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GEOCHEMICAL FACTORS GOVERNING METHYL MERCURY PRODUCTION IN MERCURY CONTAMINATED SEDIMENTS
Citation:
Fimmen, R. L., R. Darlington, V. Lal, B. Sass, S. Chattopadhyay, AND P. M. RANDALL. GEOCHEMICAL FACTORS GOVERNING METHYL MERCURY PRODUCTION IN MERCURY CONTAMINATED SEDIMENTS. Presented at Battelle Conference - The Fifth International Conference On Remediation Of Contaminated Sediments, Jacksonville, FL, February 02 - 05, 2009.
Impact/Purpose:
To investigate the transformation of Hg to MeHg and to investigate the factors that affect MeHg production.
Description:
Bench scale experiments were conducted to improve our understanding of aquatic mercury transformation processes (biotic and abiotic), specifically those factors which govern the production of methyl mercury (MeHg) in sedimentary environments. The greatest cause for concern regarding mercury contamination is bioaccumulation into higher trophic levels in aquatic food webs, from bacteria to plankton, micro- and macro-invertebrates, and ultimately to herbivorous and piscivorous fish. The dominant form of mercury in the fatty tissue of fish is MeHg, a potent mammalian neurotoxin even at very low concentrations. MeHg is formed mainly in suboxic and anoxic sediments by the interaction of inorganic Hg with organic matter and microorganisms. The conversion of Hg to MeHg is widely accepted to be governed by the action of sulfate reducing bacteria (SRB), which enzymatically catalyze the methylation of inorganic Hg. The exact mechanism of methylation is not known, but it is believed that during heterotrophic organic carbon metabolism, passive uptake of inorganic Hg occurs through the cell membrane of methylating bacteria. Due to the lack of information regarding the sedimentary biogeochemical conditions which promote mercury methylation, microcosm incubation studies were conducted to investigate the transformation of Hg to MeHg and the factors that affect MeHg production. Microcosm incubations were designed to examine the influence of ORP (oxidation/reduction potential), organic carbon (as lactate), sulfate, and 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 were analyzed for both the rate and extent of MeHg production. Methylation rates were estimated by analyzing MeHg and Hg after 48 hours, 13 d, and 27 d and 170 d. The production of dissolved gases as a proxy for metabolic utilization of carbon substrate was also monitored. In all treatments amended with sulfate and sulfate reducing bacteria, MeHg (97 ng/g-sediment dry weight (DW)) was produced after only 48 hours of incubation. MeHg concentrations then increased to 122 ng/g-sediment DW after 13 d. Concentrations of sedimentary MeHg were then observed to decline; to 110 ng/g-sediment DW and 102 ng/g after 27 and 170 d, respectively. Concentrations were found to be statistically significantly different at a 95% confidence interval. Identification of geochemical factors which govern kinetics of methyl mercury production and degradation offer valuable insight into 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 of MeHg production and degradation within areas of mercury contamination depending upon the biogeochemical condition of the sediments and sediment pore-waters.