Probing methylmercury photodegradation by different fractions of natural organic matter in water: Degradation kinetics and mercury isotope fractionation characteristics.

Recent advancements in mercury (Hg) isotopic fractionation research have evolved from conceptual demonstrations to practical applications. However, few studies have focused on revealing fractionation fingerprinting for aqueous methylmercury (MeHg) photodegradation due to its sensitivity to natural organic matter (NOM). Here, the impact of NOM fractions with varying chemical properties on MeHg photodegradation kinetics and Hg isotope fractionation characteristics was investigated. Findings reveal that reduced NOM, containing alcohol/phenol groups, slows the degradation rate compared to the oxidized. Low-molecular-weight NOM, rich in thiol groups, enhances the degradation rate more effectively than high-molecular-weight counterparts. Hydrophilic/hydrophobic-acidic/basic NOM also significantly influence the rate constant, with the highest for hydrophilic-acidic NOM. Isotopic analysis showed that NOM's redox properties affect the extent and direction of Hg isotope fractionation. NOM with various molecular weights controls mass-dependent and mass-independent fractionation by regulating MeHg-NOM triplet radical pairs reactions, likely due to differences in functional groups. Similar effects were observed for different hydrophilic/hydrophobic-acidic/basic fractions. Further experiments with scavenger addition indicated that direct photodegradation of MeHg-NOM is a possible degradation mechanism, with free radicals/reactive oxygen species playing a minor role. These findings underscore the sensitivity of both the degradation rates and Hg isotope fingerprinting to different NOM fractions.