Reactive transport in biofouled and biomineralized porous media
Schultz, Logan Nicholas
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The geologic subsurface environment contains regions of high porosity where fluid flows both naturally and during engineered technologies such as carbon sequestration, enhanced oil recovery, and bioremediation of contaminants. In these porous media regions, microbes can have both desirable and undesirable effects on the hydrodynamics and fluid chemistry by inducing the formation of microbial aggregates, which can include extracellular polymeric substance and abiotic particles such as mineral precipitates. While not the focus of this research, these analyses are likewise applicable to biomatter control scenarios in filtration systems and other industrial reactors with a high surface area to volume ratio. Microbially-induced ureolytic calcium carbonate precipitation has been suggested as a means to mitigate leakage from geologic CO 2 sequestration sites and as a means to immobilize divalent contaminants such as strontium-90 in remediation scenarios. In this process, microbes hydrolyze urea, increasing the solution pH, generating carbonate ions, and ultimately shifting the saturation state of the fluid and leading to solid calcium carbonate (CaCO 3) formation in a calcium-rich environment. Experiments were conducted to assess the distribution and effects of biofouling and biomineralization in two-dimensional flat plate reactors with 1mm pore spaces simulating a tortuous porous media environment. In biomineralization experiments, calcium carbonate was formed under flow conditions, and strontium was effectively immobilized within the crystal lattice, suggesting the applicability of subsurface biotechnical applications utilizing this technology. Image, residence time distribution, and piezometer analyses of biofouling experiments quantified porosity and hydraulic conductivity reductions. Biofilms were grown under constant flow and head conditions and were shown to be more channeled and evenly distributed along the flow path in constant flow conditions. Biofilms were challenged with chlorinated bleach, which temporarily increased the hydraulic conductivity, yet failed to remove significant biofouling unless coupled with significant fluid shear. In situ methods utilizing stereo and confocal microscopy were developed to visualize and quantify the distribution of biomatter formation and analyze the biological environment at the surface of bio-induced minerals.