Photooxidation of Hg(O) in artificial and natural waters

The oxidation of volatile aqueous Hg(O) in aquatic systems may be important in reducing fluxes of Hg out of aquatic systems. Here we report the results of laboratory and field experiments designed to identify the parameters that control the photooxidation of Hg(O)(aq) and to assess the possible importance of this process in aquatic systems. The concentrations of elemental and total Hg were measured as a function of time in both artificial and natural waters irradiated with a UV-B lamp. No change in Hg speciation was observed in dark controls, while a significant decrease in Hg(O) was observed in UV-B irradiated artificial solutions containing both chloride ions and benzoquinone. Significant photooxidation rates were also measured in natural samples spiked with Hg(O)(aq); the photooxidation of Hg(O) then follows pseudo first-order kinetics (k = 0.6 h^-1). These results indicate that the previously observed Hg(II) photoreduction rates in natural waters could represent a net balance between Hg(II) photoreduction and Hg(O) photooxidation. As calculated from Hg(O) photooxidation rates, the dominant Hg(O) sink is likely to be photooxidation rather than volatilization from the water column during summer days.