Nuclear magnetic resonance studies of biofilm - porous media systems
Kirkland, Catherine Mullinnix
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Nuclear magnetic resonance (NMR) allows for in-situ non-invasive studies of opaque systems over a wide range of length and time scales, making the method uniquely suited to studies of biofilms and porous media. The research comprising this thesis uses NMR to explore biophysical, chemical, and transport properties within heterogeneous porous media systems at both a macro- and micro-scale. The macro-scale projects validate a low-field borehole NMR instrument to monitor field-scale environmental engineering applications like subsurface biofilms and microbially-induced calcite precipitation (MICP). Subsurface biofilms are central to bioremediation of chemical contaminants in soil and groundwater whereby micro-organisms degrade or sequester environmental pollutants like nitrate, hydrocarbons, chlorinated solvents and heavy metals. When composed of ureolytic microbes, subsurface biofilms can also induce calcite precipitation. MICP has engineering applications that include soil stabilization and subsurface barriers, as well as sealing of cap rocks and well-bore regions for carbon dioxide sequestration. To meet the design goals of these beneficial applications, subsurface biofilms and MICP must be monitored over space and time - a challenging task with traditional methods. The low-field borehole NMR tool recorded changes in the T 2 relaxation distribution where enhanced relaxation indicated biofilm accumulation in a sand bioreactor and in subsurface soil. Additionally, the tool was able to detect MICP in a sand bioreactor. The changed mineral surface of the sand lead to an increase in T 2 relaxation times. The complementary high-field NMR project investigated micro-scale internal structures and mass transport within biofilm granules used for wastewater treatment. Granular sludge, composed of spherical aggregates of biofilm grown without a carrier, is an innovative biological treatment method with the potential to vastly reduce the cost of wastewater treatment without sacrificing efficiency. Large gaps remain, however, in our understanding of the fundamental formation mechanisms and the factors that control granule activity and stability. Magnetic resonance imaging (MRI) identified heterogeneous internal structures within aerobic granular sludge where relaxation rates and diffusion coefficients vary. Ultimately, these results will help improve modeling for optimization of granular sludge wastewater treatment process design.