Photo-degradation of monomethylmercury in the presence of chloride ion

Abstract

Photo-degradation (PD) of monomethylmercury (MMHg) is recognized as the most important process in mercury cycling that maintains MMHg at low concentration in water systems. Chloride ion (Cl(-)) is present widely in ecosystems and it may have an effect on MMHg PD. In this investigation, MMHg PD with various Cl(-) concentrations under different light radiation intensity and wavelength were performed in a quartz cylinder reactor. Various light radiations were obtained from xenon and ultraviolet (UV) bulbs. The results showed that the first order reaction coefficient decreased (1.35-0.37 h(-1)) with Cl(-) concentrations, and increased (0.14-0.76 h(-1)) with UV-B radiation intensity. The wavelength mediated PD rates varied from 0.29 h(-1) to 0.96 h(-1). The peak emission of Hg(0) was excited by UV-B radiation and Hg(0) flux of the reactions without Cl(-) (94.06 ng) was higher than that with Cl(-) by 1-4 times. All the results suggest that Cl(-) could block MMHg PD and subsequently influence the end products from PD. The wavelength and light intensity have a significant effect on MMHg PD and Hg(0) emission. Calculation of MMHg species (%) at initial and terminal time with various Cl(-) concentrations show that MMHg species changed by Cl(-) complexation and thus result in different PD mechanisms and pathways.