Preparation Of Covalent Organic Framework Materials And Study On Their Adsorption Properties For Iodine

Nuclear energy is one of the new energy sources vigorously developed at this stage.The biggest drawback of nuclear energy is that it will produce radioactive waste and will not be degradable for a long time.Due to the electrostatic repulsion between radioactive iodine and soil and rock components,iodine shows high fluidity in aqueous system.According to its half-life(t_1/2),the radioactive isotopes of iodine produced during the nuclear fission of uranium-235 can be divided into ¹³¹I(t_1/2=8 d)and ¹²⁹I(t_1/2=1.57×10⁷ y),of which the half-life of ¹²⁹I is as long as 1.57×10⁷ years,which will cause permanent pollution,so the removal of radioactive iodine nuclides is particularly important.（1）COF-PT material was prepared by ethanol reflux method.With p-phenylenediamine and p-phthalaldehyde as raw materials,the characteristics of the material were characterized by SEM and XPS.Explore the effects of time,concentration and temperature on iodine enrichment.The results show that the maximum adsorption capacity of COF-PT for iodine is 926.499 mg·g^-1,the adsorption process is mainly multi-molecular layer,and physical adsorption and chemical adsorption jointly control the adsorption process.The adsorption process is spontaneous,and proper temperature rise is helpful to promote the adsorption process.The existing mechanism may be that the largeπbond in benzene ring combines withσbond in iodine molecule to form ligand,that is,benzene-I₂ structure,and electron transfer occurs,and iodine is attached to the surface of adsorbent in the form of I3^-.（2）Schiff Base-UHP was prepared by ultra-high pressure synthesis method and ethanol reflux method.In this chapter,N heterocyclic ring was introduced on the basis of COF-PT to explore whether the materials after high pressure treatment can effectively capture iodine in water phase.The results show that the crystal structure of the material after high pressure treatment is complete,and there is no collapse or change of adsorption performance due to high pressure treatment.The calculation of Langmuir adsorption model shows that the saturated adsorption capacity can reach1535.450 mg·g^-1 is more in line with the pseudo-first-order kinetic model dominated by physical adsorption;And the adsorption process is a spontaneous endothermic reaction.The adsorption mechanism is inferred as follows:N heterocyclic ring will form I₃^-electron transfer with iodine guest,and with the change of electron cloud density,the structure of N heterocyclic ring,benzene ring and imine bond in the material provides effective binding sites for iodine,which realizes the effective adsorption of iodine in water phase by high-pressure materials.（3）COF-PAN was prepared by the improved solvothermal method,which greatly shortened the preparation time compared with the traditional reflux method.In this chapter,melamine and terephthalaldehyde were used as reactants,and the characteristics of the materials were characterized by FT-IR,XRD,SEM and XPS.To explore the effects of time,concentration,temperature,pressure and coexisting ions on iodine enrichment.The results show that the maximum adsorption capacity of COF-PAN for iodine calculated by Langmuir adsorption model is 1988.626 mg·g^-1,the adsorption process is dominated by multi-molecular layers and controlled by chemical action or chemical bonding.The whole adsorption process is spontaneous,and proper temperature rise is helpful to promote the adsorption process.The presumed mechanism is that the rich N content and the existence of terminal-NH₂enhance its electron-rich and alkaline properties,and the p-πconjugation effect occurs between N and ring,forming delocalization system,which mainly exists on the surface of COF-PAN in the form of I₃^-.