RESUMO
Extraction of uranium from seawater is one of the important ways to solve the shortage of terrestrial uranium resources. Thereinto, the competition between uranyl and vanadium cations is a significant challenge in the commonly used amidoxime-based adsorbents for extracting uranium from seawater. An in-depth understanding of the extraction behaviors of modified amidoxime groups with uranyl and vanadium ions is one of the effective means to design and develop efficient adsorbents for selective uranium sequestration. In this work, we have designed and systematically investigated the alkyl and amino functionalized amidoxime, (Z)-2-amino-N'-hydroxy-N,N-dimethylbenzimidamide (L1), and its phenyl and methoxy derivatives ((Z)-3-amino-N'-hydroxy-N,N-dimethyl-2-naphthimidamide (L2) and (Z)-2-amino-N'-hydroxy-4-methoxy-N,N-dimethylbenzimidamide (L3)) by quantum chemistry calculations. In the uranyl complexes, the amidoxime groups prefer to act as η2-coordinated ligands as the amidoximes increase, and there exist substantial hydrogen bond interactions, which are different from the vanadium complexes. Various bonding analyses show that the L1 ligand possesses a stronger binding affinity to UO22+, and the -C6H5 and -CH3O substituent groups seem to have no effect on the improvement of extraction ability. Thermodynamic analysis confirms that the L1 ligand has a stronger extraction capability to uranyl ion compared to L2 and L3. According to the calculations of the vanadium (V) (VO2+ and VO3+) complexes with the L1 ligand, L1 is more likely to react with [H2VO4]- and [HVO4]2- to form VO2+ complexes. Expectantly, thermodynamic analysis displays a higher extraction capacity for uranyl ions than vanadium ions. Therefore, these alkyl and amino functionalized amidoxime ligands demonstrate high selectivity for uranyl over vanadium ions, which is mainly due to the coordination mode changes of these ligands toward vanadium in conjunction with the considerable hydrogen bonds in the uranyl complexes. These results are expected to afford useful clues for the design of efficient adsorbents for uranium extraction from seawater.
RESUMO
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, L1) and 5H-cyclopenta[2,1-b:3,4-b']dipyridine-2,8-dicarboxylate acid (L3), as well as the flexible ligand [2,2'-bipyridine]-6,6'-dicarboxylate acid (L2) using density functional theory (DFT). These ligands coordinate to the uranyl cation in a tetradentate fashion, while L1 and L3 act as tridentate ligands toward VO2+ due to the smaller ionic radius of VO2+ and larger cleft sizes of L1 and L3. Bonding analyses show that the metal-ligand bonding orbitals of the uranyl complexes [UO2L(CO3)]2-, [UO2L(OH)]-, and [UO2L(H2O)] 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 L1 ligand possesses a stronger complexing ability for uranyl ions than other ligands, while the L3 ligand has weaker binding affinity than L1 and L2. All these ligands prefer to coordinate with the uranyl cation rather than vanadium ion, indicating the selectivity of these ligands to [UO2(CO3)3]4- over H2VO4- and HVO42- 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.