PhD Oral Exam - Samantha Wilcox, Civil Engineering

When studying for a doctoral degree (PhD), candidates submit a thesis that provides a critical review of the current state of knowledge of the thesis subject as well as the student’s own contributions to the subject. The distinguishing criterion of doctoral graduate research is a significant and original contribution to knowledge.

Once accepted, the candidate presents the thesis orally. This oral exam is open to the public.

Abstract

Carbonate precipitation relies on the nucleation of stable crystals. These crystals include calcium carbonate (CaCO3; calcite, aragonite, vaterite), magnesium carbonate (MgCO3; magnesite), calcium magnesium carbonate (CaMg(CO3)2; dolomite), and iron (II) carbonate (FeCO3; siderite). The focus of this work is the evaluation of chemical and biochemical reactions using simple, easily adoptable reaction designs. The study analyzes carbonate precipitation for sequestration of gaseous carbon dioxide (CO2) and stabilization of hazardous, real-world slag materials. Chemically, headspace CO2 was removed from complex, alkaline slag materials through solubility trapping, adsorption, and precipitation. Up to 99.6% of 10% v/v CO2 was sequestered and precipitation of calcite, magnesite, and siderite was confirmed through x-ray-diffraction (XRD) and field emission scanning electron microscopy (FESEM) equipped with energy-dispersive x-ray spectroscopy (EDS). Notably, the physiochemical characteristics of the slag material, specifically particle size, impacted the efficacy of carbon sequestration and the impact of initial moisture conditions on CO2 removal. This was due to diffusion barriers inhibiting transport throughout the material, which also impacted the efficacy of microbially induced carbonate precipitation (MICP) to slag materials. Only one of the slag materials achieved biocementation via MICP with precipitated calcite, dolomite and magnesite. Uniquely, calcite precipitated homogenously around the loose particles causing binding, while magnesite created a surface crust exhibiting strong resistance to physical degradation but limited chemical stabilization of metal(loid) contaminants. In addition to the impact of particle size affecting hydrology, increased pH and initial magnesium concentration were found to positively impact cementation. Carbon sequestration was further analyzed using enzymatic (urease and carbonic anhydrase) Sporosarcina pasteurii in a batch reactor to maximize carbon capture and storage (CCS) of CO2 as carbonate crystals. Carbon sequestration was maximized using the response surface methodology and achieve 94.3% CO2 removal at 20% v/v resulting in 125% uptake as vaterite crystals. The results of the current study are promising for the duality of carbonate precipitation for carbon sequestration and solidification/stabilization. The comprehensive evaluation of chemical and biochemical carbonate precipitation successfully describes the intricate mechanisms providing a better understanding of its applicability to the field of environmental engineering.