Preparation Of Covalent Organic Frameworks And Their Iodine Capture Properties

Environmental pollution and energy shortage are two major problems facing the world today.As a kind of green energy with high energy density,nuclear energy has attracted worldwide attention and development.Being an important nuclear fuel,uranium is used in most nuclear power plants in the world.129I and 131I are volatile radionuclides produced during uranium fission,mainly in the form of iodine molecules(I2)and organic iodides(such as CH3I and C2H5I).They are characterized by great radioactive harm,strong fluidity and high diffusion coefficient,posing a serious threat to the ecological environment and human health.Therefore,the development of green and efficient radioactive iodine capture materials is of great significance to human health,environmental restoration and the safe development of nuclear energy.Based on the outstanding application advantages of regular porous nitrogenrich materials in capturing radioactive iodine,a series of two-dimensional conjugated covalent organic frameworks(COFs)with rich N active sites were constructed through the design and regulation of unique multi-nitrogen nodes,and applied to the capture of I2 and CH3I.The details are as follows:(1)A series of two-dimensional conjugated COFs(COF-HFPTP-DPT,COFHFPTP-DBID and COF-HATN-DBID)with nitrogen-doped skeleton and periodic double pore structure were prepared by solvothermal method.A series of characterization methods such as FT-IR,PXRD and SEM show that all three COFs have good crystallinity,large porosity,excellent thermal stability and irradiation stability,showing great application potential in I2 and CH3I capture.(2)The ability of three COFs to capture iodine vapor was studied by gravimetric method.The results showed that COF-HFPTP-DPT,COF-HFPTPDBID and COF-HATN-DBID have good adsorption properties for iodine vapor,which are 5.07 g·g-1,5.51 g·g-1 and 6.2 g·g-1,respectively.The adsorption properties of COF-HFPTP-DPT,COF-HFPTP-DBID and COF-HATN-DBID for I2 in aqueous solution were further studied,and their maximum saturated adsorption capacities were 2447.8 mg·g-1,2048.5 mg·g-1 and 1525.2 mg·g-1,respectively.In the iodine aqueous solution with pH=1,COF-HFPTP-DPT still showed a good removal effect,with a removal rate of 92.4%within 48 h.The saturated adsorption capacity of 715 mg·g-1 was obtained by using COF-HFPTP-DPT packed column for penetration experiment of iodine solution.In addition,COFHFPTP-DPT showed ultra-fast kinetics and excellent selectivity for I3-in aqueous solution,and the removal rate reached 92.2%within 10 min.In the presence of various competitive anions,the I3-removal rate of COF-HFPTP-DPT is above 99%,which could be used to remove I3-in groundwater.The adsorption properties of COF-HFPTP-DPT,COF-HFPTP-DBID and COF-HATN-DBID for I2 in cyclohexane solution were investigated,and their maximum saturated adsorption capacities were 655.8 mg·g-1,853.0 mg·g-1 and 738.0 mg·g-1,respectively.In addition,the adsorption properties of COF-HFPTP-DPT,COFHFPTP-DBID and COF-HATN-DBID for CH3I were also studied by gravimetric method.Among them,the maximum adsorption capacity of COF-HATN-DBID reached 1.77 g·g-1 which was higher than that of two COFs,SCU-COF-2(1.45 g·g-1)and COF-TAPT(1.53 g·g-1).(3)Mechanism research shows that COFs can achieve high adsorption performance for I2 mainly through charge transfer between nitrogenous functional groups(imines,imidazoles and hexaazene)/electron-rich hyperconjugated structure and I2 molecules to form polyiodide(I3-and I5-).The adsorption experiment shows,however,that the adsorption capacity of the adsorbent for I2 in aqueous solution depends on its hydrophobicity rather than the number of its strong binding sites.In addition,CH3I molecules specifically bind to various nucleophilic N sites in COFs structure through N-methylation reaction,and the adsorption capacity of COFs materials for CH3I depends not only on the number of strong binding sites,but also on the pore size.In this paper,different adsorption behaviors of I2 under different conditions were revealed by applying the materials to the adsorption of I2 in different media systems,and the optimal adsorption materials of gaseous iodine,iodine aqueous solution and iodocyclohexane solution were selected.It is proved that the pore characteristics of porous materials are important in the adsorption process of CH3I.At the same time,the similarities and differences of the adsorption mechanism of COFs for I2 and CH3I are explored and understood,which provides theoretical basis and experimental support for the application of COFs in the capture of radioactive iodine in nuclear waste reprocessing.