Studies On The Separation Of Actinides By N-Heterocyclic Ligands And Nitrogen-containing COFs

Green and sustainable development are the main theme of contemporary society.Therefore,in order to develop the atomic energy business efficiently and safely,it is necessary to properly and safely deal with and dispose of radioactive materials produced in the process of nuclear industry.As important nuclear materials,uranium,thorium and plutonium occupy a central position in the nuclear fuel cycle,and their separation and recovery are a necessary part of achieving sustainable development of nuclear energy.Based on the current status and problems of actinides separation in the nuclear fuel cycle,this thesis synthesizes different structures of N-heterocyclic diamide ligands and uses them for the selective separation of actinides in high acid to provide basic theoretical support for the design of lanthanides and actinides extractants.In addition,the N-heterocyclic structure was further introduced into COFs materials to prepare highly crystalline nitrogen-containing sp²-carbon hybridized COFs（sp²C-COFs）on a large scale and the COFs were used for the separation of actinides in high acid systems.The details of the study are as follows:The thesis firstly briefly outlines the current status of nuclear fuel cycle and spent fuel treatment,introduces the development of nitrogen-containing ligands and their application in actinides separation,then introduces the development of covalent organic frameworks and their research progress in actinides and other radionuclides separation application.Based on the above research progress,the topic idea and research content of this thesis are proposed.In the second chapter,two N-heterocyclic ligands with tetradentate coordination structures,QL-DABP and QL-DAPhen,were successfully prepared by combining tetrahydroquinoline groups with 2,2’-bipyridine and 1,10-phenanthroline rigid skeletons using a strategy of combining hard and soft donor atoms.The extraction performance of the ligands for lanthanides and actinides was investigated by systematic extraction experiments.A combination of experimental and theoretical means such as ultraviolet-visible spectroscopy（UV-Vis）,Fourier transform infrared spectroscopy（FT-IR）,electrospray mass spectrometry（ESI-MS）,nuclear magnetic resonance（NMR）spectroscopy and the DFT theoretical calculations were also used to focus on the extraction performance and extraction mechanism of QL-DAPhen ligands for trivalent lanthanides and actinides.The research results indicate that the nitrogen donor skeleton is the key to achieving the separation of trivalent lanthanides and trivalent actinides,and the steric hindrance of amide affiliated groups affects the distribution of complex species.In the third chapter,two-dimensional covalent organic frameworks（sp²C-COFs）were successfully prepared on a large scale by solid-phase melting using industrial-grade pyridine heterocycles and aromatic aldehydes（terephthalaldehyde）as monomers for sp²C-COFs materials,and further post-modified to methyl ionize them.The two obtained sp²C-COF-1 and sp²C-COF-1-Me NO₃ with different anion exchange capacities were then used for the separation of Th（Ⅳ）in nitric acid systems.This type of COFs materials has good crystallinity,stability,porosity,and regular lamellar structure.The adsorption performance of the materials on Th（Ⅳ）was investigated by a series of adsorption experiments including acidity,time contact,ion concentration,ions selectivity,recycling,and irradiation performance tests.The sp²C-COF-1 and sp²C-COF-1-Me NO₃ exhibit good adsorption performance on Th（Ⅳ）at 9 M nitric acid.The kinetics were fast,the adsorption equilibrium is reached around 20 min,and the maximum saturation adsorption capacity is up to 35.5 mg/g.Th（Ⅳ）could be selectively separated in high level solution containing common ions.In the fourth chapter,sp²C-COF-2 and sp²C-COF-2-Me NO₃ with larger pore size and the same anion exchange function were further prepared by replacing terephthalaldehyde with 1,4-biphenyldicarboxaldehyde as the monomers of sp²C-COFs,again by solid-phase melting method,and they were used for the adsorption studies on Pu（Ⅳ）.The property characterization confirms that they also have good crystallinity,stability,porosity and regular two-dimensional lamellar structure.Their separation performance and adsorption mechanism for tetravalent actinides（Th,Np,Pu）were investigated by systematic adsorption experiments and theoretical calculations.It had good adsorption performance on Pu（Ⅳ）at 8 M nitric acid.The kinetics was fast,reaching equilibrium in about 10 min,and the removal rate of Pu（Ⅳ）at equilibrium was about 96%.The DFT theoretical analysis of the effective fragments of COFs materials shows that the affinity magnitude of methyl ionized COFs for tetravalent actinides is Pu（Ⅳ）>Np（Ⅳ）>Th（Ⅳ）and is larger than that of hydrogen protonated COFs.In the fifth chapter,with the separation of U（Ⅵ）/Mo（Ⅵ）as the research objective,sp²C-COF-1 and sp²C-COF-1-Me Cl ionized with chloromethyl were used as anion exchange materials for the separation of U（Ⅵ）and Mo（Ⅵ）in sulfuric acid systems.The separation factor(SF_Mo/U)of COFs material for U（Ⅵ）/Mo（Ⅵ）is best at around p H=1.50,which is about 84.Both sp²C-COFs hold good adsorption capacity for Mo（Ⅵ）under various anions.The adsorption results of simulated acid leaching uranium-molybdenum ore solution indicated that the COFs could selectively separate Mo（Ⅵ）from U（Ⅵ）.In addition,infrared spectroscopy（FT-IR）and X-ray photoelectron spectroscopy（XPS）show that Mo（Ⅵ）is mainly adsorbed on COFs materials as oxygenated molybdate anion.Finally,the research work of this thesis is summarized,and some views on the design of N-heterocyclic amides and functional covalent organic framework materials for the separation of actinides are put forward.