Theoretical Insights into Selective Extraction of Uranium from Seawater with Tetradentate N,O-mixed Donor Ligands

The competition of uranium and vanadium ions is a major challenge in extracting uranium from seawater. In-depth exploration of the complexation of uranium and vanadium ions with promising ligands is essential to design highly efficient ligands for selective recovery of uranium. In this work, we systematically explored the uranyl and vanadium extraction complexes with three tetradentate N,O-mixed donor analogues including the rigid backbone ligands 1,10-phenanthroline-2,9-dicarboxylic acid (PDA, L) and 5-cyclopenta[2,1-:3,4-']dipyridine-2,8-dicarboxylate acid (L), as well as the flexible ligand [2,2'-bipyridine]-6,6'-dicarboxylate acid (L) using density functional theory (DFT). These ligands coordinate to the uranyl cation in a tetradentate fashion, while L and L act as tridentate ligands toward VO due to the smaller ionic radius of VO and larger cleft sizes of L and L. Bonding analyses show that the metal-ligand bonding orbitals of the uranyl complexes [UOL(CO)], [UOL(OH)], and [UOL(HO)] mainly arise from the interactions of the U 5f, 6d orbitals and N, O 2p orbitals. Because of the rigid structure and more suitable chelate ring size, the L ligand possesses a stronger complexing ability for uranyl ions than other ligands, while the L ligand has weaker binding affinity than L and L. All these ligands prefer to coordinate with the uranyl cation rather than vanadium ion, indicating the selectivity of these ligands to [UO(CO)] over HVO and HVO in seawater. This is mainly attributed to the metal ion size-based selectivity and structural preorganization of the ligands. These results demonstrate that the backbone of these ligands affect their extraction behaviors. It is expected that this work might prove useful in designing efficient ligands for uranium extraction from seawater.