Carbon and Mercury Stable Isotope Fractionation during Aqueous Photodemethylation of CH3Hg

The photodemethylation of monomethylmercury (CH₃Hg) is one of the most important natural degradation processes of this toxic compound and is therefore key to understanding Hg exposure. The isotopic composition of CH₃Hg contains information about its sources and transformation pathways. The stable isotopes of Hg during photodemethylation of CH₃Hg bound to dissolved organic matter (DOM) have been shown to undergo unique mass-independent fractionation (MIF), as well as mass-dependent fractionation (MDF). Here we present different photodegradation experiments (DOM, Cl^– ligands) where, in addition to Hg isotopes and degradation kinetics, the δ¹³C of CH₃Hg was analyzed by compound-specific isotope analysis (CSIA): purge and trap–gas chromatography–combustion–isotope ratio mass spectrometry (PT-GC-C-IRMS). Our results show covariation of odd Hg MIF and light C isotope enrichment in CH₃Hg photodemethylation products, with δ¹³C fractionation factors from −5 to −16‰, depending on the presence of Cl^– and DOM. We also find a linear relationship between C MDF and Hg MIF, indicating that both isotope effects occur during C–Hg bond breaking. We suggest that biota Hg MIF can potentially be used to correct for photochemical C MDF, bringing us one step closer to exploring the origin of the methyl group contributing to CH₃Hg formation.