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PILOT STUDIES OF IN-SITU BIO-TRANSFORMATION OF MERCURY-CONTAMINATED GROUNDWATER IN KAZAKHSTAN UTILIZING NATIVE BACTERIA
Citation:
ABDRASHITOVA, S. A., W. J. DAVIS-HOOVER, AND R. DEVEREUX. PILOT STUDIES OF IN-SITU BIO-TRANSFORMATION OF MERCURY-CONTAMINATED GROUNDWATER IN KAZAKHSTAN UTILIZING NATIVE BACTERIA. Presented at 9th Annual International In situ and On-Site Bioremediation Symposium, Baltimore, MD, May 07 - 10, 2007.
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 directed towards determining the feasibility of developing a biological filter, which when placed into the path 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 cold-tolerant bacteria make them promising candidates for developing in situ technologies. Pure culture experiments demonstrated the aerobic bacteria were capable of accumulating Hg inside thcir cells. These bacteria grew optimally at 28°C, were hindered at high temperatures (e.g. 3SnC), and grew nearly as well at low temperatures, e.g. 4°C, as they did at 28°C. 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 law 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 4°C to 28°C 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 immabilize mercury forming insoluble mercuric sulfides, although some SRB can also methylate mercury our cultures formed minimal amounts of MeHg when grown on acetate. Labaratory 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 cause the precipitation of mercury with little to no detectable MeHg formation and potentially lower the level of Hg in the groundwater to meet water quality standards. A facultative anaerabic bacterium isolated from the contaminated site produced H2S under anaerobic conditions when grown in the sulfate 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 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 Hg pollution. Thus HgCl2 can be effectively removed on support material colonized with our isolated aerobic or anaerobic bacteria. We have continued work to develop this system. Two objectives are to test pilot scale reactors to optimize the conditions for effective removal of mercury with limited formatian of dissolved or methylated mercury, and to conduct small scale field trials at the contaminated site using the native bacterial cultures.