Bioavailability and Methylation Potential of Mercury Sulfides in Sediments
ER-1744
Objective
Methylmercury (MeHg) is the form of mercury that enters the food web from contaminated sediments. Thus, the production of MeHg by anaerobic bacteria and the bioavailability of inorganic mercury to these microbes are critical steps for MeHg bioaccumulation in benthic organisms. This research will investigate the relationship between Hg speciation and microbial methylation potential in anaerobic sediments, a relationship that remains poorly understood. The hypothesis is that kinetically limited mercury sulfide (HgS) mineralization reactions, rather than equilibrium pore water chemistry, control the concentration of bioavailable mercury to sediment bacteria that convert it to MeHg. Current models utilize equilibrium HgS chemistry to predict bioavailable Hg concentration. Such models, however, are flawed because they incorrectly overestimate the solubility of HgS minerals and also ignore the "aging" effects of mercury in sediments that reduce bioavailability over time. A known aging process for mercury in sediments is the precipitation of sulfide minerals (i.e., HgS(s) metacinnabar). Previous research has shown that nanoparticles of HgS can form as stable intermediates of slow precipitation reactions. These nanoparticles, especially amorphous phases, are inherently more soluble than bulk minerals and can be more bioavailable than larger particles.
The objective of this project is to investigate geochemical processes that control the bioavailability of mercury in contaminated sediments. In particular, researchers will quantify the microbial methylation potential of nanoparticulate HgS in relation to bulk scale HgS and dissolved HgS species. They hypothesize that nanocrystalline HgS is more bioavailable than bulk scale HgS(s) due to differences in solubility and the ability for nanoparticles to collect near the cell surfaces of methylating bacteria. The project seeks to establish a premise that links the "age" and chemical form of mercury in sediment pore water to the rate of MeHg formation. The kinetic data will be incorporated in a conceptual model describing the fate of mercury.
Technical Approach
Laboratory experiments will be performed to compare MeHg production from various forms of inorganic mercury by various cultures of methylating bacteria. The project also will investigate the formation and uptake of nanoparticulate HgS. Specific tasks include: (1) assess methylation rate of HgS nanoparticles by sulfate-reducing bacteria and compare rates to those for bulk metacinnabar; (2) investigate the mechanism of Hg delivery to the cell and accumulation of nanoparticles at the cell membrane; (3) evaluate mechanisms of nanoparticle stability, incorporating chemical complexities associated with sediment pore water chemistry; and (4) develop a mechanistic-based model that links environmental parameters (e.g., pore water chemistry) to MeHg potential in sediments. The research will involve biological methylation experiments that include sediments and microbial consortia obtained from sediments at Department of Defense (DoD) contaminated sites. The experimental results will be used to develop a model that will predict MeHg potential as a function of known environmental conditions and changes resulting from remediation.
Benefits
This project will improve the understanding of mercury bioavailability in sediments. The connection between Hg speciation and bioavailability is a knowledge gap that prevents implementation of low-cost and feasible remediation in Hg-contaminated DoD sites. With an accurate understanding of the chemical forms of mercury that are bioavailable to methylating organisms and its integration into risk assessment models, DoD will improve its ability to manage and restore contaminated sites through appropriate modifications that minimize bioavailable mercury, the formation of MeHg, and its subsequent uptake by benthic organisms. These modifications require accurate knowledge of the biogeochemical processes that accelerate the "aging" of mercury in sediments and decrease its bioavailability for the long term. (Anticipated Project Completion - 2013)
Points of Contact
Principal Investigator
Dr. Heileen Hsu-Kim
Duke University
Phone: 919-660-5109
Fax: 919-660-5219
Document Types
- Fact Sheet - Brief project summary with links to related documents and points of contact.
- Final Report - Comprehensive report for every completed SERDP and ESTCP project that contains all technical results.
- Cost & Performance Report - Overview of ESTCP demonstration activities, results, and conclusions, standardized to facilitate implementation decisions.
- Technical Report - Additional interim reports, laboratory reports, demonstration reports, and technology survey reports.
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- Workshop Report - Summary of workshop discussion and findings.
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