Effects of precipitation and UV photolysis on Ag isotope ratio: experimental studies

Abstract

First results on silver isotope fractionation observed in a number of experimental studies that mimic natural environment conditions and/or processes are reported. Precipitation of Ag as AgCl in the presence of a small amount of Cl⁻¹ induced significant isotopic fractionation. A δ^107/109Ag value of −0.17 ± 0.03‰ (1SD, n = 3) (relative to SRM 978a Ag isotopic standard) was obtained at a 96% loss of the original aqueous Ag. Thus, during the precipitation process, the supernatant is enriched in heavy ¹⁰⁹Ag whereas AgCl precipitate is enriched in ¹⁰⁷Ag. A slightly positive value of 0.052 ± 0.013 (1SD, n = 3) for δ^107/109Ag obtained in lake sediment samples collected from the St. George Lake (Ontario, Canada), wherein no industrial or domestic discharge is present, corroborates with results obtained during Cl⁻¹ precipitation laboratory experiments. This observation suggests that the natural precipitation process in the presence of Cl⁻¹ may be a major mechanism for the observed Ag isotope fractionation in uncontaminated lake sediments. The depletion of aqueous silver and its isotope fractionation during UV photolysis experiments were not significant. Only 12–25% of Ag was lost under test conditions, suggesting that the transport of Ag from the aquatic system to the atmosphere is rather limited in the natural environment. Physical mixing of a dietary Ag supplement, which has distinct isotopic composition, with sediment CRM PACS-2 was performed in order to investigate the possibility of tracking the origin of Ag through the use of its isotope ratio information. Isotope ratios of Ag measured in these mixtures gave a well-defined mixing pattern of these two end members. Results obtained in this study suggest that the Ag isotope ratio measured by MC-ICPMS may be a useful tracer for studying the natural processes and/or tracking the origin of the anthropogenic Ag in the environment.