Abstract
Photochemical oxidation of dissolved elemental mercury [Hg(0)] affects mercury chemical speciation and its transfer at the water-air interface in the aquatic environment. The mechanisms and factors that control Hg(0) photooxidation, however, are not completely understood, especially in natural freshwaters containing dissolved organic matter (DOM) and carbonate. Here, we evaluate Hg(0) photooxidation rates affected by various reactive ionic species [e.g., DOM, HCO3-, NO3-] and free radicals in a creek water and a phosphate buffer solution (pH=8) under simulated solar irradiation. We report a high Hg(0) photooxidation rate (k = 1.44 h-1) in the presence of both HCO3- and NO3-, whereas HCO3-, NO3-, or DOM alone increased the oxidation rate slightly (k = 0.1–0.17 h-1). Using scavengers and enhancers for singlet oxygen (1O2) and hydroxyl (HO•) radicals, as well as electron paramagnetic resonance spectroscopy, we identify that carbonate radicals (CO3•-) primarily drive the Hg(0) photooxidation, whereas addition of DOM resulted in a 2-fold decrease in Hg(0) oxidation. This study identifies an unrecognized pathway of Hg(0) photooxidation by CO3•- radicals and the inhibitory effect of DOM, which could be important in assessing Hg transformation and fate in water containing carbonate such as hard water and seawater.
