Theoretical Study of Hydroxyl- and Amino-substituted Amidoxime Ligands for Extraction of Uranium from Seawater

As the main fuel for the operation of nuclear power plants, uranium is mainly supplied through terrestrial mining. However, terrestrial uranium resources are insufficient and unevenly distributed, and the mining process is prone to environmental pollution. In contrast, seawater contains about 4.5 billion tons of uranium, which is 1000 times the total amount of terrestrial uranium resources. If utilized effectively, it could meet the demand for nuclear energy for thousands of years. However, it is extremely difficult to extract uranium from seawater. At present, the most effective and economical method for extracting uranium from seawater is the adsorption method, and the key lies in the development of highly selective, low-cost, simple and durable adsorbent materials. The amidoxime ligands have attracted extensive attention in the field of uranium extraction from seawater because of their better coordination capacity to uranyl cations. It was found that the introduction of hydroxyl and amino groups into amidoxime ligands could improve their adsorption capacity for uranyl cations. In order to investigate the extraction mechanism of hydroxyl- and amino-substituted amidoxime derivatives with uranyl cations, the present work systematically investigates the structures, bonding properties, and thermodynamic stabilities of four amidoxime ligands (HL1: N',3-dihydroxypropionamidine; HL2: 3-amino-N'-hydroxypropionamidine; HL3: N',2-dihydroxypropio- namidine; HL4: 2-amino-N'-hydroxypropionamidine) and its mono-, di-, and tri-substituted uranyl complexes by density functional theory (DFT). The results show that the presence of hydrogen bonding enhances the stability of the uranyl complexes, and the L-2(-) ligand has stronger covalent interaction with the uranyl cations compared to the other three ligands. However, the relatively high dissociation energy of the HL2 ligand leads the HL1 ligand to be more susceptible from substitution reactions with [UO2(CO3)(3)](4-) compared to HL2. Comparing with unmodified amidoxime (HAO) ligands, HL1 may be a potential ligand that can be applied to seawater uranium extraction. The present work provides theoretical clues for the design and development of adsorption groups for efficient seawater extraction of uranium.