Design And Synthesis Of Covalent Organic Framework Materials For The Detection/Adsorption Of Nuclide Uranium

The nuclide uranium is a key resource for the development of nuclear energy.The total amount of uranium reserves in seawater is about 4.5 billion tons,which is about 1,000 times the proven reserves of uranium ore on land.With the increasing scarcity of terrestrial uranium ore reserves,selective enrichment of uranium from natural seawater is one of the most practical and promising methods to solve the shortage of uranium resources.At present,there are many methods for enriching uranium from seawater,including adsorption,chemical precipitation,solvent extraction and separation,and biological treatment.Among them,the adsorption method is the most reliable and commonly used method.However,it is very difficult to enrich uranium from natural seawater.The concentration of uranium in natural seawater is extremely low(about 3.3 ppb),there are a large number of high concentrations of competing ions and serious biofouling.To achieve efficient enrichment of nuclide uranium from natural seawater,adsorption materials with high stability are required,which can maintain excellent adsorption performance under high ionic strength and severe biofouling conditions,and can be recycled.The adsorbents currently reported are often limited by their stability,selectivity,adsorption capacity,adsorption efficiency or reusability,and cannot achieve large-scale practical applications.In addition,the nuclide uranium has high biological toxicity,strong radioactivity and high migration ability,and its accidental leakage can cause serious health and environmental problems.Effective enrichment of uranium requires advanced adsorption materials,which not only have high stability,high affinity and selectivity,but also have fast kinetics and large adsorption capacity.Covalent organic frameworks(COFs)are a new type of porous crystalline material with the characteristics of large specific surface area,regular pore structure,high stability,rich topological structure and tunability of functional groups.Based on the structural and functional characteristics of COFs materials,this paper ingeniously designed functional monomers with selective adsorption of uranium,synthesized a variety of COFs materials with novel structures,and realized rapid and sensitive detection and high-efficiency enrichment of the nuclide uranium in complex environmental media.It is expected to be used for the enrichment and recycling of strategic resources and the treatment of nuclear pollution environmental problems.The main research contents are as follows:1.Accidental leakage of nuclide uranium will cause serious health and environmental problems.To achieve real-time monitoring and effective removal of highly toxic uranium,we synthesized amidoxime functionalized sp2 carbon conjugated fluorescent COF(TFPT-BTAN-AO)for the first time.TFPT-BTAN-AO has excellent chemical,thermal and radiation stability.TFPT-BTAN-AO shows an ultra-fast response time(2 s)and an ultra-low detection limit of 6.7 nM UO22+attributable to the abundant selective uranium-binding groups on the highly accessible pore walls of open 1D channel.At the same time,it has an exceptional uranium adsorption capacity of 427 mg g-1,indicating that the amidoxime-functionalized sp2 carbon conjugated fluorescent COFs material can not only suitable for on-site and real-time monitoring of UO22+,but also can effectively enrich UO22+.This study demonstrates great potential of fluorescent COFs for radionuclide detection and extraction.2.In this paper,an amidoxime-functionalized sp2 carbon-conjugated COF(NDA-TN-AO)with excellent photocatalytic and photoelectric activities was synthesized for the first time.The excellent photocatalytic effect endowed NDA-TN-AO with a high anti-biofouling activity by generating biotoxic reactive oxygen species and promoting photoelectrons to reduce the adsorbed U(Ⅵ)to insoluble U(Ⅳ),thereby increasing the uranium extraction capacity.The adsorption capacity of NDA-TN-AO to uranium in seawater reaches 6.07 mg g-1,which is 1.33 times of that in dark,indicating that the sp2 carbon conjugated COFs material with excellent photocatalytic and photoelectric activity can be used to efficiently enrich the nuclide uranium from natural seawater.3.The inherent features of covalent organic frameworks(COFs)make them highly attractive for uranium recovery applications.A key aspect yet to be explored is how to improve the selectivity and efficiency of COFs for recovering uranium from seawater.In this paper,a series of robust and hydrophilic benzoxazole-linked COFs materials(Tp-DBD,Bd-DBD,and Hb-DBD)have been developed for the first time as effective adsorbents for photo-enhanced uranium enrichment.Benefiting from the hydroxyl groups and the formation of benzoxazole rings,the Tp-DBD shows outstanding stability,selectivity,hydrophilicity and chemical reduction properties.Meanwhile,the synergistic effect of the hydroxyl groups and the benzoxazole rings in the π-conjugated frameworks significantly decrease the optical band gap,so that Tp-DBD has excellent photocatalytic activity,photothermal and photoelectric effect,thereby increasing the affinity and capacity for uranium.The adsorption capacity of Tp-DBD for uranium in natural seawater reaches 10.31 mg g-1,and it has excellent selectivity,indicating that COFs with excellent chemical stability,selectivity,hydrophilicity and chemical reduction properties are suitable for the selective enrichment of nuclide uranium from natural seawater.4.The design and synthesis of a uranium capture platform with high capacity,selectivity and efficiency,as well as outstanding photoactivity,stability and hydrophilicity remains a challenge.In this paper,we report the first example of covalent organic framework(DHBD-TMT)linked by unsubstituted olefin-linkages for selective loading,chemical reduction and photocatalytic reduction of uranium.The unique structures of DHBD-TMT possess all the characteristics to be well suited as a capture platform for selective ligand complexation,efficient chemical reduction and photocatalytic reduction of uranium,thus exhibiting a groundbreaking uranium capture capacity(2640.8 mg g-1).In the dark,DHBD-TMT can effectively adsorb uranium through the hydroxyl groups laced on the skeleton and reduce U(Ⅵ)to U(Ⅳ)in situ,leading to a higher adsorption capacity and selectivity of uranium.At the same time,the synergistic effect of the hydroquinone and triazine units in the extendedπ-conjugated skeleton significantly improve the photocatalytic activity of DHBD-TMT,and an additional U(Ⅵ)photocatalytic reduction mechanism can occur under visible light irradiation,allowing significantly higher the capacity and faster adsorption kinetics.