321 - Overlooked Isotope Exchange Reactions between Mercuric [Hg(II)] Species in Environmental Systems

Enriched mercury (Hg) stable isotopes have been widely used in both laboratory and field investigations to assess biogeochemical transformations of Hg. However, previous studies rarely considered isotope exchange reactions between the spiked and pre-existing Hg(II) in the environment, which can result in redistributions of the Hg in environmental matrices. This study examined the isotope exchange kinetics of divalent Hg(II) species in various soil and groundwater matrices, including metacinnabar (β-HgS), Hg-contaminated sediments, low-molecular-weight (LMW) thiols (e.g., cysteine and glutathione), and dissolved organic matter (DOM). Using enriched 198Hg, we found rapid exchange of the spiked 198Hg and mineral-bound ambient Hg(II), despite concurrent Hg(II) adsorption and immobilization on the solids. The spiked 198Hg also rapidly exchanged with 200Hg pre-equilibrated with LMW thiols and DOM in solution. While the exchange did not cause net changes in Hg(II) chemical speciation, it resulted in redistribution of Hg(II) isotopes bound to the ligands. Moreover, the impact of isotope exchange was demonstrated by assessing the methylmercury production in the presence of organic ligands with an iron-reducing bacterium Geobacter sulfurreducens PCA in a phosphate buffered solution (pH 7.4). The results show an apparently similar methylation rate and magnitude of the spiked 198Hg and pre-existing 200Hg by PCA cells when 198Hg and 200Hg were added at 1:1 ratio. These observations underscore the importance of considering isotope exchange when an enriched Hg isotope is applied in environmental matrices, as the exchange could potentially lead to biased rate calculations of Hg(II) transformation and bioaccumulation.