Xiaodong Zhao, Final Exam – AER/I Seminar

About the event

Speaker:  Xiaodong Zhao
Group:  Xiaofeng Guo
Title: Thermodynamics and Recovery Studies of Critical Elements Under Ambient and Extreme Conditions

Abstract

The safe deposit of actinide elements are important to ensure public safety and advancing nuclear energy development, and zircon (MSiO4) has been regarded as a promising candidate to immobilize large actinides due to its low solubility, high compatibility, and excellent radiation resistance. Entropy plays an important role in estimating the long-term thermodynamic stability of zircon mineral, as the temperature in deep geological site can reach to 250 C in case of granite/tuff deposits. However, debates and discrepancies exist in the standard  for the entropy of coffinite (USiO4). To address that, adiabatic calorimetry and density functional perturbation theory (DFPT) calculations were utilised 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 trantion of CeSiO4, as surrogaet to PuSiO4. Even stetindite phase is less thermodynamically stable than scheelite phase after 8.35 GPa, lthough it can maintain up to 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 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.