Grantee Research Project Results
1999 Progress Report: The Redox Cycle of Mercury in Natural Waters
EPA Grant Number: R827915Title: The Redox Cycle of Mercury in Natural Waters
Investigators: Morel, Francois M.
Institution: Princeton University
EPA Project Officer: Packard, Benjamin H
Project Period: October 11, 1999 through October 10, 2002 (Extended to October 10, 2003)
Project Period Covered by this Report: October 11, 1999 through October 10, 2000
Project Amount: $726,318
RFA: Mercury: Transport and Fate through a Watershed (1999) RFA Text | Recipients Lists
Research Category: Watersheds , Heavy Metal Contamination of Soil/Water , Water , Safer Chemicals
Objective:
The main objective of this project is to elucidate the parameters that control the flux of elemental mercury from natural waters to the atmosphere. The experimental plan consists of a series of iterative laboratory and field experiments focused on the principal chemical and biological mechanisms that transform mercury between its dissolved ionic form, Hg(II), and its volatile elemental form, Hg(0): (1) the biological reduction of Hg(II) to Hg(0) by transmembrane metal reductases in photosynthetic microorganisms; (2) the chemical reduction of Hg(II) by high energy reductants (typically formed in the light) such as the superoxide anion or an organic radical; and (3) the oxidation of elemental mercury Hg(0) to Hg(I), likely effected by radicals such as superoxide or semiquinones.
Progress Summary:
The main emphasis of our research during the first year of this project has been on the photooxidation of elemental mercury. This is the least studied and most poorly understood of the redox transformations of mercury in surface waters and it has been assumed so far to be insignificant. Using both artificial solutions and natural samples of freshwater and brackish water we have measured the rate of photooxidation of Hg(0) under various conditions. Both in the laboratory and in the field samples we observed remarkably rapid oxidation of dissolved elemental mercury, Hg(0)(aq), upon irradiation under appropriate conditions. This observation should lead us to reevaluate the results of Hg(II) photoreduction experiments in which Hg(0) reoxidation has usually been assumed to be negligible. Our laboratory experiments demonstrate that photooxidation of Hg(0)(aq), requires the simultaneous presence of chloride, of a photoreactive compound (such as benzoquinone) and of light. There is thus little doubt that this oxidation is the result of an indirect photochemical process, in which an electron acceptor produced by the chromophore is somehow able to promote effective oxidation of Hg(0)(aq) in the presence of chloride. In the experiments done with natural waters, it is notable that, unlike what is observed in the laboratory, a significant rate of Hg(0)(aq) photooxidation is observed even in the absence of added chloride. Our field data can serve to provide a first evaluation of the possible importance of Hg(0)(aq) photooxidation in the cycling of Hg in surface waters. Even in shallow coastal waters, photooxidation of Hg(0) is likely to be dominant during the summer days as compared to the volatilization of Hg(0) from the water column, except in periods of high winds. Once normalized to the photon flux, we find that the light induced redox processes, Hg(II) reduction and Hg(0) oxidation, could occur at roughly similar rates in natural waters. It is now clear that the photooxidation of Hg(0) should be taken account, both in calculating mercury budgets in aquatic systems and in sampling protocols which must consider the highly dynamic nature of the Hg(0)(aq) pool and the diel variations in sunlight. In addition to significant progress on the redox cycle of mercury in surface waters, we have also continued to address the critical question of what parameters control the rate of methylation of mercury. A collaborative project with MIT has demonstrated that while the presence of polysulfides greatly augments the concentration of soluble mercury in sulfidic media, the methylation rate by sulfate reducing bacteria remains unchanged. We are thus now examining if the biochemical properties of sulfate reducers, rather than the chemistry of the medium in which they grow, might be responsible for the high rate of mercury methylation by these organisms.
Future Activities:
Our results on the photooxidation of elemental mercury have the potential to change radically the various estimates of mercury volatilization from surface waters to the atmosphere. We thus intend to repeat and extend these laboratory and field experiments. In addition, we have formulated a novel hypothesis concerning the mechanisms of mercury methylation by sulfate reducing bacteria and hope to be able to test it during the second year of the grant.
Journal Articles on this Report : 2 Displayed | Download in RIS Format
Other project views: | All 20 publications | 7 publications in selected types | All 7 journal articles |
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Type | Citation | ||
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Jay J, Morel FMM, Hemond HF. Mercury speciation in the presence of polysulfides. Environmental Science and Technology 2000;34:2196-2200. |
R827915 (1999) R827915 (2001) R824778 (Final) R827634 (Final) |
not available |
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LaLonde JD, Amyot M, Kraepiel AML, Morel FMM. Photooxidation of Hg(0) in Artificial and Natural Waters. Environmental Science and Technology 2001;35(7):1367-1372. |
R827915 (1999) |
not available |
Supplemental Keywords:
photooxidation, photoreduction, volatilization, enzymatic reduction., Scientific Discipline, Waste, Water, Ecosystem Protection/Environmental Exposure & Risk, Hydrology, Environmental Chemistry, Fate & Transport, Microbiology, Mercury, fate and transport, photosynthetic microorganisms, emissions, phytoplankton, biogeochemical cycling, methylmercury, enzymatic reduction, atmospheric deposition, mercury vaporRelevant Websites:
Progress and Final Reports:
Original AbstractThe perspectives, information and conclusions conveyed in research project abstracts, progress reports, final reports, journal abstracts and journal publications convey the viewpoints of the principal investigator and may not represent the views and policies of ORD and EPA. Conclusions drawn by the principal investigators have not been reviewed by the Agency.