ANTIMONY ISOTOPE SYSTEMATICS FOR USE IN NATURAL SYSTEMS

Antimony (Sb) has a history of being ignored as an aqueous contaminant, but in the last decade, awareness has grown and more research has been dedicated to Sb geochemistry. For other contaminant elements, stable isotopic ratio measurements have been beneficial for source tracing and detection of environmentally critical chemical reactions (Wiederhold 2015). We seek to develop and apply isotopic methods for Sb as a potential contaminant of concern, using measurements of the 123Sb/121Sb ratio. Currently, processes that generate shifts in 123Sb/121Sb are not well studied, though they can be inferred from a few previous studies, 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.

First, we have determined that isotopic exchange between Sb(V) and Sb(III) is negligible on a timescale of weeks, at high Sb concentration. At the lower concentrations of natural systems, exchange would be much slower. Accordingly, we conclude that kinetic isotope effects are important for redox reactions. Second, we quantified isotopic fractionation during transformation of aqueous Sb(V) to amorphous Sb2S3. Varying amounts of sulfide was added to identical bottles in an acidic, anoxic, and chloride rich sequential batch experiment, initially containing 8.2 μM dissolved Sb(V). The Sb2S3 product was lower in 123Sb/121Sb, relative to the dissolved Sb(V), by 1.42‰ ± 0.09‰ (2σ). This is a surprisingly large magnitude of 123Sb/121Sb shift during reduction, a critically important reaction in geochemical settings.

References

Wiederhold, JG. Metal stable isotope signatures as tracers in environmental geochemistry. Environmental science & technology. 2015, 49(5), 2606-2624.