Theoretical Study On The Separation Of Transuranic Actinides And Ln(Ⅲ)/An(Ⅲ) By Amide Ligands

With the development of green chemistry,the extraction of minor actinides from toxic high-level liquid waste has become a critical step affecting the development of nuclear energy.Currently,the process of separating and recovering uranium and plutonium elements in spent fuel reprocessing has been perfected.However,the similarity of atomic radii and chemical properties makes it difficult to separate minor actinides from spent fuel fission products such as the lanthanides,and the operation of separating transuranic elements from adjacent actinides is even more complicated and difficult.Therefore,we want to explore and reveal the bonding nature between transuranic actinide metal cations and ligands,and compare the extraction separation performance of different amide extractants for lanthanide and actinide metal ions from a thermodynamic point of view,in order to design efficient ligands for the separation of lanthanides and transuranic actinides.However,the high radioactivity and toxicity of lanthanides and actinides have seriously hindered researchers’experimental investigations,and therefore,their theoretical studies have gradually become an important basis for guiding experiments.In this paper,the relativistic density functional theory（DFT）method is used to conduct a theoretical study on the ability of amide ligands and their derivatives for the in-group separation of transuranic elements and the separation of trivalent lanthanide and actinide elements.The main theoretical results are as follows:（1）The extraction and separation ability of the amide podded ether extractant2-（2-amino-2-oxyethoxy）acetic acid（HL）for transuranic actinides were analyzed at the BP86/6-31G（d）/RECP theoretical level.In the acidic environment of dilute hydrochloric acid,the extraction of An（III）is controlled by a cation exchange mechanism,and during the extraction process,the complex[ML₂（H₂O）₂]⁺（M=Np,Pu,Am,Cm,Bk）is formed with the deprotonated ligand L^-.The electrostatic potential and molecular orbital analysis of the ligand L^-indicate that the donor O atom on the amide is the best active site for complexation.The structure of the optimally calculated complex[ML₂（H₂O）₂]⁺and the analysis of the bonding properties indicate that there is a weak covalent interaction between the ligand L^-and the trivalent transuranic ion,and its extraction ability gradually decreases from Np to Cm,and then increases from Cm to Bk.Meanwhile,thermodynamic analysis shows that the Pu（III）complex have the highest stability,i.e.,a stronger metal-ligand bond.Compared with cyclohexanone,the organic phase of n-dodecane has better extraction efficiency for actinide metal ions.（2）The extraction and separation ability of tetradentate extractants L_triazol,L_pyrazoland L_pyrrolof the phenanthroline group for trivalent transuranic actinides and lanthanide were systematically investigated at the BP86/6-31G（d）/RECP level of theory.The coordination number of the possible extracted complexes of 1:1 type（metal:ligand）An L（NO₃）₃and the 1:2 type[An L₂（NO₃）]²⁺(An=Np,Pu,Am,Cm;L=L_triazol,L_pyrazol,L_pyrrol)is 10 in a nitric acid environment.The electrostatic potential and natural atomic charge data of the ligands indicate that L_pyrazol,which contains a pyrazole substituent,has better electron giving ability and stronger coordination ability.Analysis of the bonding properties shows that the covalent interaction between the ligand and the actinides gradually decreases from Np to Cm due to the energetic degeneration of the 5f orbital of the actinide with the 2p orbital of the donor O/N atom in the ligand.Thermodynamic analysis shows that compared to the 1:2 type[An L₂（NO₃）]²⁺,the 1:1extraction product An L（NO₃）₃is more stable.Among them,L_pyrazolexhibited better extraction selectivity for the actinides.In addition,L_triazolshowed good separation performance for the in-group separation of transuranic elements under the conditions of n-dodecane,which may be due to the fact that the increase in the number of N atoms in the five-membered ring can appropriately improve the separation selectivity of the ligand for the actinides.（3）We compared the extraction and separation selectivity of the ligands containing amide or thioamide substituents and based on the backbone structures of phenanthroline or bipyridine rings for trivalent transuranic actinides and lanthanide at the PBE0/6-31G（d）/RECP theoretical level.The electrostatic potential and molecular orbital analysis of the ligands show that the cavity structure of the flexible bipyridine backbone with the amide substituents on both sides constituting the L_O-BPyis more capable of extracting the actinide metal ions.Structural optimization of the extracted products ML（NO₃）₃(M=Np,Pu,Am,Cm,Bk,Eu;L=L_O-Phen,L_O-BPy,L_S-Phen,L_S-BPy)was carried out.Geometric configuration and bonding analysis show that the covalent interactions between the transuranic actinides and the ligands gradually weaken from Np to Cm,and increase from Cm to Bk.The special electronic arrangement of the 5f orbitals of Cm（III）as half-filled（5f ⁷）makes the weakest stability of the complex.The solvation effect was fully considered in the thermodynamic analysis,and the results show that the ligand L_O-BPyhas good separation selectivity for Am/Eu.