Theoretical Insights on Improving Amidoxime Selectivity for Potential Uranium Extraction from Seawater.

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 (L 1 ), and its phenyl and methoxy derivatives (( Z )-3-amino- N' -hydroxy- N,N -dimethyl-2-naphthimidamide (L 2 ) and ( Z )-2-amino- N '-hydroxy-4-methoxy- N,N -dimethylbenzimidamide (L 3 )) by quantum chemistry calculations. In the uranyl complexes, the amidoxime groups prefer to act as η 2 -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 L 1 ligand possesses a stronger binding affinity to UO 2 2+ , and the -C 6 H 5 and -CH 3 O substituent groups seem to have no effect on the improvement of extraction ability. Thermodynamic analysis confirms that the L 1 ligand has a stronger extraction capability to uranyl ion compared to L 2 and L 3 . According to the calculations of the vanadium (V) (VO 2 + and VO 3+ ) complexes with the L 1 ligand, L 1 is more likely to react with [H 2 VO 4 ] - and [HVO 4 ] 2- to form VO 2 + 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.