Functionalized Covalent Organic Frameworks Materials For The Adsorption Of Radioactive Heavy Metal Oxyanions

With the development of nuclear energy,radioactive nuclear waste and its derived pollutants have caused great environmental hazard.Technetium-99(⁹⁹Tc)is the main product（～6%）during nuclear fission of ²³⁵U or ²³⁹Pu.⁹⁹Tc is beta-radioactive and has a half-life of up to 2.13×10⁵ years.In the natural environment,Tc exists mainly as the oxyanions form,TcO₄^-,which is stable over almost the entire p H range.With high water-solublility（11.3 mol/L）and almost no complexing ability,TcO₄^-has extremely high mobility in the environment,and can enter the food chain easily,causing serious health hazards.Therefore,the development of efficient and low-cost adsorbents for the enrichment,separation or removal of ⁹⁹Tc is important for water purification and improvement of the cycling and recovery efficiency of the spent fuel and nuclear resources.Cationic covalent organic materials have a covalently bonded and positively charged robust backbone,possessing tunable structure and pore size,good hydrolytic stability and radiation resistance.Furthermore,their conjugated backbones can be easily functionalized to withstand extreme conditions（e.g.,high acidity and alkalinity）,thus they have wide application perspectives in environmental monitoring,chemical analysis,and nuclear waste treatment.In this thesis,new covalent organic materials with strong affinity for TcO₄^-and ReO₄^-and good interference resistance are designed,which are capable of highly selective removal of TcO₄^-and ReO₄^-under strong acid or base conditions and are expected to be used for the enrichment and separation of Tc and Re in nuclear waste streams and real waters.The specific research work is as follows:（1）Design of highly selective covalent organic polymers（COPs）for the adsorption of TcO₄^-/ReO₄^-:The cationic ligand Bpa-Br was firstly synthesized using dibromo-p-xylene and4-formylpyrimidine,then three spherical cationic organic polymers（COPs）,COP-TAB,COP-TAT and COP-TAE were fabricated by a mild solvothermal method using Schiff base reaction.The obtained COPs were characterized by Fourier transform infrared spectroscopy（FT-IR）,scanning electron microscopy（SEM）,transmission electron microscopy（TEM）,and polycrystalline X-ray diffraction（PXRD）techniques.Results showed that COP-TAB,COP-TAT and COP-TAE had excellent adsorption effects on ReO₄^-owing to the porosity and hydrophobicity of their skeletons and their positive charged centers.The adsorption of ReO₄^-was mainly monolayer chemisorsorption,which followed the Langmuir isothermal adsorption model.The adsorption capacity of COP-TAB,COP-TAT and COP-TAE was 522.30±15.66,461.89±19.91 and 519.36±11.39 mg/g respectively.And the adsorption equilibrium could be reached within 2 h.All three COPs showed excellent resistance to interfering ions;even 1000-fold PO₄^3-did not cause significant interference.The satisfying enrichment/removal efficiency ReO₄^-was achieved in actual environmental water samples and the simulated wastewater of the Hanford LAW melter recirculation stream.（2）Design of hydrazone-bonded guanidinium-based ionic covalent organic frameworks（i COFs）for the rapid adsorption/removal of TcO₄^-/ReO₄^-uner high acidic condition:Using the condensation reaction between the trialdehyde-based m-phenylene triol（Tp）and diaminoguanidine hydrochloride(DG_Cl),the guanidinium-based material DG_Cl-Tp COF was synthesized by a solvothermal method.The obtained materials were characterized by Fourier transform infrared spectroscopy（FT-IR）,scanning electron microscopy（SEM）,transmission electron microscopy（TEM）,polycrystalline X-ray diffraction（PXRD）,and N₂ adsorption and desorption.The experimental studies showed that the porous DG_Cl-Tp COF produced excellent adsorption of ReO₄^-mainly through hydrogen bonding,electrostatic interactions and ion exchange.The saturation adsorption capacity of DG_Cl-Tp COF was 418.15±23.47 mg/g,and the adsorption equilibrium could be chieved in a very short time（<1 min）.The DG_Cl-Tp COF was capable of fast and efficient adsorption of TcO₄^-and ReO₄^-in a strongly acidic environment and was applied succefully for the enrichment/removal of ReO₄^-in actual environmental water samples and the simulated wastewater of the Hanford LAW melter recirculation stream.Therefore,the method has significant potential for the removal of TcO₄^-from strongly acidic nuclear waste streams.