Chemical structures of dissolved organic matter control mercury isotopic fractionation during methylmercury photodegradation.

Methylmercury (MeHg) photodegradation can induce mass-dependent (MDF) and mass-independent fractionation (MIF) of mercury (Hg) isotopes, providing useful information for fingerprinting the transformation and transport of Hg species. However, Hg isotopic fractionation during dissolved organic matter (DOM)-mediated MeHg photodegradation remains poorly understood, constraining environmental application of this isotopic tracer. In this study, several low-molecular-weight aromatic thiols were selected to investigate the influence of DOM chemical structures on Hg isotope fractionation during MeHg photodegradation. Findings reveal that the type and position of functional groups are responsible for the MDF and MIF processes, exhibiting distinct fractionation magnitude: carboxyl>hydroxyl>amino and carboxyl para>meta>ortho for MIF while the opposite for MDF. This role is primarily attributed to the variation in electron distribution of Hg influenced by DOM based on density functional theory calculations. Such electron redistribution affects the recombination probability of MeHg-DOM complex triplet-singlet radical pair by modulating hyperfine coupling and spin-orbit coupling, and the subsequent fractionation. Additionally, environmental factors (pH, DOM/MeHg ratio, and co-existing Cl) have a distinct influence on the fractionation process, likely involving a combination of various factors. This study provides novel insights into Hg isotope fractionation during DOM-mediated photodegradation of MeHg.