Studies of Adsorbents for Actinide Separations Relevant to the Nuclear Fuel Cycle

Our planet is experiencing unprecedented warming due to levels of anthropogenic CO2 in the atmosphere never before attained. Global climate change can only be addressed through a combination of policy, education, and technological effort. Central to this is the rapid adoption of energy sources which emit much less carbon dioxide than conventional sources such as coal and natural gas. Nuclear energy remains the only mature, low carbon technology capable of baseload power generation and represents our best chance at decreasing emissions without drastically lowering living standards. Chapter 1 introduces the very basics of the nuclear fuel cycle while emphasizing the need for a robust uranium supply in the front end. A brief survey of previous work sets the basis for a discussion on the design, synthesis, and testing of advanced materials for actinide separations found in later chapters. Chapter 2 introduces the bifunctional chelator, a ditopic amidoxime/carboxylic acid functionalized diaryl ether molecule identified by computational work to exhibit ultrahigh uranium affinity.1 Uranium isotherms indicate an equilibrium capacity of 553 mg U g-1 ligand, one of the highest reported to date. The next chapter further develops the ideas presented in Chapter 2. Chapter 3 discusses the synthesis of bis-amidoxime functionalized ligands2 as well as their incorporation into robust polymer materials. These polymers show high selectivity for U over V achieving a 1:1 molar uptake of U:V after 37 days in seawater simulant. Raman spectroscopy used to gauge the extent of ligand-UO22+ binding finds a dramatic redshift of the ν1 U═O stretching frequency suggestive of strong amidoxime binding in the equatorial sphere.3 The following chapter provides details for in silico work4 investigating the origins of selectivity within the functionalized polymer materials. Density functional theory is used to ascertain the geometrically optimized structures of functionalized ligands with the nuclear energy relevant isotopes 90Sr, 137Cs, and 233U. Radionuclide uptake studies confirm the trend born from calculations—U can associate with bis-amidoxime ligands under a broad range of pH. Uptake of Sr is negligible and Cs is less than 30% under all conditions. Chapter 5 introduces the poly(HIPE), a new adsorbent platform with potential to dramatically increase efficiencies in industrial-scale actinide separations. Several amidoxime-functionalized, crosslinked poly(styrene) based poly(HIPEs) are described and characterized via FTIR, N2 uptake, and SEM. Separation experiments show a reversal in conventional trends for amidoxime-based materials—the poly(HIPEs) display remarkable selectivity for Th over U over a pH range relevant to groundwater decontamination. High energy X-ray scattering (HEXS) data provide insight into the speciation of the bound metal providing evidence for the presence of Th oligomers bound to the polymers. This observation is used to inform upon the experiments presented in the concluding chapter. The last chapter describes work on a series of benzamidoxime functionalized poly(HIPE) materials for Th, U, and Pu separations. A summary and outlook into proposed work in this field is provided towards the end of this chapter