Synthesis Of A Cationic Covalent Organic Framework For Selective Removal Of ⁹⁹TcO₄- And The Study Of Mechanism

Technetium-99(99Tc)is one of the most problematic fission products in the nuclear fuel cycle.The main species of technetium-99 is radioactive anion 99TcO4-,which has the characteristics of large reserves,long half-life,potential radiation damage,high environmental mobility and redox activity.It is estimated that 1990 kg of 99Tc has been produced since 1943-1987,when the Hanford plant,a retired nuclear power plant in the United States,began to produce raw materials for nuclear weapons.Due to the rapid development of nuclear power technology,this number will continue to grow rapidly.Ideally,when the fuel rods are dissolved in high concentration nitric acid solution,99TcO4-should be removed in the first stage because of its redox activity,which seriously hinders the valence state conversion of other key actinides in the traditional Purex Process,greatly reducing its interference in the solvent extraction process and the emission of volatile substances in the waste vitrification process.In addition,99Tc as a typical long-life(half-life 2.13 × 105 years)? radiation source,has chemical toxicity and potential radiation hazards.However,the anion exchange materials reported up to now,including traditional anion exchange resin,inorganic cation framework materials and recently reported cation metal organic framework materials(MOFs),are difficult to maintain structural stability under both of high acidity and strong radiation field.According to Hofmeister selectivity(PO43->SO42->OH->Cl->NO3->I->TcO4->ClO4-),most of the materials show poor selectivity toward 99TcO4-in the presence of a large number of competitive anions such as NO3-and SO42-.Therefore,it is of great scientific significance to develop a new type of adsorbent material towards 99Tc with acid resistance,irradiation resistance,fast kinetics and high selectivity.In this paper,a novel cationic two-dimensional covalent organic framework(COF)was synthesized by using the cationic aromatic amino modified violet base ligands(viologen-NH2)and 2,4,6-trialdehyde-m-triphenyltriphenol(TP).After a series of characterization,the adsorption properties toward 99TcO4-and the adsorption mechanism were studied.The specific work is as follows:(1)In the second chapter,the cationic Zincke salt(BDB)was synthesized by zincke reaction of 4,4'-bipyridine and 1-chloro-2,4-dinitrobenzene via ligand pre modification,and the cationic aromatic violet base ligand(viologen-NH2)was further synthesized by reaction with p-phenylenediamine.Then a cationic two-dimensional COF material named SCU-COF-1 was synthesized by solvothermal method with 2,4,6-trialdehyde-m-triphenyltriphenylphenol(TP)and aromatic amino violet base ligand,and the micro size of the material was observed in micron level by high resolution scanning electron microscopy(SEM)and transmission electron microscopy(TEM).In addition,the spectrum obtained by powder X-ray diffraction corresponds to the simulation value,and the special ABC stacking mode of SCU-COF-1 is determined experimentally and theoretically.Then we verified its chemical composition and structure by infrared spectrum and 13C solid-state NMR,and studied the stability and porosity of the material.It was found that the material had good strong acid stability(3 M HNO3)and irradiation stability(600 kGy ? rays,600 kGy ? rays)due to the high conjugation of the skeleton.Since the pyridine buzzing unit in the structure is difficult to keep stable in the presence of a large number of nucleophiles(such as OH-),the material is unstable in strong alkali(1M NaOH).In addition,nitrogen adsorption and pore size distribution test showed that SCU-COF-1 has a lower specific surface area due to its disordered ABC stacking mode and ionized framework,and formed a pore size of?1.44 nm.In conclusion,SCU-COF-1 can be used as a material with good adsorption capacity for 99TcO4-under environmental conditions or high acidity and high irradiation field conditions.(2)We studied the adsorption of 99TcO4-by the material under the environmental conditions,simulating the conditions of high acidity and high irradiation field.It was demonstrated that SCU-COF-1 has very fast adsorption kinetics(it can reach the adsorption equilibrium within 1 min),very high adsorption capacity(the adsorption capacity of the ReO4-,a surrogate of 99TcO4-,is 367.6 mg/g at room temperature,and the maximum adsorption capacity can reach 702.4mg/g when the temperature is elevated).It has good competitive ion selectivity towards 99TcO4-.Further experiments show that SCU-COF-1 has the ability to separate 99TcO4-from simulated high acid spent fuel reprocessing solution(3 M HNO3)and low activity waste liquid from U.S.nuclear sites.(3)This work studied the anion exchange process and explain the mechanism of high selectivity clearly through molecular dynamics simulation.On the one hand,the binding free energy between 99TcO4-anion and SCU-COF-1 is strong.Over time in the adsorption process,the anion binding site in SCU-COF-1 is replaced,and 99TcO4-successfully "squeezed" Cl-into the skeleton;On the other hand,due to the direct electrostatic repulsion between 99TcO4-and Cl-,the occupation of 99TcO4-gradually weakens the interaction between SCU-COF-1 and Cl-,thus Cl-is gradually dissociated from the skeleton.In conclusion,this work not only provides a new idea for the synthesis of a new cationic COF framework by ligand pre-modification,but also proves that this kind of highly conjugated COFs as a new material platform for nuclear fuel reprocessing has a considerable prospect and potential application in future.