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Bench Scale Experiments To Understand The Biogeochemical Factors For Production Of Methyl Mercury At The Sediment-Water Interface
Citation:
RANDALL, P. Bench Scale Experiments To Understand The Biogeochemical Factors For Production Of Methyl Mercury At The Sediment-Water Interface. Presented at 26th Annual International Conference on Soils, Sediments, Water, and Energy, Amherst, MA, October 18 - 21, 2010.
Impact/Purpose:
To study the complex behavior of Hg in natural sediments, as well as using bauxite as an in-situ sediment cap.
Description:
Mercury (Hg) is a naturally-occurring element that is ubiquitous in the environment. Although an effort has been made in recent years to decrease Hg emissions, historically-emitted Hg may be retained in the sediments of aquatic systems where they may be slowly converted to methylmercury (MeHg). Consequently, archived Hg in historically-contaminated sediments can result in high levels of Hg exposure for aquatic organisms, wildlife and human populations consuming fish from Hg-contaminated systems. It has been observed that under oxidizing conditions (such as in the freshwater environment), inorganic Hg is predominately present as Hg(OH)2 and HgOHCl, while under reducing conditions, the sulfur-based Hg species, (such as HgS), predominate. In addition to inorganic forms, Hg also can exist in organic forms, such as CH3Hg+ and (CH3)2Hg. MeHg is generally a product of biotic transformation of inorganic Hg by sulfate reducing bacteria (SRB) in the aquatic environment. Even if source control of contaminated wastewater is achievable, it may take a very long time, perhaps centuries, for Hg-contaminated aquatic systems to reach relatively safe Hg levels in both water and surface sediment naturally. Therefore, a better understanding of the Hg methylation mechanism is needed in order to more effectively clean up mercury contamination in sediments. Due to the lack of information regarding the sedimentary biogeochemical conditions, which promote mercury methylation, microcosm incubation studies have been studied to investigate the transformation of Hg to MeHg and the factors that affect MeHg production. Microcosm incubations have been designed to examine the influence of ORP (oxidation/reduction potential), organic carbon (as lactate) sulfate, and a bacterial amendment (SRB) on the production of MeHg. Sediments used in this study were obtained from a site previously known to be impacted by mercury contamination. Incubations will be analyzed for both the rate and extent of MeHg production. The production of dissolved gases as a proxy for metabolic utilization of carbon substrate will also be monitored. This project conducted laboratory experiments to study the complex behavior of Hg in natural sediments, as well as studying bauxite as an in-situ sediment cap. Identification of geochemical factors, which govern kinetics of MeHg production and degradation, offer valuable insight into the potential remediation approaches, such as strategic selection of appropriate sediment capping materials, which are effective on the time-scales of sedimentary mercury geochemical dynamics. Applications of these results will enable researchers to make predictive estimates of the time-scale MeHg production and degradation within areas of mercury contamination depending upon the geochemical condition of the sediments and sediment pore-waters, and to provide baseline information for the possible demonstration of an in-situ cap.