Re-assessment of mercury isotope mass-independent fractionation during monomethylmercury photodegradation pathways in freshwater

Abstract

Photodegradation of monomethylmercury (MMHg) in surface waters induces mass-dependent and mass-independent fractionation (MIF) of mercury (Hg) isotopes, providing information on the fate of Hg. We aim to understand the mechanisms of MMHg photodegradation and Hg isotopic fractionation affected by physico-chemical factors in freshwater. We investigated the MMHg photodegradation under anoxic and oxic conditions. In addition, MMHg solution was exposed to simulated solar radiation wavelength including or not short UVB (280-305nm). Results for Hg species remaining in the solution showed lower ε_Δ¹⁹⁹_Hg in anoxic conditions within the range from -8.5±0.6‰ to -16.9±1.9‰ and -10.8±0.7‰ to -29.3±6.0‰ with and without short UVB, respectively, than oxic conditions from -16.8±4.0‰ to -19.6±4.1‰ and -29.1±4.5‰ to -33.0±7.7‰ regardless of purging mode. This is probably caused by slower spin interconversion in oxic conditions. In anoxic conditions, the average Δ¹⁹⁹Hg/Δ²⁰¹Hg slope increased from 1.42±0.04 to 1.67±0.08 with short UVB. Our study supports the finding that Δ¹⁹⁹Hg/Δ²⁰¹Hg generated by the photochemical magnetic isotope effect (MIE) varies significantly according to the UV spectral ranges. This highlights the importance of UV light interactions for Hg and other elements featuring photochemical MIE, providing a new basis for interpreting Hg odd-MIF signature.