Photoinduced Oxidation of Hg⁰(aq) in the Waters from the St. Lawrence Estuary

The oxidation of volatile aqueous Hg⁰ in aquatic systems may be important in decreasing the fluxes of Hg out of the water column. Using incubations of natural samples from the St. Lawrence River, we examined some of the parameters that control this oxidation. Hg° was found to be chiefly mediated by UV radiation since (i) "dark" oxidation was not found to be statistically significant; (ii) visible light induced a significant but slow photooxidation (k = 0.09 h^-1); and (iii) visible + UV radiation led to a faster photooxidation (k = 0.6-0.7 h^-1), mainly because of UV-A induced reactions. Doubling UV irradiation did not increase the reaction rate of Hg⁰ photooxidation in natural water samples, indicating that some factor other than photon flux was rate limiting and suggesting that the reaction involves intermediate photoproduced oxidant(s). The addition of methanol, a ^.OH scavenger, decreased Hg photooxidation rates by 25% in brackish waters and by 19% in artificial saline water containing semiquinones, indicating that ^.DH may be partly responsible for Hg⁰ oxidation. Photooxidation rates were not affected by oxygen concentrations and did not decrease when samples were heat-sterilized, treated with chloroform, or filtered prior to exposure to light.