Mechanisms of mercury oxidation in natural water: Dominant pathways, and role of sunlight spectrum and reactive oxygen species/free radicals

Mercury (Hg) is one of the globally concerned persistent toxic pollutants, posing a serious threat to ecosystems and human health due to its high toxicity and bioaccumulation in the food chain. In aquatic environments, Hg exists in a variety of forms, including elemental Hg (Hg(0)), divalent ionic Hg (Hg(Ⅱ)) and methylmercury. The toxicity of Hg depends on its species, and the transformations of Hg are the key processes affecting its risk. The oxidation of elemental Hg in natural water is one of the key processes controlling Hg levels in water bodies. However, the reaction mechanisms of this process remain unclear. In this study, we investigated the mechanisms of Hg oxidation in natural water (Shilaoren seawater, Jiaozhou Bay seawater, Ink River water, Dagu River water) by simulated incubation experiments. The dominant pathways of this process were identified and the relative contributions of different wavelengths of sunlight (ultraviolet A (UVA), ultraviolet B (UVB), visible light) and reactive oxygen species/free radicals to this process were evaluated. The results indicate that photochemical (22.08%—80.11%) and non-photochemical (12.90%—67.53%) reactions are the major pathways of Hg oxidation in natural water, whose contributions were much higher than that of microbial pathway (1.82%—10.39%). Ultraviolet (UV) radiation was identified to be the key sunlight spectrum for mercury oxidation reactions, contributing to 80.82%—91.30% of Hg oxidation, with UVB playing a more important role compared to UVA. Among the common reactive oxygen species/free radicals, hydroxyl radical (·OH) and triplet excited state of dissolved organic matters (3DOM*) were found to play a key role in the oxidation of Hg, while singlet oxygen (1O2) and superoxide radical (O2·−) contribute little to this process.