Theoretical Insights Into The Selective Separation Of Lanthanides And Actinides By Hydrophilic Ligands

The reprocessing of spent fuel is one of the severe challenges facing the nuclear energy industry,in which the separation of trivalent actinides（An（Ⅲ））and lanthanide（Ln（Ⅲ））elements is very important.Theoretical calculations can be used as effective and convenient compensation methods to study the complexation and separation mechanisms of An（Ⅲ）over Ln（Ⅲ）due to the strong radiotoxicities of An（Ⅲ）which demand well equipped experimental infrastructures.The main results are summarized as follows:（1）We designed four hydrophilic sulfonated ligands based on the phenanthroline and bipyridine skeleton to elucidate the effects for the Am（Ⅲ）/Eu（Ⅲ）separation of sulfonation,sulfonated groups and the rigidity and flexibility of skeletons.We explored the complexation behavior and selectivity of these four hydrophilic sulfonated ligands for Am（Ⅲ）/Eu（Ⅲ）separation using quasi-relativistic density functional theory.The M-N_Land M-O_Lbonds in the sulfonated[ML（NO₃）₃]complexes are longer compared to the corresponding hydrophobic non-sulfonated according to the previous results,probably because that the introduction of sulfonated groups increase steric hindrance of the[ML（NO₃）₃]complexes.Properties analysis and thermodynamic analysis show that all four ligands have Am（Ⅲ）/Eu（Ⅲ）separation ability.The tetrasulfonated ligand L²has the strongest complexing ability to metal ions,while the disulfonated ligand L¹has the highest selectivity to Am（Ⅲ）.The results suggest that the covalent difference between the Am（Ⅲ）/Eu（Ⅲ）complexes may be the main factor in separation of Am（Ⅲ）/Eu（Ⅲ）ligands.Moreover,the introduction of sulfonate increases the Am（Ⅲ）/Eu（Ⅲ）selectivity of the ligand.（2）The preferred ligands containing only C,H,O,N atoms have completely incinerability,which reduces secondary pollution.Recently,a type of hydrophilic hydroxylated ligands have been proved to have excellent back-extraction capability for An（Ⅲ）/Ln（Ⅲ）separation.In this work,in order to explore the influence of different bridging group of the three ligands（the skeleton of pyridine L^a,bipyridine L^b,and phenanthroline L^c）on the separation of An（Ⅲ）/Ln（Ⅲ）,we investigated the complexing ability and selectivity towards Am（Ⅲ）over Eu（Ⅲ）with three hydrophilic hydroxylated triazolyl-based ligands using scalar relativistic density functional theory.The results show that all the M-N_trzlbonds are shorter than the M-N_pybonds in the same complex,indicating that the N_trzlhas a stronger coordination ability to the metal ions.The analysis of bonding nature shows that the Am-L bond has a stronger interaction in the same type of complexes.N_trzlhas a stronger coordination ability to metal ions,which is consistent with the result of bond length analysis.Thermodynamic results demonstrate that the ligands with bipyridine and phenanthroline as the bridging group have the same complexing ability,while the ligand with pyridine has higher Am（Ⅲ）/Eu（Ⅲ）selectivity.