Chemical kinetic isotope fractionation of mercury during abiotic methylation of Hg(II) by methylcobalamin in aqueous chloride media

Mercury (Hg) is assumed to be predominantly methylated by microorganisms in the environment. However, the mechanisms and extent of abiotic methylation are poorly appreciated. The understanding of the mechanisms leading to abiotic methylation and demethylation in the aquatic environment is of special concern since methylmercury (MeHg) biomagnifies in the food web. Bioaccumulating organisms have also been found to preserve specific Hg isotopic signatures that provide direct insight into aquatic Hg transformations. In this study we investigated the influence of chloride on the magnitude of Hg isotope fractionation during abiotic methylation of inorganic Hg (Hg(II)) using methylcobalamin as methyl donor compound. Coupling of gas chromatography with multi-collector inductively coupled plasma mass spectrometry has allowed to determine simultaneously isotopic ratios of inorganic and methyl-Hg species. Kinetic experiments demonstrated that the presence of chloride not only slowed the chemical alkylation of Hg(II) by methylcobalamin, but also decreased the extent of the methylation, which it is especially significant under visible light conditions due to the enhancement of MeHg photodecomposition. Abiotic methylation of Hg(II) by methylcobalamin in the presence of chloride caused significant Hg mass-dependent isotope fractionation (MDF) for both Hg(II) substrate (δ202Hg(II) from − 0.74‰ to 2.48‰) and produced MeHg (δ202MeHg from − 1.44‰ to 0.38‰) both under dark and visible light conditions. The value of this MDF under such saline conditions was higher than that previously reported (δ202MeHg from − 0.73‰ to 0.09‰) in the absence of chloride and appeared mainly related to inorganic Hg speciation in solution, which is predominantly mercuric chloro-complexes (i.e. HgCl42−). Different isotopic signatures were observed for the different Hg species at the same time of reaction for either dark or visible light (450–650 nm wavelengths) conditions. However, no significant mass-independent fractionation (MIF) was induced under any conditions within the analytical uncertainties (− 0.17 ± 0.31 < ∆201Hg < 0.17 ± 0.28‰), suggesting that photo-induced demethylation does not always involve MIF. These results also suggest that methylation by methylcobalamin can be an experimental model to study Hg isotope fractionation extent during elementary reaction of methyl transfer in biotic systems.