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.