[Characteristics of monomethylmercury photodegradation in water body]

To investigate the photochemical process of monomethylmercury (MMHg) in the water environment, laboratory experiments were conducted using artificial and visible light sources to confirm the effects of wavelength and light intensity on MMHg photodegradation (PD), and the reaction process of MMHg PD was discussed based on Hg(0) emission rate. The results indicated that Hg(0) was the end product of MMHg PD, and the light conditions had an effect on the rate constant of MMHg PD and Hg(0) flux. When the reactor was exposed to UV light conditions, the rate constant of MMHg PD increased with the decrease in wavelength and the increase in light intensity. The rate of MMHg PD was first-order with respect to MMHg concentration in the reactor, and the rate constants were K(UVA) 0.403-0.562 h(-1), K(UVB) 0.961 h(-1), K(UVC) 1.221 h(-1) and K(VL+UVA+UVB) 1.346 h(-1), respectively and the emission rates of Hg(0) were 0.166-0.392 ng x min(-1). When the reactor exposed to visible light conditions, the rate of MMHg PD was only 0.061 h(-1), and the emission rate of Hg(0) was 0.008 ng x min(-1). In the experiments conducted under dark conditions, we did not observe MMHg PD and no Hg(0) was found. All the results suggest that the degradation of MMHg in artificial waters is mediated by ultraviolet (UV) radiation, and UV radiation plays a key role in the photochemical process of MMHg in freshwater lakes.