Kinetics of calcite precipitation by ureolytic bacteria under aerobic and anaerobic conditions
Parks, Stacy Leigh
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Carbonate precipitation is a natural phenomenon with a great importance in many chemical and engineering applications. Precipitation can be induced by bacteria as a by-product of common microbial processes, such as ureolysis. In this process, bacteria hydrolyze urea through a series of reactions which raise the pH of the system. In the presence of calcium ions, this rise in pH shifts the saturation state of the system, allowing for solid calcium carbonate (CaCO 3) to form. The use of these bacteria in biotechnical applications is appealing because urea is a fairly inexpensive substrate, and ureolytic bacteria are common in soil and aquatic environments. Bacteriogenic mineral plugging is an innovative use for this process. This technique controls subsurface fluid movement through the reduction of porosity and permeability of geologic formations, such as oil wells and aquifers. A potential use of this technology is in geologic carbon sequestration, which involves capturing CO 2 and storing it underground in deep saline aquifers. The goal of this project is to determine the kinetics of urea hydrolysis and CaCO 3 precipitation for use in the deep subsurface to mitigate potential leakage pathways of sequestered CO 2. To achieve this goal, three species of ureolytic bacteria, S. pasteurii, B. sphaericus strain 21776, and B. sphaericus strain 21787, were grown in batch systems under static conditions. Kinetic analysis was performed on the data gathered in these experiments. Due to the potential lack of oxygen in the deep subsurface, experiments using S. pasteurii were also carried out under anaerobic conditions. Because of the potential need to manipulate the rate of CaCO 3 precipitation to allow maximum distribution in the deep saline aquifers, the rates of urea hydrolysis and CaCO 3 precipitation among species and between aerobic and anaerobic conditions were compared. All three species studied were capable of inducing calcite precipitation. B. sphaericus strain 21776 exhibited the highest rate coefficient for both ureolysis and CaCO 3 precipitation, while B. sphaericus strain 21787 showed the lowest. S. pasteurii is capable of hydrolyzing urea and inducing calcite precipitation in anaerobic environments, although growth in these environments could not be shown conclusively.