Isotope exchange between mercuric [Hg(II)] chloride and Hg(II) bound to minerals and thiolate ligands: Implications for enriched isotope tracer studies

Lijie Zhang, Xujun Liang, Quanying Wang, Yaoling Zhang, Xiangping Yin, Xia Lu, Eric M. Pierce, Baohua Gu

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19 Scopus citations


Enriched mercury (Hg) stable isotopes have been widely used as tracers in field and laboratory investigations of Hg(II) biogeochemical transformations such as methylation and demethylation. Few studies, however, have considered concurrent isotope exchange reactions between newly spiked and pre-existing Hg(II) in environmental matrices, which may alter redistribution and thus transformation of the spiked and pre-existing Hg(II). Using enriched 198Hg [as mercuric Hg(II) or HgCln species], this study investigated isotope exchange between 198Hg and pre-existing Hg(II) bound to metacinnabar (β-HgS), sediments, low-molecular-weight (LMW) thiols, and dissolved organic matter (DOM). The impact of isotope exchange on methylmercury production in the presence of organic ligands was also evaluated with an iron-reducing bacterium Geobacter sulfurreducens PCA in a phosphate buffered solution (pH 7.4). We found that spiked 198Hg readily exchanged with mineral-bound ambient Hg(II) despite concurrent Hg(II) adsorption and immobilization on the solids. Rapid exchange (<3 min) was also observed between spiked 198Hg and 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 and thus 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 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 and thus risk assessments of new Hg(II) input to the natural ecosystems.

Original languageEnglish (US)
Pages (from-to)468-481
Number of pages14
JournalGeochimica et Cosmochimica Acta
StatePublished - Jan 1 2021
Externally publishedYes

All Science Journal Classification (ASJC) codes

  • Geochemistry and Petrology


  • Isotope exchange
  • Mercury methylation
  • Natural organic matter
  • Thiol ligands
  • Tracer studies


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