Theoretical Insights On Selective Separation Of Uranium From Seawater By N,O Donor Ligands

Uranium is the main raw material for nuclear reactor fuel.However,uranium resources on land are limited,and the uranium reserves in seawater are abundant.It is expected to alleviate the shortage of uranium resources on land by extraction of uranium from seawater.But the competitive adsorption between uranyl and vanadium ions is a major challenge in extracting uranium from seawater.An in-depth understanding of the complexation behavior of various functional groups with uranyl and vanadium ions is helpful for the design and research of highly selective functional groups to complexe uranium,improving the efficiency to develop high-efficient seawater uranium adsorbents.In this study,the structures and geometric properties of two kinds of N,O donor ligands are studied by the density functional theory（DFT）.The extraction complexes with uranyl and vanadium（V）were also systematically studied.The main contents and conclusions of this thesis are as follows: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））were investigated by quantum chemistry calculations.The extraction complexes with uranyl and vanadium（V）were also systematically studied.In the uranyl complexes,the amidoxime groups prefer to act asη²-coordinated ligands as the increase of amidoximes,and there exist substantial hydrogen bond interactions.Various bonding analyses show that the L¹ ligand possesses stronger binding affinity to UO₂²⁺,and the-C₆H₅and-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 L²and L³.According to the calculations of the vanadium（V）(VO₂⁺and VO³⁺)complexes with the L¹ligand,L¹is more likely to react with[H₂VO₄]^-and[HVO₄]^2-to form VO₂⁺complexes.Expectantly,thermodynamic analysis displays the 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.We systematically explored the uranyl and vanadium extraction complexes with three nitrogen-heterocyclic ligands including the preorganized 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.These ligands coordinate to uranyl cation in a tetradentate fashion.Bonding analyses show that the metal-ligand bonding orbitals of uranyl complexes mainly arise from the interactions of the U 5f,6d orbitals and N,O 2p orbitals,and[UO₂（L）（OH）]^-possesses stronger metal-ligand bonding relative to[UO₂（CO₃）L]^2-.The L¹and L²ligands have higher binding affinity for uranyl cation than the L³ligand.Thermodynamic analysis confirms that L¹ ligand has a strongest complexing ability for uranyl ions.The results of vanadium（V）complexes show that L¹ and L³ act as tridentate ligands toward VO₂⁺due to the small ionic radius of VO₂⁺and large cleft sizes of L¹ and L³.Comparing the thermodynamic analysis of uranyl and vanadium（V）complexes,it is found that these ligands prefer to coordinate with the uranyl cation rather than vanadium ion,mainly attributable to the metal ion size based selectivity of the ligands.These results demonstrate the selectivity of these two kind of ligands to[UO₂（CO₃）₃]^4-over H₂VO₄^-and[HVO₄]^2-in seawater.In addition,the energy required for the reaction of nitrogen heterocyclic ligands with[UO₂（CO₃）₃]^4-are lower than that of alkyl and amino functionalized amidoxime ligands.It is expected that this work might prove useful in designing efficient ligands for uranium extraction from seawater.