2001 Progress Report: Understanding the Role of Sulfur in the Production and Fate of Methylmercury in Watersheds

EPA Grant Number: R827653
Title: Understanding the Role of Sulfur in the Production and Fate of Methylmercury in Watersheds
Investigators: Mason, Robert P. , Gilmour, Cynthia C.
Institution: Chesapeake Biological Laboratory , Academy of Natural Sciences
EPA Project Officer: Hunt, Sherri
Project Period: October 1, 1999 through September 30, 2002 (Extended to October 1, 2003)
Project Period Covered by this Report: October 1, 2000 through September 30, 2001
Project Amount: $779,786
RFA: Mercury: Transport and Fate through a Watershed (1999) RFA Text |  Recipients Lists
Research Category: Water and Watersheds , Mercury , Water , Safer Chemicals

Objective:

The principal objective of this research project is to understand the role of sulfur, especially in the form of sulfide, in mercury (Hg) methylation, and methylmercury (MeHg) fate and transport in watersheds, including the factors controlling the efflux of MeHg from sediments and soils. To understand the role of sulfur species, it is necessary to understand the complexation of Hg in the dissolved and solid phase, because Hg bioavailability to methylating bacteria depends on its availability for uptake. The fate, transport, and bioacccumulation of MeHg depends on its complexation and binding in the solid phase. To understand the controls over the methylation process, the more specific objectives are to: (1) investigate and contrast Hg complexation and sulfide with the role of microbial community structure and microbial activity in net MeHg production; (2) investigate the controls over biotic demethylation; and (3) investigate the controls over the abiotic decomposition of MeHg in natural waters. A model of Hg and MeHg cycling in aquatic systems will be constructed, which incorporates the factors controlling bioavailability, methylation, and MeHg fate and decomposition.

Progress Summary:

Work in Project Year 2 was aimed at answering the questions of how speciation in water and bioavailability from sediments control Hg methylation. Studies of interactions between organic matter, iron oxide, and Hg and MeHg have continued. Investigators will explore the potential interactions of Hg and MeHg with solid oxide phases and with solid sulfide phases. They have made substantial progress but research has not yet been completed. Results to date support our hypothesis, that the oxic surface layer acts as a barrier to MeHg transport, and that complexation to organic matter bound to oxide phases is controlling the movement of MeHg at the sediment-water interface. The information gained from the laboratory investigations on Hg and MeHg binding to various ligands is necessary to model MeHg movement across a redox gradient, and for determining the conditions under which MeHg becomes mobile. In concert with the laboratory studies, the speciation (partitioning) of Hg to sediment phases has been examined using sequential extraction experiments with Florida Everglades sediment, estuarine sediment, and lake sediment. In all cases, the organic matter appears, in the solid phase, to be an important locale for Hg sequestration, especially in the high organic content Everglades sediment. The Hg is more evenly distributed between fractions in sediments from the Experimental Lakes Area in Canada, which have less than 5 percent organic matter. Clearly, other phases increase in importance in high sulfide or low organic content sediments. To complete this aspect of the research, studies of Hg binding to sulfide phases, in the presence or absence of organic matter, need to be examined.

We continue to use stable isotopes of Hg in both methylation and demethylation studies in a wide variety of environments (lakes sediments, wetlands, uplands, the Everglades, and estuarine systems) and have demonstrated some important tenets about the factors controlling methylation. The development of techniques necessary to measure the low levels of Hg and associated isotope ratios by ICP-MS continues, and our group successfully participated in a laboratory intercalibration where Hg and MeHg isotopic distribution was measured in a variety of matrices. We continue to use two ICP-MS instruments, one at each of the Principal Investigator's laboratories. Methods are now in place for the analysis of both Hg and MeHg isotopes in water, sediment, and biota.

Overall, our understanding on how sulfides influence Hg methylation has enormously increased, and is detailed in the American Chemical Society, currently in press. Methylation often is rapid upon addition of an Hg spike to an environmental sample; overall the rate of production is higher than expected based on in situ concentrations, suggesting higher bioavailability. However, strong relationships (r2 ~ 0.5 to 0.8) have been found between in situ MeHg concentration and methylation of added Hg isotopes (equilibrated prior to injection with site water) in wetlands, lakes, and estuarine sediment. To interpret these measured short-term methylation rates, consideration of both thermodynamic and kinetic aspects of the experimental protocols need to be considered to assess if and why the added ("new") Hg is more bioavailable than the existing Hg pool. The complexation of added Hg will alter the rate of thermodynamic equilibration, and thereby influence short-term methylation rates. Consideration of complexation kinetics, and the likely abundance of weak binding ligands, such as carboxylic acid groups on dissolved organic carbon (DOC) in the environment, suggests that the added Hg will be in a more "bioavailable" pool immediately after addition (i.e., more weakly bound) than the "older," more strongly complexed Hg. Further studies will examine the kinetics of bioavailability of added Hg to soils and sediments, because this is crucial to our understanding of Hg bioavailability to methylating bacteria. Our collaboration with the Canadian group on the "bioreporter" studies continue, and this has produced some surprising results. The measure of bioavailability provided by the bioreporter does not coincide with other measures of Hg reactivity (e.g., the rate of production of elemental Hg in surface waters). The reasons for these differences are being examined. Abiotic demethylation studies in estuarine water suggest that MeHg relatively is stable to degradation in contrast to studies in freshwater, which suggest that abiotic degradation of MeHg is confined to the surface 0.5 m. Thus, its importance is limited. Additional studies are needed to examine if the enhanced stability in low-DOC saline waters is due to the stability of the chloride complex relative to that of the hydroxide. Also, the presence of DOC will be important if MeHg complexation to DOC dominates, and if DOC may enhance degradation by acting as a mediator for electron transfer.

Future Activities:

Activities in the next year will focus on completing the laboratory investigations of the role of DOC and solid oxide phases in Hg and MeHg complexation. Additionally, further sequential extraction studies of the binding of Hg to the solid phase in sediments and soils is planned for the summer of 2002, as well as kinetic studies of Hg bioavailability ?post-spiking? of sediments and soils. These experiments will complete the first section of the proposal plan, which focused on the role of complexation in modifying mercury methylation. The experiment also will complete the objectives under the section of the proposal examining factors controlling the exchange of Hg and MeHg at the sediment-water interface. One of the major activities for the latter part of the project is to focus on the role of microorganisms and microbial activity in modifying methylation rates in sediments. Currently, it is not clearly known whether limitation of microbial processes by such factors as carbon supply limitation is more important in determining Hg methylation in sediments, in contrast to changes in bioavailability as a result of changes in Hg speciation. These biotic experiments will be complimented with experiments further exploring the importance of abiotic demethylation processes. The mesocosm-dosing experiments and the field work related to examining controls on methylation will continue in the summer of 2002 and proceed until the end of the project. Model development will continue.


Journal Articles on this Report : 4 Displayed | Download in RIS Format

Other project views: All 20 publications 6 publications in selected types All 5 journal articles
Type Citation Project Document Sources
Journal Article Benoit JM, Mason RP, Gilmour CC, Aiken GR. Constants for mercury binding by dissolved organic matter isolates from the Florida Everglades. Geochimica et Cosmochimica Acta 2001;65(24):4445-4451. R827653 (2000)
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R827631 (2000)
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  • Full-text: ScienceDirect-Full Text PDF
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  • Abstract: ScienceDirect-Abstract and Full Text HTML
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  • Other: Smithsonian Library Online - Full Text PDF
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  • Journal Article Benoit JM, Gilmour CC, Mason RP. The influence of sulfide on solid-phase mercury bioavailability for methylation by pure cultures of Desulfobulbus propionicus (1pr3). Environmental Science & Technology 2001;35(1):127-132. R827653 (2000)
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    R827653 (Final)
    R827631 (2000)
    R827631 (2001)
    R827631 (Final)
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  • Abstract: ACS - Abstract
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  • Journal Article Benoit JM, Gilmour CC, Mason RP. Aspects of bioavailability of mercury for methylation in pure cultures of Desulfobulbus propionicus (1pr3). Applied and Environmental Microbiology 2001;67(1):51-58. R827653 (2000)
    R827653 (2001)
    R827653 (Final)
    R827631 (2001)
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  • Other: AEM-Full Text PDF
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  • Journal Article Benoit JM, Gilmour CC, Heyes A, Mason RP, Miller CL. Geochemical and biological controls over methylmercury production and degradation in aquatic ecosystems. Biogeochemistry of Environmentally Important Trace Elements ACS Symposium Series 2003;835:262-297. R827653 (2001)
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    Supplemental Keywords:

    water, sediment, estuary, watersheds, coastal, bioavailability, mercury, Hg, methylmercury, MeHg, sulfate, sulfide., Scientific Discipline, Waste, Water, Ecosystem Protection/Environmental Exposure & Risk, Bioavailability, Hydrology, Contaminated Sediments, Remediation, Environmental Chemistry, Fate & Transport, Air Deposition, Ecology and Ecosystems, Mercury, fate and transport, contaminated sediment, soils, sulfur, biogeochemical cycling, methylmercury, watershed influences, terrestrial and aquatic fate, sulfide, atmospheric deposition, microbiological aspects

    Relevant Websites:

    http://www.cbl.umces.edu/~mason/welcome.html Exit
    http://www.acnatsci.org/research/anserc/index.html Exit
    Synthesis Report of Research from EPA’s Science to Achieve Results (STAR) Grant Program: Mercury Transport and Fate Through a Watershed (PDF) (42 pp, 760 K)

    Progress and Final Reports:

    Original Abstract
  • 2000 Progress Report
  • 2002
  • Final Report