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

Abstract

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.