Mercury Isotope Exchange Kinetics and Implications for Stable Isotope Tracer Studies

Enriched mercury (Hg) stable isotope tracers have been widely used in both laboratory and field experiments to assess the biogeochemical transformation processes of Hg. However, previous studies rarely considered isotope exchange reactions between the newly added Hg isotope with the previously deposited ambient Hg ("old"), as these reactions may affect Hg species distribution of the new and "old" Hg. In this study, we evaluated the exchange kinetics between newly added Hg(II) isotope and Hg(II) bound to various water and soil matrices, including low molecular weight (LMW) organic ligands (e.g., thiols or methyl), dissolved organic matters (DOM), mercuric sulfide (HgS), and a Hg-contaminated sediment. We found that Hg(II) bound to LMW thiol ligands (e.g., cysteine and glutathione) and naturally dissolved organic matter (DOM) can be rapidly exchanged by the addition of another Hg(II) isotope in a homogeneous solution. The exchange resulted in identical distributions of Hg(II) isotopes when added at equal molar concentrations. Likewise, we observed rapid isotope exchange between newly added Hg(II) and the mineral-associated "old" Hg(II), such as cinnabar, metacinnabar, and a Hg(II)-contaminated sediment, although the exchange was slower on cinnabar than on metacinnabar. However, isotope exchange did not occur between inorganic Hg(II) and methylmercury, likely due to the strong binding between Hg(II) and the methyl group. We therefore present evidence for potentially wide occurrence of isotope exchange reactions when an enriched Hg isotope is added to natural water and sediments. These reactions could influence species distributions of Hg(II) bound to organic ligands and minerals and thus have important implications for Hg stable isotope tracer studies in environmental systems.