THERMODYNAMICS AND RECOVERY STUDIES OF CRITICAL ELEMENTS UNDER AMBIENT AND EXTREME CONDITIONS

Xiaodong Zhao

doi:10.7273/000006489

This thesis delves into the thermodynamics of minerals associated with critical elements, primarily focusing on actinides and rare earth elements. Safe disposal of actinides is important to ensure public safety and advancing nuclear energy development, and ceramic waste form such as zircon (MSiO4) has been considered as a promising candidate to immobilize large actinides due to its low solubility, high compatibility, high stability, and excellent radiation resistance. Both enthalpy and entropy are important in estimating the long-term thermodynamic stability of zircon mineral and ceramics, as the temperature in deep geological site can reach to 250 C in case of granite/tuff deposits. However, discrepancies exist in the thermodynamic parameters of zircon type materials, such as coffinite (USiO4), which prevents further prediction of its stability under relevant disposal conditions. To address that, adiabatic calorimetry and density functional perturbation theory (DFPT) calculations were used and determined the standard entropy values for coffinite (127.29 ± 1.27 J·mol-1·K-1) and stetindite (109.16 ± 1.09 J·mol-1·K-1). In addition to thermodynamic investigations, a DFT+U methodology was employed to reveal the mechanical stability and stetindite → scheelite phase transition of CeSiO4, which is a surrogate to PuSiO4. We found out that even stetindite phase is less thermodynamically stable than scheelite phase after 8.35 GPa, its transition does not occur till 12 GPa. A phonon spectra investigation by DFPT indicated the Eg1 mode is being softened with pressure and becomes imaginary after 12 GPa, which concluded that the phase transition is dynamically driven, instead of being thermodynamically driven. Besides the deep geological disposal, the interaction between rare earth elements (REE) and gibbsite was also investigated, which is an important reaction in the DOE Hanford waste tank site. Specifically, attention is given to the adsorption and doping behaviors of scandium, yttrium, and lanthanum, revealing the favorable incorporation of scandium into gibbsite through evidence of crystal lattice expansion, local octahedron perturbation, and DFT calculations on incorporation energies. Lastly, the work investigated the potential use of covalent organic frameworks (COFs) for uranium removal in nuclear wastewater treatment and environmental remediation. Through testing COF-316 and its functionalized derivatives, the study identified COF-316-AO as a good uranium removal candidate with notable selectivity and radiation resistance. All the insights from thermodynamic investigations and removal techniques could contribute to the field of nuclear waste management.

THERMODYNAMICS AND RECOVERY STUDIES OF CRITICAL ELEMENTS UNDER AMBIENT AND EXTREME CONDITIONS

Files and links (1)

Abstract

Metrics

Details