New Cationic Organic Frameworks For Efficient And Selective Removal Of ⁹⁹TcO₄^-

Aiming at the "carbon neutral" national development strategy,the nuclear power with virtues of clean and low-carbon has entered a rapid development period.With the development of nuclear power,how to safely dispose of the large number of radionuclides produced in uranium/plutonium fission reaction has become a crucial problem.Technetium-99（99Tc）is one of the most problematic β radionuclides with a long half-life（2.13×105 years）and high fission yield（6%）and is predominately present as the most stable pertechnetate anion( ⁹⁹TcO₄^-)in aerobic nuclear wastes.In spent fuel recycling processes,the redox property of 99Tc seriously affects the control of chemical valence states of neptunium and plutonium,generating a notable barrier on the separation of neptunium and plutonium in the PUREX process.Besides,it is irksome that Tc（VII）complexes tend to evaporate in the form of Tc2O7 during the vitrification process of nuclear wastes.Moreover,due to its high solubility and nearly non-complexing nature, ⁹⁹TcO₄^-is likely to leak out with the migration of groundwater during the long-term underground storage of nuclear wastes.Therefore,direct removal of ⁹⁹TcO₄^-from unclear wastes before the PUREX process and vitrification process can not only help to eliminate the adverse effects of 99Tc on the spent fuel cycle,but also can fundamentally solve the leakage problem of ⁹⁹TcO₄^-during the vitrification process and long-term storage of nuclear wastes.However,considering the high salinity,high radioactivity,and high acidity or alkalinity of the unclear wastes in the PUREX process or underground tanks,this target is very challenging and sets a very high threshold for the stability and selectivity of sorbents.Anion exchange method is one of the most effective and widely used methods for the removal of anionic pollutants.A variety of anion-exchange materials have been extensively studied for the removal of anionic pollutants from nuclear wastes,including purely inorganic materials,metal-organic frameworks（MOFs）,and cationic organic frameworks.Among them,cationic organic frameworks have been widely used in the removal of ⁹⁹TcO₄^-due to their high stability,designability controllable pore size,shape and charge,and modifiable structure.However,most of the traditional adsorption materials（such as anion exchange resin）exhibit poor removal efficiency of ⁹⁹TcO₄^-in this extreme condition,as most of them usually are destroyed in strong radioactive and highly acidic/alkaline condition.Therefore,the design and synthesis of new stable cationic organic frameworks for efficient and selective removal of ⁹⁹TcO₄^-from highly acidic used fuel wastes and strong alkaline high-level radioactive wastes are of great importance.In this work,a series of new cationic organic frameworks have been designed and synthesized for efficient and selective removal of ⁹⁹TcO₄^-from different types of high-level radioactive wastes.The adsorption performance of these cationic organic frameworks on ⁹⁹TcO₄^-/ReO₄^-has been evaluated through batch and dynamic adsorption experiments.More importantly,the practical applications of these cationic organic framework materials have been fully discussed.The details are as follows:Chapter 2:In Chapter 2,the adsorption performance of two commercially produced cationic resins（Purolite A532E and Purolite A530E）for ⁹⁹TcO₄^-/ReO₄^-was systematically evaluated by a series of batch adsorption experiments.The results show that this kind of resins exhibits excellent adsorption performance for ⁹⁹TcO₄^-/ReO₄^-removal,including high adsorption capacity（Purolite A532E:706 mg/g and Purolite A530E:446 mg/g）and excellent adsorption selectivity.However,the adsorption kinetics of these two resins for ⁹⁹TcO₄^-/ReO₄^-removal is very slow,and both of them need at least 150 min to reach adsorption equilibrium.In addition,the two resins exhibit poor irradiation resistance that the sorption capacities decrease gradually after suffering from irradiation.Based on these results,these two resins are not suitable for practical applications in high-level radioactive wastes.Chapter 3:A cationic polymeric network（named SCU-CPN-1）with high acid and radiation resistance was designed and synthesized for efficient and selective removal of ⁹⁹TcO₄^-from highly acidic used fuel wastes.SCU-CPN-1 exhibits excellent adsorption performance for ⁹⁹TcO₄^-/ReO₄^-removal,including the fastest adsorption kinetics,high adsorption capacity,and excellent adsorption selectivity.Additionally,SCU-CPN-1 exhibits excellent uptake performances for the removal of ⁹⁹TcO₄^-/ReO₄^-under the condition of 3 M HNO3 solution.Those forgoing fortune demonstrates the powerful application potentials of SCU-CPN-1 in highly acidic used fuel wastes before the PUREX process.In addition,the theoretical calculation results show that the imidazolium salt ring in SCU-CPN-1 can form a strong p-π interaction with ⁹⁹TcO₄^-,which reveals the intrinsic reason for the high selectivity of SCU-CPN-1.Based on this material,a ⁹⁹TcO₄^-separation process can be introduced before the PUREX process,which is not only highly beneficial for the separation of uranium and plutonium,but also solve the leakage problem of 99TcO4during nuclear wastes volatilization and long-term storage processes fundamentally.Chapter 4:To reducing the generation of secondary radioactive solid wastes,a rationally designed cationic polymeric network（SCU-CPN-2）with high positive charge density and excellent radiation resistance was synthesized.SCU-CPN-2 exhibits fast adsorption kinetics for ⁹⁹TcO₄^-removal,excellent pH stability,excellent radiation resistance,and good adsorption selectivity for ⁹⁹TcO₄^-/ReO₄^-.More impressively,the adsorption capacity of SCU-CPN-2 for ReO₄^-is as high as 1467 mg/g,setting a new record in adsorption capacity for ⁹⁹TcO₄^-/ReO₄^-removal.Furthermore,SCU-CPN-2 shows excellent recyclability,which makes its practical applications more cost-effective.Overall,the use of this material is not only beneficial for improving the removal efficiency of ⁹⁹TcO₄^-,but also contributes to reducing the generation of secondary radioactive solid waste.Chapter 5:A new cationic polymeric network（SCU-CPN-4）with excellent alkaline and radiation resistance was designed and synthesized for efficient and selective removal of ⁹⁹TcO₄^-from highly alkaline high-level radioactive wastes.Herein,we overcome the poor alkaline stability of cationic quaternary ammonium salt materials by the installation of adjacent bulky groups at imidazolium to inhibit the reaction of nucleophilic attack by OH-.SCU-CPN-4 not only exhibits excellent alkaline stability and good radiation resistance,but also shows excellent sorption performance in the removal of ⁹⁹TcO₄^-/ReO₄^-,including fast adsorption kinetics,high adsorption capacity,and outstanding adsorption selectivity.In addition,SCU-CPN-4 can efficiently separate ⁹⁹TcO₄^-from simulated highly alkaline Savannah River Site high-level radioactive waste（SRS HLW,1.3 M NaOH）and is fully reusable under this extreme condition.Most importantly,SCU-CPN-4 shows excellent dynamic adsorption performance in both highly alkaline solution（1M NaOH）and simulated SRS wastes.These unique features fully indicated the vast potential application of SCU-CPN-4 for ⁹⁹TcO₄^-removal in highly alkaline high-level radioactive wastes.