Shandong Key Laboratory of Environmental Processes and Health, School of Environmental Science and Engineering, Shandong University, Qingdao 266237, China.
Shandong Key Laboratory of Environmental Processes and Health, School of Environmental Science and Engineering, Shandong University, Qingdao 266237, China.
Sci Total Environ. 2022 Nov 20;848:157550. doi: 10.1016/j.scitotenv.2022.157550. Epub 2022 Jul 27.
Photochemical degradation acts as the principal sink for methylmercury (MeHg) in surface water, which is regulated by light and solution matrix (especially the presence of dissolved organic matter, DOM). The spectral composition of light irradiation and the light absorption properties of reaction media (often exerted by DOM) are important in MeHg photodegradation, which has not yet been clearly resolved. Aiming to understand the role of light in MeHg photodegradation from the perspectives of both light irradiation and absorption, we investigated the photodegradation of MeHg under different simulated sunlight sources, with and without DOM model compounds of different molecular structures. The results show that the photodegradation of MeHg under different sunlight irradiation yields distinct first-order date constant, mainly due to the slight difference in UVB composition. The degradation of MeHg without DOM under a light source with high intensity in the UV region and no MeHg degradation under the UV-filtered light even in the presence of DOM showed the importance of UV lights in MeHg photodegradation. The use of ultrapure water as a reaction medium may be subject to MeHg loss through vessel adsorption, not just photolysis. Additionally, this work found that the type and position of coexisting substituents on aromatic thiols play a critical role in improving the photodegradation of MeHg, following amino > hydroxyl > carboxyl, para > meta > ortho. Based on the characterization of ultraviolet-visible absorption spectra and our previous work, it was concluded that the presence of DOM could induce red-shift in light absorption and reduce the electronic transition energy of the CHg bond, facilitating MeHg photodegradation. The structures of DOM affect the light absorption properties, which are related to MeHg photodegradation.
光化学降解是水体中甲基汞（MeHg）的主要消除途径，受光照和溶液基质（尤其是溶解态有机物，DOM）的调控。光照的光谱组成和反应介质的光吸收特性（通常由 DOM 决定）对 MeHg 的光降解很重要，但这一点尚未得到明确解决。为了从光照和吸收两方面来理解光在 MeHg 光降解中的作用，我们研究了不同模拟太阳光源、有无不同分子结构的 DOM 模型化合物存在时 MeHg 的光降解情况。结果表明，不同太阳光照射下 MeHg 的光降解生成明显的一级数据常数，主要是由于 UVB 组成略有不同。在 UV 区强度高的光源下，无 DOM 存在时 MeHg 降解，而在 UV 滤光光源下即使有 DOM 存在时 MeHg 也不降解，这表明 UV 光在 MeHg 光降解中很重要。使用超纯水作为反应介质时，可能会因容器吸附而导致 MeHg 损失，而不仅仅是光解。此外，这项工作发现，芳香硫醇中相邻取代基的类型和位置对提高 MeHg 的光降解起着关键作用，顺序为氨基＞羟基＞羧基，对位＞间位＞邻位。基于紫外可见吸收光谱的表征和我们之前的工作，我们得出结论，DOM 的存在会导致光吸收的红移，并降低 CHg 键的电子跃迁能，从而促进 MeHg 的光降解。DOM 的结构会影响光吸收特性，这与 MeHg 的光降解有关。
