Novel phenanthroline-derived pyrrolidone ligands for efficient uranium separation: Liquid-liquid extraction, spectroscopy, and molecular simulations

A challenge in nuclear waste remediation is the construction of extraction systems with exceptional stability and remarkable selectivity under harsh conditions, to achieve efficient uranium separation from nuclear wastewater. In this work, two novel N, O-hybrid extractants, L1 and L2 attaching two or one pyrrolidone groups to the phenanthroline skeleton were synthesized and applied in uranium extraction. Solvent extraction tests showed that these ligands in 3-nitrobenzotrifluoride can extract uranium with fast kinetics, high selectivity over twelve competing ions, and facile stripping. Slope analysis indicated that each uranyl ion binds to one ligand during extraction, and the extraction conforms to the cation exchange mechanism. The effect of temperature on uranium extraction was also carried out to determine the thermodynamic parameters. Notably, outstanding radiation stability was demonstrated for these extractants even after being exposed to 500 kGy γ ray irradiation. In addition, NMR titration and DFT calculations were performed to understand the complexation behaviors between ligands and uranyl ions. This study reveals the potential of these two phenanthroline-based extractants for the use in uranium separation from nuclear wastewater.