Mercury isotope fractionation during photoreduction in natural water is controlled by its Hg/DOC ratio

Photoreduction of Hg in natural water plays a crucial role in the production of elemental Hg and its biogeochemical cycle. Solar irradiation and dissolved organic carbon (DOC) in water are considered to be the major factors inducing Hg photoreduction. We investigated Hg isotope fractionation during photoreduction and its relationship with Hg/DOC ratios. Both mass dependent (MDF) and mass independent fractionation (MIF) was observed. MIF enriched ¹⁹⁹Hg and ²⁰¹Hg in the reactant Hg(II) and thus, significantly enhanced the fractionation between odd and even isotopes. This direction of MIF is consistent with magnetic isotope effect as the underlying cause for the odd isotope enrichment in reactants. MIF also occurred in dark controls. But in the absence of light, ¹⁹⁹Hg and ²⁰¹Hg were enriched in the product Hg(0), which is not explained by magnetic isotope effects. We propose that nuclear volume effect dominated Hg isotope fractionation under these conditions. The reduction kinetics and isotope fractionation during photoreduction strongly correlated to Hg/DOC concentration ratios. Although different reduction kinetics and fractionation factors were measured at different Hg/DOC ratios, the same Hg/DOC ratios led to almost identical results. The degree of MIF for the two odd isotopes was also affected by Hg/DOC ratios. For this reason, it is critical to study Hg photoreduction at a near-natural Hg/DOC ratio in order to better simulate natural conditions. We suggest that differences in Hg-DOC binding, which varies with Hg/DOC ratios, may be responsible for the relationship between Hg/DOC ratios and Hg photoreduction.