Thesis (Ph.D)--Earth and Atmospheric Sciences, Georgia Institute of Technology, 2010. / Committee Chair: Taillefert, Martial; Committee Member: DiChristina, Thomas; Committee Member: Sobecky, Patricia; Committee Member: Van Cappellen, Philippe; Committee Member: Webb, Samuel. Part of the SMARTech Electronic Thesis and Dissertation Collection.

Uranium contamination of soils and groundwater at Department of Energy facilities across the United States is a primary environmental concern and the development of effective remediation strategies is a major challenge. Bioremediation, or the use of microbial enzymatic activity to facilitate the remediation of a contaminant, offers a promising in situ approach that may be less invasive than traditional methods, such as pump and treat or excavation. This study demonstrates for the first time the successful biomineralization of uranium phosphate minerals as a result of microbial phosphatase activity at low pH in both aerobic and anaerobic conditions using pure cultures and soils from a contaminated waste site. Pure cultures of microorganisms isolated from soils of a low pH, high uranium- and nitrate-contaminated waste site, expressed constitutive phosphatase activity in response to an organophosphate addition in aerobic and anaerobic incubations. Sufficient phosphate was hydrolyzed to precipitate 73 to 95% total uranium as chernikovite identified by synchrotron X-ray absorption spectroscopy and X-ray diffraction. Highest rates of uranium precipitation and phosphatase activity were observed between pH 5.0 and 7.0. Indigenous microorganisms were also stimulated by organophosphate amendment in soils from a contaminated waste site using flow-through reactors. High phosphate concentrations (0.5 to 3 mmol L-1) in pore water effluents were observed within days of organophosphate addition. Highest rates of phosphatase activity occurred at pH 5.5 in naturally low pH soils in the presence of high uranium and nitrate concentrations. The precipitation of uranium phosphate was identified by a combination of pore water measurements, solid phase extractions, synchrotron-based X-ray spectroscopy, and a reactive transport model. The results of this study demonstrate that uranium is biomineralized to a highly insoluble uranyl phosphate mineral as a result of enzymatic hydrolysis of an organophosphate compound over a wide range of pH, in both aerobic and anaerobic conditions, and in the presence of high uranium and nitrate concentrations. The nonreductive biomineralization of U(VI) provides a promising new approach for in situ uranium bioremediation in low pH, high nitrate, and aerobic conditions that could be complementary to U(VI) bioreduction in high pH, low nitrate, and reducing environments.

Aquifers are an important storage location and source of fresh groundwater. They may become polluted by a number of contaminants including mobile divalent radionuclides such as strontium-90 which is a byproduct of uranium fission. A method for remediating such divalent radionuclides is sequestration through co-precipitation into calcium carbonate. Calcium carbonate precipitation occurs naturally but can be enhanced by the use of ureolytic microorganisms living within the aquifer. The microbial enzyme urease cleaves ammonia from urea (added as a stimulant to the aquifer) increasing the pH and subsequently pushing the bicarbonate equilibrium towards precipitation. Laboratory experimentation is necessary to better predict field scale outcomes of remediation that is driven by ureolytic calcium carbonate co-precipitation. To aid in such laboratory experiments, I constructed two ureolytic organisms which contain green fluorescent protein (GFP) so that the location of the microbes in relation to media flow paths and precipitation can be viewed by microscopy in a 2- dimensional porous medium flow cell reactor. The reactor was operated with a parallel flow regime where the two influent media would not promote microbially induced calcium carbonate precipitation until they were mixed in the flow cell. A demonstration study compared the results of parallel flow and mixing in the reactor operated with and without one of the GFP-containing ureolytic organisms. The growth and precipitation of calcium carbonate within the reactor pore space altered flow paths to promote a wider mixing zone and a more widely distributed overall calcium carbonate precipitation pattern. This study will allow optimization of remediation efforts of contaminants such as strontium-90 in aquifers. / Graduation date: 2012