甲基汞的光降解是水环境中甲基汞最重要的一个"汇"。自然环境中甲基汞降解的可能途径主要有三个：活性氧/自由基主导的MeHg-DOM络合物的降解，活性氧/自由基主导的MeHg的降解和MeHg-DOM络合物的直接光解。DOM既参与活性氧/自由基的产生又可以与MeHg形成MeHg-DOM络合物，因而DOM在甲基汞光降解的三个途径中都起着至关重要的作用，但对于这一关键因素在甲基汞光降解中的作用目前认识尚少，亟需系统深入研究。本课题将综合运用同位素稀释、同位素示踪、同位素分馏、自由基捕获剂添加等技术，深入研究DOM在甲基汞光降解中的作用及机制，具体包括以下几个研究目标：明确DOM在甲基汞光降解中的作用及系统间的差异性；确定DOM中参与甲基汞光降解的主要组分和官能团；明确DOM参与甲基汞光降解的化学过程机理。本研究将为理解水环境甲基汞的生物地球化学循环、合理评价和有效控制甲基汞污染提供科学依据。

Photodegradation is a major sink for methylmercury (MeHg) in aquatic environments. Three potential pathways could be responsible for MeHg photodegradation: 1) photodegradation of MeHg-DOM complexes by free radicals/reactive oxygen species, 2) photodegradation of MeHg by free radicals/reactive oxygen species, 3) direct photodegradation of MeHg-DOM complexes. Dissolved organic matter (DOM) is expected to play a crucial role in all of these three pathways, as DOM is exclusively involved in the production of free radicals/reactive oxygen species and formation of DOM-MeHg complexes. However, the role of DOM in the photodegradation of MeHg remains unclear. Focused on this research gap, this project is aimed to provide an in-depth understanding towards the mechanisms of DOM-involved MeHg photodegradation. By using various techniques, including isotope dilution, isotope tracer, isotope fractionation, and scavenger addition, the following three specific goals will be accomplished: 1) Determine the importance of DOM in MeHg photodegradation in different aquatic environments; 2) Identify the fractions and functional groups of DOM that are involved in the photodegradation of MeHg; and 3) Elucidate the fundamental chemical processes of DOM-involved MeHg photodegradation. The accomplishment of this project is expected to provide a better understanding of the biogeochemical cycling of mercury in aquatic environments. Such studies are also necessary for assessing and making effective measures to minimize the risk of methylmercury contamination in aquatic environments.