The production of dissolved gaseous mercury from methylmercury photodegradation at different salinity

Abstract

Photodegradation of methylmercury (MeHg) is an important process in mercury cycling that maintains low concentrations of MeHg in freshwater lakes; however, less is known about importance of this process in marine waters. The photo-induced formation of dissolved gaseous mercury (DGM, Hg-0) from MeHg removal was investigated. This study examined the effect of various environmental factors (i.e. light wavelength and intensity, and MeHg concentration), and primary water constituents on the abiotic photodegradation of MeHg, especially under different salinity. MeHg photodegradation rates were positively correlated with the UV light intensity, implying that the attenuation of UV radiation had a significant effect on MeHg photodegradation. However, a high dissolved organic matters concentration and salinity inhibited MeHg photodegradation. DGM was always produced during the photodegradation of MeHg. Photodegradation rates of MeHg and DGM production decreased with increasing salinity, suggesting that the presence of chloride ions inhibited MeHg photodegradation. Therefore, this study implies that MeHg in freshwater could be more rapidly demethylated than that in seawater. In other words, MeHg flowing into the lake or river would be almost removed by photo demethylation. However, MeHg flowing to seawater would be hardly removed, which could have more chance for bioaccumulation in seawater.