Key Laboratory of Marine Chemistry Theory and Technology, Ministry of Education and College of Chemistry and Chemical Engineering, Ocean University of China, Qingdao 266100, China.
Key Laboratory of Marine Chemistry Theory and Technology, Ministry of Education and College of Chemistry and Chemical Engineering, Ocean University of China, Qingdao 266100, China.
Water Res. 2018 Jul 1;138:264-271. doi: 10.1016/j.watres.2018.03.055. Epub 2018 Mar 26.
Photodegradation is the main depletion pathway for methylmercury (MeHg) in surface water. The formation of MeHg-dissolved organic matter (DOM) complexes has been found to be a key step in MeHg photodegradation. However, the major functional groups involved in the DOM-mediated process have yet to be clearly resolved. In this work, we systematically investigated the effects of DOM molecular structures on MeHg photodegradation by using a variety of organic ligands with different functional groups (e.g., thiosalicylate, thiophenol, and thioaniline). The results showed that thiol and phenyl groups may be the major functional groups governing DOM-mediated MeHg photodegradation, with photodegradation rates also dependent on the type (carboxyl, hydroxyl, and amino group) and position (ortho-, meta-, and para-) of other chemical substituents. The addition of "non-photochemically active" thiol ligands (e.g., mercaptoethanol and dithiothreitol) and high concentrations of Cl can significantly inhibit the o-thiosalicylate-induced MeHg photodegradation, indicating that complexation of MeHg with these ligands is necessary for MeHg photodegradation. Sparging with O had a negligible effect on MeHg photodegradation, while sparging with N significantly enhanced MeHg photodegradation. This finding suggests that MeHg photodegradation may be a reductive process, which was further supported by identification of the degradation products of MeHg. A possible protonolysis mechanism of MeHg photodegradation in the presence of o-thiosalicylate was then proposed based on the findings of this study.
光降解是地表水甲基汞（MeHg）的主要消耗途径。已发现 MeHg-溶解有机质（DOM）配合物的形成是 MeHg 光降解的关键步骤。然而，参与 DOM 介导过程的主要官能团尚未得到明确解决。在这项工作中，我们使用具有不同官能团的各种有机配体（例如，硫代水杨酸、巯基苯酚和硫代苯胺）系统地研究了 DOM 分子结构对 MeHg 光降解的影响。结果表明，巯基和苯基可能是控制 DOM 介导的 MeHg 光降解的主要官能团，光降解速率还取决于其他化学取代基的类型（羧基、羟基和氨基）和位置（邻位、间位和对位）。添加“非光化学活性”巯基配体（例如，巯基乙醇和二硫苏糖醇）和高浓度的 Cl 可以显著抑制邻位硫代水杨酸诱导的 MeHg 光降解，表明 MeHg 与这些配体的络合对于 MeHg 光降解是必要的。O 的鼓泡对 MeHg 光降解的影响可以忽略不计，而 N 的鼓泡则显著增强了 MeHg 光降解。这一发现表明 MeHg 光降解可能是一个还原过程，这进一步得到了鉴定 MeHg 降解产物的支持。根据本研究的结果，提出了邻位硫代水杨酸存在下 MeHg 光降解的可能质子分解机制。
