Research On The Removal Of Radionuclide Uranium(Ⅵ) By Covalent Organic Frameworks

As an efficient and clean energy source,nuclear energy provides a practical solution to the fossil energy shortage and global warming crisis.Uranium is crucial in the process of nuclear power generation.However,due to its radioactivity and chemical toxicity,the contamination of uranium has become an urgent environmental problem.In recent years,covalent organic frameworks(COFs),as an emerging adsorbent,have been applied in environmental remediation due to its large specific surface area,high porosity,superior crystallinity and stability.In this thesis.batch adsorption experiments and density functional theory(DFT)calculations were used to systematically explore the adsorption performance and interaction mechanism of covalent organic frameworks with β-ketoenamine structure(noted as DQTP COF)for uranyl ions.The details of the study are as follows:(1)Covalent organic frameworks with β-ketoenamine structure were synthesized by the Schiff base reaction of 1,3,5-triformy lmethanetriol with 2,6diaminoanthraquinone via a solvothermal synthesis technique.A series of characterization were used,such as scanning electron microscopy(SEM),X-ray diffraction(XRD),Fourier transform infrared(FT-IR)spectroscopy,N2 adsorptiondesorption,simultaneous thermal analysis(TGA),and zeta potential analysis.The fundamental characteristics of DQTP COF were described in terms of morphology,crystal structure,and chemical composition.The results demonstrated that the C-N and C=C stretching vibrational peaks in FT-IR verified the Schiff base reaction and the irreversible β-ketoenamine structure.And the successfully synthesized DQTP COF displayed a high crystallinity and uniform nanofiber structure.The DQTP COF also had a substantial specific surface area as well as high thermal and chemical stability.(2)Further adsorption experiments were conducted to investigate the effects of reaction time,pH,ionic strength,competing ions,initial concentration of contaminants and reaction temperature on the removal of uranyl ions.A series of experimental results showed that DQTP COF reached adsorption equilibrium within one hour and the adsorption kinetics was in accordance with the pseudo-second-order kinetic model.The adsorption of DQTP COF on uranyl ions was an internal spherical complexation process because the adsorption performance was primarily affected by pH values.The effect of ionic strength on the adsorption of uranyl ions by DQTP COF could be neglected.The adsorption performance of DQTP COF was excellent at room temperature,with a saturation adsorption capacity of 517.62 mg/g fitted by the Langmuir model.And DQTP COF still demonstrated a high removal rate(>90%)after six adsorption-desorption cycles.(3)The adsorption mechanism of uranyl ions by DQTP COF was thoroughly investigated by experiment and theoretical calculations.The O-U peak at 531.55 eV in the X-ray photoelectron spectroscopy(XPS)indicated that uranyl ions was successfully adsorbed towards DQTP COF.Secondly,DFT calculations revealed that the mechanism of adsorption was mostly promoted by electrostatic interactions between uranyl ions and DQTP COF.In addition,carbonyl functional groups in βketoenamine and quinone units also had a significant impact on adsorption process.In this thesis,the adsorption process and interaction mechanism between covalent organic frameworks and uranyl ions are comprehensively investigated from batch adsorption experiments and theoretical calculations,which provides experimental and theoretical support to promote the wide application of covalent organic frameworks in the field of environmental remediation.