Institute of Resources and Environment, Henan Polytechnic University , Jiaozuo, 454000, China.
Environ Sci Technol. 2014 Jul 1;48(13):7333-40. doi: 10.1021/es500316d. Epub 2014 Jun 18.
Photodegradation is the major pathway of methylmercury (MeHg) degradation in many surface waters. However, the mechanism of MeHg photodegradation is still not completely understood. Dissolved organic matter (DOM) is expected to play a critical role in MeHg photodegradation. By using several techniques, including N2/O2 purging and the addition of stable isotope (Me(201)Hg), scavengers, competing ligands, and a singlet oxygen ((1)O2) generator, the role played by MeHg-DOM complexation in MeHg photodegradation of Everglades surface water was investigated. DOM appeared to be involved in MeHg photodegradation via the formation MeHg-DOM complexes based on three findings: (1) MeHg was quickly photodegraded in solutions containing DOM extracts; (2) degradation of MeHg did not occur in deionized water; and (3) addition of competing complexation reagents (dithiothreitol-DTT) dramatically prohibited the photodegradation of MeHg in Everglades water. Further experiments indicated that free radicals/reactive oxygen species, including hydroxyl radical (·OH), (1)O2, triplet excited state of DOM ((3)DOM*), and hydrated electron (e(-)aq), played a minor role in MeHg photodegradation in Everglades water, based on the results of scavenger addition, (1)O2 generator addition and N2/O2 purging. A pathway, involving direct photodegradation of MeHg-DOM complexes via intramolecular electron transfer, is proposed as the dominant mechanism for MeHg photodegradation in Everglades water.
光降解是许多地表水体系中甲基汞（MeHg）降解的主要途径。然而，MeHg 光降解的机制仍不完全清楚。溶解有机质（DOM）预计将在 MeHg 光降解中发挥关键作用。通过使用几种技术，包括 N2/O2 吹扫和添加稳定同位素（Me(201)Hg）、猝灭剂、竞争配体和单线态氧（(1)O2）发生器，研究了 MeHg-DOM 络合物在大沼泽地地表水的 MeHg 光降解中的作用。基于以下三个发现，DOM 似乎通过形成 MeHg-DOM 络合物参与 MeHg 光降解：（1）DOM 提取物存在的溶液中 MeHg 迅速光降解；（2）去离子水中未发生 MeHg 降解；（3）添加竞争络合试剂（二硫苏糖醇-DTT）极大地阻止了大沼泽地水中 MeHg 的光降解。进一步的实验表明，基于猝灭剂添加、(1)O2 发生器添加和 N2/O2 吹扫的结果，包括羟基自由基 (·OH)、(1)O2、DOM 的三重激发态 ((3)DOM*) 和水合电子 (e(-)aq) 在内的自由基/活性氧物质在大沼泽地水中的 MeHg 光降解中只起次要作用。提出了一条途径，即 MeHg-DOM 络合物通过分子内电子转移的直接光降解，作为大沼泽地水中 MeHg 光降解的主要机制。
