Abstract
Mercury (Hg) isotope exchange is a common process in biogeochemical transformations of Hg in the environment, but it is unclear whether and at what rates dissolved elemental Hg(0)aq may exchange with divalent Hg(II) bound to various organic and inorganic ligands in water. Using enriched stable isotopes, we investigated the rates and dynamics of isotope exchange between 202Hg(0)aq and 201Hg(II) bound to organic and inorganic ligands with varying chemical structures and binding affinities. Time-dependent exchange reactions were followed by isotope compositional changes using both inductively coupled plasma mass spectrometry and Zeeman cold vapor atomic absorption spectrometry. Rapid, spontaneous isotope exchange (<1 h) was observed between 202Hg(0)aq and 201Hg(II) bound to chloride (Cl-), ethylenediaminetetraacetate (EDTA), and thiols, such as cysteine (CYS), glutathione (GSH), and 2,3-dimercaptopropanesulfonic acid (DMPS) at a thiol ligand-to-Hg(II) molar ratio of 1:1. Without external reductants or oxidants, the exchange resulted in transfer of two electrons and redistribution of Hg isotopes bound to the ligand but no net changes of chemical species in the system. However, an increase in the ligand-to-Hg(II) ratio decreased the exchange rates due to the formation of 2:1 or higher thiol:Hg(II) chelated complexes, but had no effects on exchange rates with 201Hg(II) bound to EDTA or Cl-. The exchange between 202Hg(0)aq and 201Hg(II) bound to dissolved organic matter (DOM) showed an initially rapid followed by a slower exchange rate, likely resulting from Hg(II) complexation with both low- and high-affinity binding functional groups on DOM (e.g., carboxylates vs bidentate thiolates). These results demonstrate that Hg(0)aq readily exchanges with Hg(II) bound to various ligands and highlight the importance of considering exchange reactions in experimental enriched Hg isotope tracer studies or in natural abundance Hg isotope studies in environmental matrices.
| Original language | English |
|---|---|
| Pages (from-to) | 15534-15545 |
| Number of pages | 12 |
| Journal | Environmental Science and Technology |
| Volume | 54 |
| Issue number | 23 |
| DOIs | |
| State | Published - Dec 1 2020 |
Funding
This research was sponsored in part by the Office of Biological and Environmental Research within the Office of Science of the U.S. Department of Energy (DOE), as part of the Mercury Science Focus Area project at the Oak Ridge National Laboratory (ORNL), which is managed by UT-Battelle, LLC under Contract No. DE-AC05-00OR22725 with DOE. DOE will provide public access to these results of federally sponsored research in accordance with the DOE Public Access Plan ( http://energy.gov/downloads/doe-public-access-plan ). The isotopes used in this research were supplied by DOE Office of Science, the Isotope Program in the Office of Nuclear Physics at ORNL. Q.W. and Y.Z. were supported by the Chinese Scholarship Council (CSC) of China. We also thank the four anonymous reviewers for constructive comments and suggestions to improve our manuscript.
| Funders | Funder number |
|---|---|
| Chinese Scholarship Council | |
| Office of Biological and Environmental Research | |
| U.S. Department of Energy | |
| Oak Ridge National Laboratory | DE-AC05-00OR22725 |