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SUNLIGHT AND IRON(III)-INDUCED PHOTOCHEMICAL PRODUCTION OF DISSOLVED GASEOUS MERCURY IN FRESHWATER. (R827632)
Citation:
Zhang, H. AND S. Lindberg. SUNLIGHT AND IRON(III)-INDUCED PHOTOCHEMICAL PRODUCTION OF DISSOLVED GASEOUS MERCURY IN FRESHWATER. (R827632). ENVIRONMENTAL SCIENCE & TECHNOLOGY. American Chemical Society, Washington, DC, 35:928-935, (2001).
Description:
Mechanistic understanding of sunlight-induced natural processes for
production of dissolved gaseous mercury (DGM) in freshwaters has remained
limited, and few direct field tests of the mechanistic hypotheses are available.
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
freshwater spiked with fresh Fe(III) (~5 or 10 src="/ncer/pubs/images/mgr.gif">M) 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. 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.