| Summary: | Development of technology for sustainable, low-carbon, base-load power generation is one of the great challenges for this generation of scientists. While significant developments have been made for using renewable sources, namely solar and wind-power, they are intrinsically intermittent energy sources and unable to reliably meet peak energy demands. Nuclear energy remains the only mature, low-carbon source capable of sustained base-load power generation.
This work was performed to address challenges at both front and back ends of the nuclear fuel cycle. Chapter I provides an introduction to the nuclear fuel cycle and summarizes the current state of technology. Chapter II discusses work performed by density functional theory (DFT) calculations to improve binding of uranium to amidoxime, the current state-of-the-art sorbent group for extracting uranium from seawater. It was observed the bond strength is directly related to the electron donation from the substituent adjacent to the oxime functionality, providing a convenient means to tune bond strength and a route to develop new sorbent functionalities. In Chapter III, X-ray absorption fine structure spectroscopy (XAFS) is used to characterize the coordination environment of uranyl as bound to amidoxime-functionalized polymer fibers. This constitutes the first direct investigation of the uranyl coordination environment as bound to a fiber, and reveals the anticipated eta 2-coordination motif proposed by small molecule studies does not contribute significantly to uranyl extraction. Chapter IV delineates the first instance a metal-organic framework (MOF) was used for extracting uranium from seawater. By functionalizing with pendant phosphate groups, cooperative sorbent interactions create a binding pocket for uranyl, resulting in sorption capacities in excess of 200 mg uranium g-1 sorbent. DFT calculations were used to investigate these interactions, and cooperative binding between two phosphates was found to be most thermodynamically favorable.
To address the back side of the nuclear fuel cycle, a novel, thermodynamically-driven ligand exchange process was developed to transform MOF templates into highly porous and stable inorganic sorbents for extraction of radionuclides, as presented in Chapter V. These new materials out-performed the current state-of-the-art sorbent for high level waste decontamination and displayed remarkable affinity for extraction of Sr, U, Np, and Pu from highly caustic media. Select sorbents were also investigated for partitioning of lanthanides from actinides, with the best performing material possessing distribution coefficients in excess of 1x106 mL g-1. Finally, the MOF-templated materials were used to decontaminate cooling water simulant from the Fukushima disaster. The second generation of these MOF-templated materials are presented in Chapter VI. While not directly related to the nuclear fuel cycle, a series of sulfide-based materials were developed and used to extract the heavy metals Hg, Cd, and Pb from water. The best performing material extracted over twice its mass in Hg, more than doubling the uptake of the best performing sorbent previously in the literature. Investigation of the Hg coordination environment by XAFS suggests three sulfides bind to each Hg within the cavities of this exciting new material.
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