Grantee Research Project Results
Final 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 Amount: $349,950
RFA: Water and Watersheds (1995) RFA Text | Recipients Lists
Research Category: Watersheds , Water
Objective:
Mercury is a highly toxic and mobile pollutant. As a gas in the atmosphere, it is rapidly transported all over the globe from its natural and anthropogenic sources. In water and sediments, it can be transformed by bacteria to the organo-metallic methylmercury form. Methylmercury is efficiently accumulated in the food chain, reaching high concentrations in fish that pose health risks to their predators, including human consumers.
According to our understanding of the mercury cycle in natural waters, the following two distinct processes determine the rate of formation of methylmercury and its eventual accumulation in fish: (1) mercury uptake and methylation by bacteria control the transformation of inorganic mercury into its biologically accumulated organic form; and (2) mercury reduction and volatilization control the overall concentration of mercury in many natural water bodies. The ultimate aim of this project was to elucidate the parameters that control the concentration and bioaccumulation of methylmercury.
This project was undertaken on the basis of two hypotheses, one regarding mercury methylation, the other regarding mercury volatilization:
Hypothesis 1. It is known that sulfate-reducing bacteria (bacteria that live in anoxic environments and respire sulfate—i.e., reduce sulfate to sulfide with organic matter) are the principal methylators of mercury in the environment. It also is known that mercury forms a very stable solid with sulfide (cinnabar = HgS), which should make it virtually insoluble and unavailable to the methylating bacteria. To resolve this dilemma, we hypothesized that, in sulfide-containing waters, mercury may be kept in solution by formation of soluble complexes with polysulfides. (Polysulfides, Sn2-, are sulfur species of intermediate oxidation state between sulfate and sulfide.) We hypothesized further that these polysulfide complexes of mercury may be soluble in lipids and may thus make mercury available to the methylating bacteria.
Hypothesis 2. Although the important—and often dominant—input of mercury from the atmosphere to water bodies has been well documented, we know much less about the processes that control the reverse mercury flux: the volatilization of mercury into the atmosphere. This volatilization requires the reduction of the mercuric ion, Hg(II), to the elemental mercury form, Hg(0). We hypothesized that the reduction and volatilization of mercury from water bodies was effected by both enzymatic processes mediated by microorganisms and by photochemical reactions induced by sunlight.
As discussed below, we have effectively confirmed the first part and disproved the second part of Hypothesis 1: polysulfides form mercury complexes that increase the solubility of mercury in the presence of sulfide, but these complexes are too large to diffuse rapidly through lipid membranes and are thus unavailable to the bacteria. Polysulfides tend to decrease rather than increase the rate of methylation of mercury. In contrast, we have confirmed Hypothesis 2: mercury reduction is effected by both microorganism and by light. However, we have shown that the net rate of reduction of Hg(II) to Hg(0) also depends on a photochemically induced re-oxidation of Hg(0) to Hg(II), which is particularly effective in seawater. This new mechanistic insight has led to new hypotheses regarding the redox cycle of mercury in natural waters, which now are being tested in a new U.S. Environmental Protection Agency (EPA) Science to Achieve Results (STAR) project.
Summary/Accomplishments (Outputs/Outcomes):
Solubility and Speciation of Hg(II) in Sulfidic 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, extent and rate of dissolution of HgS.
We have hypothesized that Hg(II) may form stable complexes with polysulfides, Sn2-. Polysulfides can form as an intermediate in the oxidation of sulfide or by reaction of sulfide with elemental sulfur. As part of her doctoral thesis, J. Jay (publication 6 below) performed a systematic study of the solubility of cinnabar as a function of pH and polysulfide concentration. She confirmed that the solubility of HgS was increased markedly in the presence of polysulfides—about 10-fold for [Sn2-] = 0.1 nM and pH = 8-10. She also was able to model her data by including two new chemical species—Hg(Sx)22- and HgSxOH-—in thermodynamic models of mercury speciation and to quantify the effect of these species on the lipid solubility of mercury.
In addition, 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. 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 in the context of our new STAR project.
ncerqa/progress/morel98.html. In the first study to have been completed during this project, we tested the general hypothesis that the formation of lipid soluble complexes of mercury determined the bioavailability of mercury and its accumulation in aquatic microorganisms. A systematic study of the effect of chloride complexation of mercury and methylmercury on their accumulation and toxicity in phytoplankton demonstrated the central roles of the neutral chloride complexes, HgCl2 and CH3HgCl, which are soluble in lipids (see publications 1 and 3 below). In these studies, we also were able to demonstrate that the much greater biomagnification of methylmercury compared to inorganic mercury in the food chain was caused in large part by the much greater association of methylmercury with the cellular membranes of phytoplankton.
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 (in a mechanism similar to the way chloride complexation leads to Hg accumulation in phytoplankton) and eventual methylation of the metal. After resolving a number of experimental problems (and setting up analytical systems for methylmercury and polysulfides), J. Jay was 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 (see publication 7 below). The sulfate reducing bacterium Desulfovibrio desulfuricans, in equilibrium with cinnabar, exhibits no increase in mercury methylation rate on the presence of polysulfides in the submillimolar range. Although the polysulfide complexes of mercury result in a higher lipid solubility of the metal, they apparently are too large to diffuse effectively through the bacterial membrane.
Redox Transformation of Mercury. In two different series of field studies, one conducted by Mason and one by Amyot, we have confirmed that mercury reduction can be effected by microbial processes (publication 2 below) and by photochemistry (publication 4 below). Microbial reduction of mercury was demonstrated in the field by Mason, who showed that filtration of the ambient flora resulted in a large decrease in the rates of reduction of added mercury. Studies with cultures of phytoplankton confirmed the existence of the microbial reduction process, but also showed that reduction in situ must have been dominated by bacteria. In contrast to this, Amyot found that light was necessary to effect mercury reduction. We have now reconciled these apparently contradictory results by noting that the studies of Mason were conducted at concentrations of mercury (Hg? 0.5 nM) that were sufficient to induce the synthesis of the mercuric reductase in bacteria, while the studies of Amyot were conducted at natural Hg concentrations, below the threshold for induction of the bacterial MER operon.
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 many aquatic systems. We have undertaken a systematic study of the kinetics of this process (see publication 4 below). The presence of high chloride concentrations and of appropriate particle surfaces catalyze the oxidation of Hg(0) by oxygen, resulting in rates of approximately 10 percent per hour, in natural seawater. 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 that may be present in seawater as a result of some human activity should be oxidized relatively efficiently in oxic seawater. An extensive study of the mechanisms and rate of elemental mercury in the field is a central aspect of our new STAR project.
Journal Articles on this Report : 7 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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Jay JA, Murray KJ, Gilmour CC, Mason RP, Morel FMM, Roberts AL, Hemond HF. Mercury methylation by Desulfovibrio desulfuricans ND132 in the presence of polysulfides. Applied and Environmental Microbiology 2002;68(11):5741-5745 |
R824778 (Final) R827631 (Final) |
not available |
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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. |
R824778 (Final) R827634 (Final) R827915 (1999) R827915 (2001) |
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:
mercury, methylation, bioaccumulation, sulfide, polysulfide, oxidation, reduction, redox, volatilization, speciation., RFA, Scientific Discipline, Water, Waste, Ecosystem Protection/Environmental Exposure & Risk, Water & Watershed, Bioavailability, Ecology, Hydrology, Contaminated Sediments, Environmental Chemistry, Chemistry, Fate & Transport, Watersheds, Mercury, 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 aspectsRelevant Websites:
http://geoweb.princeton.edu/people.html Exit
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.