Mercury transformations by reactive oxygen species: Occurrence, detection, evidence, and challenges

Mercury (Hg) is a global pollutant that has attracted widespread attention due to its potent toxicity to humans. The transformations of Hg are critical to its global cycling and human exposure risk, considering that different Hg forms possess various mobility and toxicity. It is, therefore, essential to explore the mechanisms of Hg transformations. Both biotic and abiotic factors mediate Hg transformations in the environments, while the latter has not been sufficiently recognized, among which reactive oxygen species (ROS) are ubiquitous yet potentially overlooked drivers. This insufficient recognition of ROS-mediated Hg transformations impeded our understanding of Hg biogeochemistry. Herein, we summarized the generation mechanisms of ROS in the atmosphere, natural water, and soil, and elucidated ROS involvement in Hg transformations, including MeHg degradation, Hg0 oxidation, and Hg(II) reduction. Then, approaches for exploring the role of ROS in Hg transformations were introduced, including ROS detection, quenching, generation, and DFT calculations. By summarizing the implications of ROS-mediated Hg transformations for Hg biogeochemical cycling and proposing potential challenges in further studies, we highlight the importance and necessity of studying this driving force in Hg biogeochemistry.