Photo-degradation of monomethylmercury in the presence of chloride ion

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 Hg0 was excited by UV-B radiation and Hg0 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 Hg0 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.

► We calculated MMHg PD rates with various Cl− concentrations.
► We investigated the effects of wavelength and light intensity on MMHg PD with Cl−.
► Cl− could alter MMHg species.
► We proposed underlying mechanisms and pathways for PD of MMHg species.
► We discussed photochemical characters of PD products with Cl−.