Once deposited in natural waters, Hg undergoes an aquatic redox cycling between oxidized Hg(II), dissolved gaseous Hg° (DGM), and methylmercury. Mechanistic understanding of sunlight-induced natural processes for production of DGM in freshwaters (Figure 1) has remained limited and few direct field tests of the mechanistic hypotheses are available. Recent field sunlight-incubations of lake waters in Teflon bottles further suggest natural production of DGM through sunlight-induced photochemical reduction of Hg(II) in freshwater. This is of significance for aquatic Hg biogeochemical cycling and its environmental impacts, because the competition of this sunlight-driven pathway for Hg(II) substrate with the methylation pathway might reduce Hg toxic hazards in local aquatic ecosystems through removal of Hg(II) as a result of its reduction to Hg° and its subsequent evasion from the aquatic system. Based on its photochemical interactions with natural DOC, we hypothesize that Fe compounds could influence DGM dynamics. We exposed ferric-iron-salt-spiked fresh surface lake water (Whitefish Bay, Lake Superior, MI) in Teflon bottles and pond water (Oak Ridge, TN) in quartz bottles to sunlight in the field to infer if sunlight and Fe(III)-induced photochemical production of DGM could mechanistically contribute partly to natural photochemical production of DGM in freshwaters. We found that exposure of fresh water spiked with fresh Fe(III) (~5 or 10 µM) to sunlight led to repeatable, significantly larger increases in DGM production (e.g., 380% in 1 h, 420% in 2 h, and 470% in 4 h for Whitefish Bay water) than exposure without the spike (e.g., 200% in 6 h). DGM increased with increasing exposure time and then often appeared to approach a steady state in the tests. Higher Fe(III) spike levels resulted in the same, or even less, DGM production. Storage of the water with or without Fe(III) spike in the dark after sunlight exposure led to significant, apparently first-order, decreases in DGM (Figure 2). These phenomena were hypothetically attributed to sunlight-induced photochemical production of highly reducing organic free radicals through photolysis of Fe(III)-organic acid coordination compounds and subsequent reduction of Hg(II) to Hg(0) by the organic free radicals; the reduction was also accompanied by dark oxidation of Hg(0) by photochemically originated oxidants (e.g., ·OH). This study suggests that sunlight and Fe(III)-induced photochemical reduction of Hg(II) could be one of the mechanisms responsible for natural photochemical production of DGM in freshwaters and that Fe species may be influential in mediating Hg chemodynamics and its subsequent toxicity in aquatic ecosystems. _________________*Supported by a USEPA STAR Grant in collaboration with J. Nriagu, G. Keeler, and F. Marsik, University of Michigan. Reference: Zhang, H., and S.E. Lindberg. 2001. Sunlight and iron(III)-induced photochemical production of dissolved gaseous elemental mercury in fresh water. Envir. Sci. & Technol. 35:928-935. |
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