title: Equilibrium mercury isotope fractionation between dissolved Hg(II) species and thiol-bound Hg.
abstract: Stable Hg isotope ratios provide a new tool to trace environmental Hg cycling. Thiols (-SH) are the dominant Hg-binding groups in natural organic matter. Here, we report experimental and computational results on equilibrium Hg isotope fractionation between dissolved Hg(II) species and thiol-bound Hg. Hg(II) chloride and nitrate solutions were equilibrated in parallel batches with varying amounts of thiol resin resulting in different fractions of thiol-bound and free Hg. Mercury isotope ratios in both fractions were analyzed by multiple collector inductively coupled plasma mass spectrometry (MC-ICPMS). Theoretical equilibrium Hg isotope effects by mass-dependent fractionation (MDF) and nuclear volume fractionation (NVF) were calculated for 14 relevant Hg(II) species. The experimental data revealed that thiol-bound Hg was enriched in light Hg isotopes by 0.53 per thousand and 0.62 per thousand (delta(202)Hg) relative to HgCl(2) and Hg(OH)(2), respectively. The computational results were in excellent agreement with the experimental data indicating that a combination of MDF and NVF was responsible for the observed Hg isotope fractionation. Small mass-independent fractionation (MIF) effects (<0.1 per thousand) were observed representing one of the first experimental evidences for MIF of Hg isotopes by NVF. Our results indicate that significant equilibrium Hg isotope fractionation can occur without redox transition, and that NVF must be considered in addition to MDF to explain Hg isotope variations.
authors.affiliation: Institute of Biogeochemistry and Pollutant Dynamics, Soil Chemistry, and Institute of Geochemistry and Petrology, Isotope Geochemistry, ETH Zurich, Switzerland. wiederhold@env.ethz.ch
authors.forename: Jan G
authors.initials: JG
authors.lastname: Wiederhold
authors.forename: Christopher J
authors.initials: CJ
authors.lastname: Cramer
authors.forename: Kelly
authors.initials: K
authors.lastname: Daniel
authors.forename: Ivan
authors.initials: I
authors.lastname: Infante
authors.forename: Bernard
authors.initials: B
authors.lastname: Bourdon
authors.forename: Ruben
authors.initials: R
authors.lastname: Kretzschmar
mesh_terms: D007554:Isotopes / Q000737:chemistry*; D008628:Mercury / Q000737:chemistry*; D012995:Solubility; D013438:Sulfhydryl Compounds / Q000737:chemistry*
chemical_list: D007554:Isotopes; D013438:Sulfhydryl Compounds; D008628:Mercury