Photoreduction of mercury in sea water and its possible implications for Hg0 air-sea fluxes

Mercury photoreduction in the presence of humic substances was investigated in laboratory experiments by means of a radiochemical technique. The seawater samples used were from the North Sea, and the light source was natural sunlight on a summer's day at temperate latitude. The reduction reaction Hg2+ --> HgO was observed to occur at different humic concentrations and light regimes. The results presented here are in the form of the percent of initial total mercury which was reduced and degassed from the solution with time. The reaction rate increased with humic concentration and the presence of light. The reaction rates varied from near zero in the case of the dark reaction in the: absence of humic material to around 2% h(-1) in a 5-mg l(-1) of dissolved organic carbon (DOC) irradiated solution. Elemental mercury fluxes calculated from data obtained during this work are compared with fluxes reported in the literature. The consequences of mercury reduction and evasion from the solution are discussed in the light of the fluxes of gaseous mercury which can potentially be generated from the water when the photochemical reduction reaction takes place. The photochemically driven fluxes calculated are shown to be potentially significant contributors to the fluxes reported in the literature for at least some of the environments tested. (C) 1999 Elsevier Science B.V. All rights reserved.