Application Of Ionic Liquid Extraction Systems And Covalent Organic Framework Materials In The Separation Of Radioactive Elements

Ionic liquids(ILs)have many attractive physical and chemical properties,such as negligible vapor pressure,high thermal and chemical stabilities,good solubility and excellent electrochemical properties,which are expected to replace tradiational organic solvents in the separation of radioactive elements.Heterocyclic N-donor ligands and calixarene crown ethers are effective extracting agents for Ln/An and Cs⁺,respectively.Researches on the radiation stability and radiolysis mechanism of the systems composed of these two extracting agents and ILs can provide an important theoretical basis for the development of the spent nuclear fuel reprocessing.Covalent organic frameworks(COFs)are new types of porous crystalline materials.Connected by light elements through covalent bonds,COFs have many advantages such as stable chemical structure,large specific surface area,low framework density,and high designability,showing a broad application prospect in the adsorption separation of radioactive elements.It is necessary to study the grafting modification of COFs to expand their application in the separation of radioactive element Tc.However,the research on the radiation modification of COFs has not been reported.The research status of spent nuclear fuel reprocessing as well as the concepts and applications of ILs and COFs were introduced.The research progress in the separation of radioactive elements by using ILs systems(heterocyclic N-donor ligands or calixarene crown ethers)and COFs materials were reviewed.And the research contents and aim of the thesis were proposed.The extraction performances and gamma radiation effects of 2,6-bis(5,6-diisobutyl-1,2,4-triazin-3-yl)pyridine(IB-BTP),2,6-bis-(5,9,9-trimethyl-5,6,7,8-tetrahydro-5,8-methanobenzo[1,2,4]triazin-3-yl)pyridine(CA-BTP)and 2,9-bis(5,9,9-trimethyl-5,6,7,8-tetrahydro-5,8-methanobenzo[1,2,4]triazin-3-yl)-1,10-phenanthroline(CA-BTPhen)in combination with 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide([C₂mim][NTf₂])(IB-BTP/[C₂mim][NTf₂],CA-BTP/[C₂mim][NTf₂]and CA-BTPhen/[C₂mim][NTf₂])were systematically studied.Under low and high acidity conditions,the dominant extraction mechanisms were cation exchange and neutral complex,respectively.The results of Eu³⁺extraction experiments combined with theoretical calculations showed that IB-BTP had the fastest extraction kinetics,while under neutral and 0.01 M HNO₃ conditions,CA-BTPhen had the highest extraction distribution ratio.The order of the radiation stability of the ligands in the three systems was CA-BTP>IB-BTP?CA-BTPhen.The main radiolytic products were substitution products formed by heterocyclic N-donor ligands and radicals from[C₂mim][NTf₂].The gamma radiation effect of bis(2-propoloxy)calix[4]crown-6(BPC6)in 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide([C₄mim][NTf₂])was studied by using a hydrophilic IL,1-butyl-3-methylimidazolium trifluoromethanesulfonate([C₄mim][OTf]).The main water-insoluble radiolytic products of BPC6/[C₄mim][NTf₂]were easily obtained by water-washing after irradiation,and the changes in the chemical structure and their influence on extraction performances of Cs⁺were studied.It was found that the main liquid radiolytic products were substitution products formed by BPC6 and radicals(such as[C₄mim]·,[C₄mim]²⁺·,CF₃·and H·)from ILs,in which the relative content of trifluoromethyl substituted BPC6 was the highest.Theoretical calculations indicated that the radiolytic products still remained a certain coordination ability,so that the system can maintain a good extraction ability for Cs⁺with a distribution ratio of 7.9 even under the condition of 500 k Gy and 1 M HNO₃.1-vinyl-3-ethylimidazolium bromide(C₂vim Br)was successfully grafted onto the vinyl-rich COFs material(Tb Da-COF)by means of gamma ray simultaneous radiation for the efficient anion exchange capture of ⁹⁹Tc O₄^–.The prepared materials,designated[C₂vim Br]_x%-Tb Da-COF,possessed good crystallinity,high porosity,excellent stability and nanofibrous morphology.The controllable adjustment of the content of imidazolium groups was realized,and the excellent adsorption performance of ReO₄^–was obtained.The adsorption equilibrium can be reached within 2 min,and the adsorption capacity was as high as 952 mg g^–1.Dynamic adsorption column experiments showed that even after four adsorption–desorption cycles,as high as 99.98%of ReO₄^–can be efficiently separated and more than 90%of them can be recovered by the resultant adsorbents.In addition,the efficient column separation of ⁹⁹Tc O₄^–was successfully achieved.4-chloromethylstyrene(VBC)was grafted onto the hydroxyl-rich COFs material(3DCOF)by means of gamma ray pre-radiation,and then triphenylphosphine(PPh₃)was used to react with the chloromethyl group to obtain super hydrophobic quternary phosphonium modified three-dimensional COFs,designated 3DCOF-g-VBPPh₃Cl.The prepared materials exhibited good crystallinity,high porosity,excellent stability and microcrystalline morphology.The super hydrophobic quaternary phosphonium contributed to overcome the Hofmeister sequence rule and achieve high adsorption selectivity for ReO₄^–.The adsorption equilibrium can be reached within 1 min for ReO₄^–solution with low concentration,and the removal rate was as high as 99.995%.Under the condition of 6000-fold excess of SO₄^2–and PO₄^3–,the removal rate of ReO₄^–were as high as 93.9%and 92.5%,respectively.In the simulated groundwater environment of Beishan,Gansu,the removal rate of ReO₄^–can reach more than 94.2%,with an excellent dynamic column separation performance.The results showed the great potential of the resultant COFs in real application environments.