Zinc Isotope Fractionation as an Indicator of Geochemical Attenuation Processes

Abstract

Isotope ratio measurements have been used to trace environmental processes, especially in subsurface environments. In this study, we evaluated the potential to use zinc (Zn) stable isotope ratios as indicators of attenuation processes, including sorption and precipitation. Zn isotope fractionation was observed during distinctly different precipitation processes. Isotope measurements confirmed an increasing trend in aqueous δ66Zn in solution during sphalerite (ZnS) formation, but a decreasing trend in δ66Zn during the precipitation of hydrozincite [Zn5(CO3)2(OH)6] and hopeite [Zn3(PO4)2·4H2O]. In contrast, time-dependent sorption of Zn onto ferrihydrite at a fixed pH did not cause isotopic fractionation in the solution over the duration of the experiments. These findings suggest potential applications of stable isotope measurements in aqueous environments for determining reaction pathways (e.g., precipitation with common groundwater constituents) leading to Zn attenuation.