Light parameters and iron ions regulate mass-independent fractionation via modulating degradation pathways of methylmercury.

Methylmercury (MeHg) is a neurotoxic pollutant threatening aquatic ecosystems and human health. Photodegradation is a key abiotic pathway for MeHg elimination in aquatic systems, yet its underlying mechanisms and controlling factors remain poorly understood. We investigated MeHg photodegradation in the presence of dissolved organic matter (DOM) under varying light conditions and iron ion concentrations. Our results demonstrate that the indirect pathway, mediated by reactive oxygen species (ROS), significantly contributes to MeHg degradation but yields negligible mass-independent fractionation (MIF). In contrast, MIF signatures originate primarily from the direct photodegradation pathway involving MeHg-DOM complexes. Short-wavelength and intense light irradiation enhance the direct pathway, driving the overall MIF enrichment factor more negative (ε199Hg up to -2.51‰). Iron addition promotes the indirect pathway of MeHg and alters MIF characteristics (ε199Hg from -1.40‰ to -0.12‰). Using an isotopic binary mixing model, we provided the first quantitative constraints on the relative contributions of these two pathways. These findings offer new insights into interpreting mercury isotope signatures in natural systems and emphasize the need to consider pathway-specific responses to environmental drivers when using MIF to quantify MeHg photodegradation, which is crucial for accurately assessing its MeHg environmental fate.