The Experimental And Theoretical Study On The Extraction And Separation Of Americium And Europium With Amide Derivations

In the closed nuclear fuel cycle,�Partition and Transmutation�?P&T?is the most advanced strategy for spent fuel reprocessing at present.To significantly reduce the difficult of geological disposal,the core idea of P&T is to convert the long-lived,high radioactive nuclides into short-lived,low radioactive by the neutron induced-nuclear transmutation reaction.Because lanthanides have very high neutron capture cross sections and content in spent fuels,they must be removed before the transmutation.However,due to the very similar chemical properties,the separation of lanthanides from actinides is one of the most important and challenging task in the P&T system.In recent years,various kinds of extractants for the separation of trivalent lanthanides and actinides have been developed.These ligands?eg.BTP,BTBP,BTPhen and Cyanex derivations?,containing nitrogen,oxygen,sulfur and phosphorus atoms for complexation,are employed for the investigation of the separation.However,most of the conclusive achievement remains lab-scale test,and many of the extractant systems have their own advantages and disadvantages.Objective gaps still exist in laboratary simulations and the practical application of nuclear industry.In the other aspects,international and domestic academics are focusing on developing more effective ligands for the separation of lanthanides and actinides.On the basis of studying a lot of references and advances,the present work aims at the effective extraction and separation of the representative nuclides,americium?III?and europium?III?,from nitric acid media by means of solvent extraction technique experimentally and theoretically.The new series of soft-hard combined C_n-PTA?N,N-dialkyl-1,10-phenanthroline-2-amide?ligands with different length of substituted alkyl chains are designed,synthesized and characterized.In a tridentate PTA ligand molecule,two �soft� nitrogen atoms in phenanthroline ring are fixed on cis-conformer,which is beneficial for the complexation with metal ions.Meanwhile,the �hard� oxygen atom in the amide group can enhance the ability of complexation,and could possibly contribute to higher distribution ratios of metal ions.The influences of external conditions,such as the length of substitution alkyl chains,nitric acid concentration,salting-out agent concentration,PTA concentration and the shaking time,on distribution ratios and separation factors are investigated experimentally.UV-vis spectroscopic titrations are used to measure the protonation constant of the ligands and the stable constants of the complexes.The distribution of the species formation in solutions is calculated to explain the regulations of the variation of the distribution ratios in different condition.By quantum chemistry and the density functional theory?DFT?,the conformers and the protonation energy of the free PTA ligands are calculated.The relationship between the chemical properties and the geometry,electron structures as well as binding energy of the complexes are analyzed.The thermodynamic state functions are also calculated to give a reasonable explanation.The above work provides lots of information on the designing and optimizing of the novel extractants for the future.The soft-hard combined tetradentate H_n-NTA?N,N,N',N',N'',N''-hexaalkylnitrilotriacetamide?ligands are synthesized,and their extraction of Am?III?and Eu?III?in room-temperature ionic liquid?IL?is investigated.It is found that NTA can well dissolved in 1-alkyl-3-methylimidazolium bis-?trifluoromethylsulfonyl?imide.In low acid concentration,NTA exhibits much higher distribution ratios and separation factors?D?Am?= 93,SF = 23?in IL than those in molecular diluents,and can be used for effective separation.In high acid concentrations however,the distribution ratios dramatically decrease,and the metal ions can be easily stripped from the organic phase.By varying the temperature the slow extraction dynamics can be significantly improved.It is also found that the metal ion extraction exhibits two different mechanisms in NTA/IL mixtures simultaneously: the neutral complexation mechanism in low acid concentration,and the cation exchange mechanism in high acid concentration.This is different from the mechanism in molecular extractants.To give a further understanding of the properties of the sophisticated NTA/IL mixtures,molecular dynamics?MD?simulations are carried out.The radial distribution function?RDF?is used to analyze the interactions between the ligand molecules and the IL ions.The diffusion properties of NTA in IL are investigated.From the MD results,the differences of the behavior of NTA in IL or molecular diluents are discussed from the perspective of structure,energy and physical properties.The obtained results and conclusions can provide valuable information for the optimization and improvement of the extraction systems.In the current work,the binary mixtures of Cn-DGA?N,N,N',N'-tetraalkyl-3-oxydiglycolamide?and Et-BTP?2,6-bis?5,6-diethyl-1,2,4-triazin-3-yl?pyridine,n=4,6,8,10?extractant systems are proposed,and their synergistic extraction behaviors of Am?III?and Eu?III?form nitric acid media are studied.By varying the experimental parameters,the constitution of the systems are well optimized for the extraction.It is found that C₆-DGA/Et-BTP system exhibits the highest synergistic extraction coefficient among all the binary systems,when the molar ratio of the two extractants is set to be 1:3.In such the condition,the extraction efficiency can be greatly enhanced,while the selectivity of the two metal ions maintains favorable.Thus,the advantages of each extractant can be utilized.The distribution ratio values with mixed extractants are more than twice that with Et-BTP alone,while the separation factor values remain as high as Et-BTP in 1.5 mol/L nitric acid and 1.5 mol/L sodium nitrate.When the constitution of the extractants is changed,the distribution ratios can be reduced for stripping.The conclusions may shed light on research of the development of novel extractants for the separation of lanthanides and actinides in the treatment and disposal of HLLW.