. Accordingly, we conclude that kinetic isotope effects that occur during Sb redox reactions are not overprinted by isotopic exchange, which would drive coexisting Sb(III) and Sb(V) toward isotopic equilibrium. Second, we quantified isotopic fractionation during reduction of 8.2 μM aqueous Sb(V) by sulfide to form amorphous Sb2S3. The data mostly conform to a Rayleigh distillation model, with 5 out of 11 data points deviating from the model slightly more than the measurement uncertainty. During experimentation it was found that the dissolved portion contained about 75% Sb(V) and 25% Sb(III). The best-fit model corresponds to an isotopic fractionation factor of 0.9986; Sb2S3 product was lower in 123Sb/121Sb, relative to the dissolved portion. This is a larger magnitude of 123Sb/121Sb shift during Sb(V) reduction than observed by Rouxel et. al. (2003)