Grantee Research Project Results
Final Report: Understanding the Role of Sulfur in the Production and Fate of Methylmercury in Watersheds
EPA Grant Number: R827653Title: 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: Chung, Serena
Project Period: October 1, 1999 through September 30, 2002 (Extended to October 1, 2003)
Project Amount: $779,786
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 objectives of this research project were to examine the controls over mercury (Hg) methylation and methylmercury (CH3Hg) fate and transport and demethylation in aquatic systems and understand the role of sulfur, especially sulfide, in Hg methylation and CH3Hg fate and transport in watersheds. The specific objectives were to examine: (1) how Hg complexation with sulfide affects bioavailability of Hg to methylating microorganisms; (2) how Hg and CH3Hg complexation in solution and to the solid phase influenced their fate and transport; (3) how CH3Hg production and accumulation may change for a given change in loading of Hg, sulfur, and other constituents to ecosystems; and (4) the controls over demethylation in environmental matrices. Laboratory studies and modeling efforts to incorporate the factors controlling bioavailability, methylation, and CH3Hg fate and decomposition into an ecosystem framework also were performed. The role of ligands and Hg complexation, especially the role of sulfide, in influencing Hg methylation was examined in detail. The important relationships and factors influencing Hg methylation at the cellular level are illustrated in Figure 1. Neutral sulfide Hg complexes were shown to be important in controlling the rate of Hg methylation, using laboratory experiments, pure culture experiments, and modeling studies. For example, Figure 2 shows the relationship between the relative methylation rate and sulfide for a number of locations in the Florida Everglades, confirming that as sulfide increases, methylation rate decreases. The rate of change is similar to the rate of change in concentration of neutral sulfide complexes, confirming the hypothesis on the role of sulfide in Hg methylation. Overall, experiments have assessed how Hg complexation in solution and with the solid phase influences Hg methylation in sediments and, subsequently, how it influences the fate of the CH3Hg produced.
Figure 1. Conceptual Diagram Showing the Factors Important in Influencing Mercury Methylation at the Cellular Level
Figure 2. Effect of Sulfide on Hg Methylation Experiments at Three Sites in the Everglades
Summary/Accomplishments (Outputs/Outcomes):
To measure Hg speciation in situ in field samples, we have refined the technique of estimating bioavailability through measurement of the octanol-water partitioning of Hg that was previously used in laboratory studies at high concentration. By the use of clean techniques and other modifications, it now can be used in environments where Hg levels are in the pM range. Also, to improve the ability to understand the role of sulfide at low sulfide concentrations found in the environment, it was necessary to develop a new technique of measuring low levels of sulfide (nM-μM) in natural waters. This was accomplished during this research project.
Studies have examined the importance of reaction kinetics, in addition to thermodynamic controls, over Hg methylation as aquatic systems often are not at thermodynamic equilibrium. Laboratory projects focused on determining the binding constants for Hg and CH3Hg to solid phases, whereas field projects measured the distribution of Hg across a diverse range of ecosystems, and the relative methylation potential of these systems. Our results have shown that there is a relationship between measured short-term methylation rate upon spiking samples with Hg and the in situ CH3Hg concentration, as demonstrated by the data in Figure 3. This relationship can be demonstrated from consideration of thermodynamic relationships and first order kinetics. Furthermore, the relationship between Hg methylation and demethylation has been examined from a kinetic standpoint in a number of ecosystems and this has lead to insights into the relative bioavailability of inorganic Hg(II) and CH3Hg(II) to these microbial processes. For a number of estuarine ecosystems, it has been found that the magnitude of the methylation and demethylation rate constants is similar and related to environmental condition. Furthermore, the ratio of the rate constants is of the same order as the ratio of CH3Hg to total Hg in sediments, suggesting that both have similar bioavailability to the organisms responsible for methylation and demethylation. Although overall there is relatively good agreement between methylation rates measured by stable isotopes and by radioisotopes, this is not true for demethylation assays. Rate constants determined from stable isotope measurements are much higher than those determined using radiolabelled-C14 CH3Hg. The reason for this is unclear and is being investigated. The importance of microbial community structure in Hg methylation is another important variable that was investigated using studies across ecosystems and both spatially and temporally within an ecosystem. In summary, the overall focus of the project was to assess if and why added (“new”) Hg is more bioavailable than the existing Hg pool. This was investigated in a number of ecosystems ranging from wetlands, such as the Florida Everglades, to estuarine systems and lakes in temperate North America. A model that incorporates speciation and other factors to predict Hg bioavailability for methylation and CH3Hg fate is being developed.
Figure 3. Data From Estuarine Mesocosm Experiments. The relationship between the in situ CH3Hg concentration (x-axis) and the relative amount of CH3Hg produced during a short-term incubation after the addition of a mercury stable isotope spike (y-axis).
Journal Articles on this Report : 6 Displayed | Download in RIS Format
Other project views: | All 23 publications | 8 publications in selected types | All 7 journal articles |
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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) R827653 (2001) R827653 (Final) R827631 (2000) R827631 (2001) R827631 (Final) |
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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) R827653 (2001) R827653 (Final) R827631 (2000) R827631 (2001) R827631 (Final) |
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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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Heyes A, Miller C, Mason RP. Mercury and methylmercury in Hudson River sediment: impact of tidal resuspension on partitioning and methylation. Marine Chemistry 2004;90(1-4):75-89. |
R827653 (Final) |
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Heyes A, Mason RP, Kim EH, Sunderland E. Mercury methylation in estuaries:Insights from using measuring rates using stable mercury isotopes.MARINE CHEMISTRY2006;102(1-2):134-147. |
R827653 (Final) |
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Rearick MS, Gilmour CC, Heyes A, Mason RP. Measuring sulfide accumulation in diffusive gradients in thin films by means of purge and trap followed by ion-selective electrode. ENVIRONMENTAL CHEMISTRY2009;24(12):3043-3047 |
R827653 (Final) |
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Supplemental Keywords:
mercury, methylation, methymercury, complexation, bioavailability, reaction kinetics, thermodynamic controls, isotopes, radioisotopes estuarine ecosystems, microbial community,, Scientific Discipline, Water, Waste, Ecosystem Protection/Environmental Exposure & Risk, Hydrology, Bioavailability, Environmental Chemistry, Remediation, Contaminated Sediments, 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 aspectsRelevant 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.