Congratulations to Sooyeon Kim who successfully defended her thesis on Wednesday July 14th, 2021.
Advisor: Udo Becker
Geochemical reactions at the mineral-fluid interfaces are key factors controlling the mobility of energy-related contaminants. Adsorption/desorption, oxidation/reduction, and crystal growth/dissolution are examples of fundamental reactions that take place on mineral surfaces or are catalyzed by them. This dissertation explores these phenomena at a molecular and electronic level using two fundamental approaches: a basic science one to develop a first-principles theory of kinetics, and an applied science one that addresses energy-related and environmental reactions regarding radioactive elements and oil compounds.
The transport of actinides (An: U, Np. and Pu) significantly decreases by reduction from An(VI) to An(IV). In Chapter 2, the reduction kinetics of actinyls (AnO2+/2+) complexed with EDTA by ferrous iron (Fe2+) is investigated using quantum-mechanical calculations. The whole redox process is divided into four sub-processes to calculate the reaction rates for each sub-process. The results show that EDTA complexation does not prohibit but only slow down the reduction of actinyl-EDTA in the presence of ferrous iron with the OSC to ISC transition being the rate-limiting step. Since the An(IV)-EDTA complexes are soluble, EDTA complexation overall increases soluble actinides in solutions that increase their mobility in the environment.
Chapter 3 investigates the surface diffusion of uranyl and reductants on pyrite using quantum-mechanical calculations. Redox pairs on mineral surfaces come closer together by surface diffusion before transferring electrons, thereby strongly influencing the overall reaction kinetics. Energy curves along the diffusion paths are derived to calculate diffusion energy barriers and attempt frequencies to overcome these barriers; these two entities determine surface diffusion coefficients and diffusion rates. Also, interdependent multiple-particle diffusion can be approximated using screened Coulomb interactions due to near-surface water with reduced dielectric constant.
Wettability of mineral surfaces, i.e., whether they prefer to contact water (water-wet) or oil (oil-wet), is a key factor determining the mobility and distribution of oil compounds in groundwater and soil in oil reservoirs. Chapter 4 describes the process of wettability change on calcite surfaces at the molecular level using both quantum-mechanical and classical-mechanical calculations. Quantum-mechanical calculations show preferential adsorption of the hydroxyl group onto step edges of the calcite surface. Classical molecular dynamics simulations indicate a rapid separation of water and oil in their mixture with phenol molecules at the water/oil interface. And phenol molecules facilitate further accumulations of hydrocarbons close to the surface. Our results that the hydroxyl functional group is a good wettability modifier from water-wet to oil-wet agrees well with the experimental results using atomic force microscopy (AFM) described in Chapter 5. Asphaltene-removed crude oil adsorption onto the calcite surface significantly increased when asphaltene surrogates (phenol and benzoic acid) were pre-adsorbed in Chapter 5. Findings from Chapters 4 and 5 extend our fundamental understanding of wettability changes in the oil reservoir to design a better way of oil extraction or removal in the field.