The Separation Of Actinides For Lanthanides By N-donor Ligands: A Theoretical Study

The separation of actinides from lanthanides is a very important issue for the processing of spent fuels and the development of nuclear power. In this paper, several N-donor ligands are investigated by the density functional theory(DFT) systematically. Here are some important conclusions:(1) According to the investigation of the complexes of PhenBHPPA with Am(Ⅲ) and Eu(Ⅲ), the ligand is coordinated with metal ions in hexane dentate in all of the considered complexes. And among them, the Gibbs free energy of the reaction with [ML(NO3)2]+ formed is more negative than other reactions. Thus, it may be the most probable product extracted from a two phase extraction system. The QTAIM analysis and WBIs of the M-N and M-O bonds suggest them to be ionic bonds dominated, and exhibit weakly covalent bond character. Besides, the AmN bonds are stronger than the corresponding Eu-N bonds. Thermodynamic analysis proved the ligand is capable of separating actinides from lanthanides. More importantly, it has been found that nitrates play a very important role during the extraction process.(2) In the study of DHDIPhen, BQPhen, Ph2-BTPhen and CyMe4-BTPhen complex with Am(Ⅲ) and Eu(Ⅲ), the hardness of these ligands exhibit a trend form the softest to the hardest:Ph2-BTPhen < CyMe4-BTPhen < BQPhen <DHDIPhen, and their separation ability yield an order, from the smallest to the biggest, as follows: Ph2-BTPhen > Cy Me4-BTPhen > BQPhen >DHDIPhen. Therefore, we can conclude that the hardness of ligand may help to determine their selectivity toward actinides over lanthanides. It has also been found that the cavity of ligand play an important role in the complexation of ligand with metal ions.NPA(natural population analysis)reveal that the d orbital of metal ions accepted more electrons from ligands during the metal-ligand comlexation. But PDOS(Partial Density of States) reveal that the interaction between d orbital of metal ions and 2p orbital of N donors exhibits an antibonding character. The solvent effect is considered by COSMO model. According to the calculated Gibbs free energy, ligand protonation have an apparent influence on the complexation of lignads with metal ions, but it does not change the selectivity of these ligands. The 1:2 type extraction complexes, [ML2(NO3)]2+, have also been studied. The Gibbs free energy of these extraction reactions are much more positive than the reactions with 1:1 type complexes formed. This could be attributed to the dissociation of one NO3-from[M(NO3)2(H2O)4]+ and the decreased number of NO3-in[ML2(NO3)]2+ compared to ML(NO3)3.