Grantee Research Project Results
1998 Progress Report: The Role of Hg(II) Reduction and Chemical Speciation in Controlling the Concentration of Mercury and its Methylation in Natural Waters
EPA Grant Number: R824778Title: The Role of Hg(II) Reduction and Chemical Speciation in Controlling the Concentration of Mercury and its Methylation in Natural Waters
Investigators: Morel, Francois M.
Institution: Princeton University
EPA Project Officer: Packard, Benjamin H
Project Period: November 1, 1995 through October 31, 1998 (Extended to October 31, 1999)
Project Period Covered by this Report: November 1, 1997 through October 31, 1998
Project Amount: $349,950
RFA: Water and Watersheds (1995) RFA Text | Recipients Lists
Research Category: Watersheds , Water
Objective:
According to our understanding of the mercury cycle in natural waters, two distinct processes determine the rate of formation of methylmercury and its eventual accumulation in fish: 1) mercury reduction and volatilization control the overall concentration of mercury in many natural water bodies; 2) mercury uptake and methylation by bacteria control the transformation of inorganic mercury into its biologically accumulated organic form. The ultimate aim of this project is to elucidate the parameters that control the concentration and bioaccumulation of methylmercury.Progress Summary:
Over the past two and half years, we have been able to make progress on both chemical and microbiological aspects of the project: i) we have continued to quantify the rates of oxidation and reduction of dissolved mercury and ii) the rate of dissolution of mercuric sulfide under various conditions; and iii) we have finally disproved a seductive hypothesis according to which polysulfide complexation of ionic mercury could have enhanced mercury methylation rates in anoxic sediments. These achievements are described succinctly below.
- Redox Transformation of Mercury
It is generally thought that the major fate of mercury in water is reduction of Hg(II) to Hg(0) and volatilization of Hg(0) to the atmosphere. We have discovered that oxidation of Hg(0) in oxic waters is in fact relatively rapid (hours to days). This process, which seems to have been ignored in the environmental literature, should effectively decrease the net rate of Hg(0) volatilization and maintain higher mercury concentrations in may aquatic systems. We have begun a systematic study of the kinetics of this process as well as of the (somewhat) better documented reduction of Hg(II). The presence of high chloride concentrations and of appropriate particle surfaces catalyze the oxidation of Hg(0) by oxygen, resulting in rates of about 10 % per hour, in natural seawater (Amyot et al., 1997). One should note that one of the effective surfaces for the catalysis of Hg(0) oxidation is that of liquid mercury. Thus, pools or droplets of liquid mercury which may be present in seawater as a result of some human activity should be oxidized relatively efficiently in oxic seawater. - Solubility and Speciation of Hg(II) in Anoxic Waters
The major form of mercury in anoxic sediments is mercuric sulfide (HgS), as cinnabar (red) or metacinnabar (black). These minerals are extremely insoluble and a major question relating to the mobility of Hg in anoxic environments is that of the mechanisms and rate of dissolution of HgS. We have performed a systematic study of the kinetics of HgS dissolution examining the effects of such factors as pH, oxygen concentration, light and the presence of organic acids. As we hypothesized, visible light greatly enhances the kinetics of dissolution, especially in the presence of sulfides. Of particular interest is the formation of elemental mercury during the photodissolution of HgS.In addition to the difficulties posed by facile redox reactions, the chemistry of mercury in sediments is complicated by the multiple forms of inorganic sulfur. In particular we have hypothesized that Hg(II) may form stable complexes with polysulfides, Sn2-. Indeed we have found that addition of 0.1mM polysulfide increases the solubility of HgS about 10 fold at pH's 8-10.
Preliminary field experiments testing the possible importance of the photochemical dissolution of HgS under natural conditions, have been performed in Lake Ontario. The data are suggestive but not conclusive and the experiments will be repeated next summer.
- Microbial Methylation of Mercury
A central hypothesis of this project was that the formation of polysulfide complexes of mercury may not only enhance the solubility of Hg(II) in anoxic waters but also lead to rapid bacterial uptake and methylation of the metal. After resolving a number of experimental problems (and setting up analytical systems for methylmercury and polysulfides) we have finally been able to perform a direct test of the hypothesis: measuring methylmercury formation in identical cultures of methylating bacteria in the presence and absence of polysulfides. Despite positive preliminary results, we have now convincingly disproved this hypothesis. Polysulfide appears to play only an insignificant and indirect role in the mercury methylation.
Journal Articles on this Report : 5 Displayed | Download in RIS Format
Other project views: | All 7 publications | 7 publications in selected types | All 7 journal articles |
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Type | Citation | ||
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Amyot M, Gill GA, Morel FMM. Production and loss of dissolved gaseous mercury in coastal seawater. Environmental Science & Technology 1997;31:3606-3611. |
R824778 (1998) R824778 (Final) |
not available |
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Mason RP, Reinfelder JR, Morel FMM. Bioaccumulation of mercury and methylmercury. Water, Air and Soil Pollution 1995;80:915-921. |
R824778 (1998) R824778 (Final) |
not available |
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Mason RP, Morel FMM. The role of microorganisms in elemental mercury formation in natural waters. Water, Air and Soil Pollution 1995;80:775-787. |
R824778 (1998) R824778 (Final) |
not available |
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Mason RP, Reinfelder JR, Morel FMM. Uptake, toxicity, and trophic transfer of mercury in a coastal diatom. Environmental Science & Technology 1996;30:1835-1845. |
R824778 (1998) R824778 (Final) |
not available |
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Morel FMM, Kraepiel AML, Amyot M. The chemical cycle and bioaccumulation of mercury. Annual Review Ecology and Systematics 1998;29:543-566. |
R824778 (1998) R824778 (Final) |
not available |
Supplemental Keywords:
RFA, Scientific Discipline, Waste, Water, Ecosystem Protection/Environmental Exposure & Risk, Bioavailability, Hydrology, Ecology, Water & Watershed, Contaminated Sediments, Environmental Chemistry, Chemistry, Fate & Transport, Mercury, Watersheds, fate and transport, aquatic, mercury uptake, natural waters, bacteria control, contaminated sediment, chemical speciation, mercury loading, mercuric sulfide, polysulfide, fish consumption, methylation, methylmercury, lake sediment, mercury concentrations, bioaccumulation, heavy metals, microbiological aspectsProgress 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.