title: Mercury isotope fractionation during precipitation of metacinnabar (β-HgS) and montroydite (HgO).
abstract: To utilize stable Hg isotopes as a tracer for Hg cycling and pollution sources in the environment, it is imperative that fractionation factors for important biogeochemical processes involving Hg are determined. Here, we report experimental results on Hg isotope fractionation during precipitation of metacinnabar (β-HgS) and montroydite (HgO). In both systems, we observed mass-dependent enrichments of light Hg isotopes in the precipitates relative to the dissolved Hg. Precipitation of β-HgS appeared to follow equilibrium isotope fractionation with an enrichment factor ε(202)Hg(precipitate-supernatant) of -0.63‰. Precipitation of HgO resulted in kinetic isotope fractionation, which was described by a Rayleigh model with an enrichment factor of -0.32‰. Small mass-independent fractionation was observed in the HgS system, presumably related to nuclear volume fractionation. We propose that Hg isotope fractionation in the HgS system occurred in solution during the transition of O- to S-coordination of Hg(II), consistent with theoretical predictions. In the HgO system, fractionation was presumably caused by the faster precipitation of light Hg isotopes, and no isotopic exchange between solid and solution was observed on the timescale investigated. The results of this work emphasize the importance of Hg solution speciation and suggest that bonding partners of Hg in solution complexes may control the overall isotope fractionation. The determined fractionation factor and mechanistic insights will have implications for the interpretation of Hg isotope signatures and their use as an environmental tracer.
authors.affiliation: †Soil Chemistry Group, Institute of Biogeochemistry and Pollutant Dynamics, ETH Zurich, CH-8092 Zurich, Switzerland.|‡Isotope Geochemistry Group, Institute of Geochemistry and Petrology, ETH Zurich, CH-8092 Zurich, Switzerland.
authors.forename: Robin S
authors.initials: RS
authors.lastname: Smith
authors.affiliation: †Soil Chemistry Group, Institute of Biogeochemistry and Pollutant Dynamics, ETH Zurich, CH-8092 Zurich, Switzerland.|‡Isotope Geochemistry Group, Institute of Geochemistry and Petrology, ETH Zurich, CH-8092 Zurich, Switzerland.
authors.forename: Jan G
authors.initials: JG
authors.lastname: Wiederhold
authors.affiliation: †Soil Chemistry Group, Institute of Biogeochemistry and Pollutant Dynamics, ETH Zurich, CH-8092 Zurich, Switzerland.
authors.forename: Ruben
authors.initials: R
authors.lastname: Kretzschmar
mesh_terms: D005591:Chemical Fractionation / Q000379:methods; D011232:Chemical Precipitation; D007700:Kinetics; D017669:Mercury Compounds / Q000737:chemistry*; D008629:Mercury Isotopes / Q000737:chemistry*; D008956:Models, Chemical; D010087:Oxides / Q000737:chemistry*; D012996:Solutions
chemical_list: D017669:Mercury Compounds; D008629:Mercury Isotopes; D010087:Oxides; D012996:Solutions; C019468:mercuric oxide; C034211:cinnabar