Roles of chloride ion in photo-reduction/oxidation of mercury

Photo-reduction of divalent mercury (Hg(II)) in aquatic systems plays a key role in global biogeochemistry cycling of mercury (Hg) in the Earth's surface environment. The mechanisms of this process with various Cl- concentrations ([Cl-]) under different pH values and irradiation wavelength ranges are still unclear. In this work, photo-reduction and photo-oxidation experiments of Hg with different [Cl-] and pH values under various light conditions were conducted. The results show that photo-reduction rate constants of Hg(II) decrease with the increasing of [Cl-] in neutral solution under full light spectrum. Photo-reduction rate constants of Hg(II) with Cl- is highly dependent on Hg(II) species, which is determined by [Cl-] and pH value. Irradiation wavelength ranges have significant effects on reaction processes of photo-reduction of Hg(II) and photo-oxidation of Hg(0) in the presence of Cl-. When cut off ultraviolet (UV) radiation (280-400 nm), the reduction rate constants decrease without Cl-, and increase with higher [Cl-], and the photo-oxidation rates of Hg(0) decrease with or without Cl-. Except Cl- complexation stabilize the reducible Hg ions in solutions, photo-oxidation is an important reason for Cl- lowering photo-reduction rate of Hg(II). The results are of great importance for understanding the photo-redox characters of Hg(II) with Cl-.