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Contribution of dissolved organic matter to the photolysis of methylmercury in estuarine water

机译：溶解有机物对河口水中甲基汞光解的贡献

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The photodemethylation (PD) is a dominant degradation process of methylmercury (MeHg) in sunlit surface seawater, leading to the decreased bioaccumulation of MeHg in pelagic organisms. This study investigated the PD rate constant of MeHg in artificial estuarine water under UV-A irradiation, with the addition of various types of dissolved organic matter (DOM) to understand the PD pathway. In the PD simulation with -180 mu mol C L-1 of DOM, microbial fulvic acid showed a higher rate constant (k = 0.16 m(2) E-(1)) than that of terrestrial humic acid (k = 0.11 m(2) E-(1)) and fulvic acid (k = 0.12 m(2) E-1). Adding sodium azide and isopropyl alcohol decreased the PD rate constant by 44-74% in the presence of humic and fulvic acids, implying the considerable involvement of O-1(2) and center dot OH. On the contrary, the direct PD of MeHg-C-1 was a major process in the presence of non-thiol aromatic ligands with the rate constant ranging from 0.033 to 0.036 m(2) E-1. When glutathione was added to the artificial estuarine water, the rate constant increased to 0.15 m(2) E-1 with MeHg-glutathione as a major MeHg species, implying that as the Hg-C bond is weakened by the formation of MeHg-thiol complex, the PD rate constant is increased. It is noteworthy that the addition of thiosalicylic acid to simulated estuarine water produced an exceptionally high rate constant (k = 4.1 m(2) E-1); however, such a high value was not obtained from natural coastal water (k = 0.15-0.18 m(2) E-1). The results suggest that the ordinary contents of DOM in estuarine water are likely to increase the PD rate of MeHg via the production of O-1(2) and center dot OH, and the formation of MeHg-aliphatic thiol complexes facilitating intramolecular charge transfer.
机译：光脱甲基化（PD）是阳光照射的海水中甲基汞（MeHg）的主要降解过程，导致中上层生物体中MeHg的生物累积减少。这项研究调查了UV-A辐射下人工河口水中MeHg的PD速率常数，并添加了各种类型的溶解有机物（DOM）来了解PD途径。在使用-180μmol C L-1的DOM进行的PD模拟中，微生物富富酸比陆基腐殖酸（k = 0.11 m（k = 0.16 m（2）E-（1））显示出更高的速率常数。 2）E-（1））和富里酸（k = 0.12 m（2）E-1）。在腐殖酸和富里酸存在的情况下，添加叠氮化钠和异丙醇会使PD速率常数降低44-74％，这意味着O-1（2）和中心点OH的大量参与。相反，MeHg-C-1的直接PD是存在非硫醇芳香族配体且速率常数介于0.033至0.036 m（2）E-1范围内的主要过程。当将谷胱甘肽添加到人工河口水中时，速率常数增加到0.15 m（2）E-1，其中MeHg-谷胱甘肽是主要的MeHg种类，这意味着随着Hg-C键被MeHg-硫醇的形成削弱复杂时，PD速率常数增加。值得注意的是，向模拟河口水中添加硫代水杨酸会产生极高的速率常数（k = 4.1 m（2）E-1）；但是，不能从天然沿海水获得如此高的价值（k = 0.15-0.18 m（2）E-1）。结果表明，河口水中DOM的普通含量可能通过O-1（2）和中心点OH的产生而增加MeHg的PD率，并形成促进分子内电荷转移的MeHg-脂族硫醇复合物。

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