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IN SITU BIO TRANSFORMATION OF MERCURY-CONTAMINATED GROUNDWATER IN KAZAKHSTAN UTILIZING NATIVE BACTERIA
Citation:
ABDRASHITOVA, S. A., S. JACKMAN, W. J. DAVIS-HOOVER, AND R. DEVEREUX. IN SITU BIO TRANSFORMATION OF MERCURY-CONTAMINATED GROUNDWATER IN KAZAKHSTAN UTILIZING NATIVE BACTERIA. Presented at Fifth International Conference on Remediation of Chlorinated and Recalcitrant Compounds, Monterey, CA, May 22 - 25, 2006.
Impact/Purpose:
to present information
Description:
Several regions in the Republic of Kazakhstan and throughout the former USSR are contaminated with mercury resulting from industrial releases. Our studies conducted under the ISTC K-756 Project were directed towards determining the feasibility of developing a biological filter, which when placed into the path of the groundwater, would remove the soluble mercury contaminant. The filter would combine biological sequestration by the introduced bacteria, chemical sorption by the support material, and further sequestration when the filter is colonized by the indigenous microbial community. This project resulted in the isolation and characterization of aerobic, facultative anaerobic and anaerobic sulphate-reducing bacteria from soils and sediments taken from contaminated areas on the outskirts of Pavlodar. These isolates were tolerant to 0.005 mM, 0.02 mM and 0.05 mM concentrations of HgCl2. Several properties of these psychrophilic bacteria (which to our knowledge have been isolated for the first time) make them promising candidates for developing in situ technologies. Pure culture experiments demonstrated the aerobic bacteria were capable of accumulating Hg inside their cells. These bacteria grew optimally at 28oC, were hindered at high temperatures (e.g. +35oC), and grew nearly as well at low temperatures, e.g. 4oC, as they did at 28oC. In addition, the strains were found to harbor plasmids and contain genetic determinants for mercury resistance. Thus, these bacteria are adapted to the high mercury concentrations and low temperatures prevalent at the site. We modeled the process for removing Hg from groundwater using these aerobic bacteria immobilized on claydite and other media. These bacteria absorbed Hg within temperatures ranging from 4oC to 28oC and lowered Hg concentrations in the culture to what would be acceptable levels for water quality. Sulfate-reducing bacteria (SRB) release hydrogen sulfide during growth. Sulfide can effectively immobilize mercury forming insoluble mercuric sulfides, although some SRB can also methylate mercury. Our cultures formed minimal amounts of MeHg when grown on acetate. Laboratory studies were carried out to simulate the clean-up of Hg contaminated water using SRB growing with acetate and a flow rate approximating conditions at the contaminated site. The results showed that our SRB isolates could effectively immobilize mercury with little to no detectable MeHg formation and lower the level of Hg in the groundwater to meet water quality standards. A facultative anaerobic bacterium isolated from the contaminated site produced H2S under anaerobic conditions when grown in thiosulfate media in the presence of HgCl2. These conditions could lead to the formation of insoluble mercuric sulfide precipitates. However, although no methyl mercury was formed, the culture broth retained relatively high levels of residual dissolved Hg compounds. This suggests the formation of soluble Hg polysulfides which is important for understanding the ecology of mercury mobility and identifying conditions that could increase the risk of pollution. Thus HgCl2 can be effectively removed on support material colonized with our isolated aerobic or anaerobic bacteria. We will continue work to develop this system. Two objectives are to test pilot scale reactors to optimize the conditions for effective removal of mercury with limited formation of dissolved or methylated mercury, and to conduct small scale field trials at the contaminated site using the native bacterial cultures.