Composites Modified By Organic Molecules Or Covalent Organic Frameworks For The Removal Of U(Ⅵ)

With the increase of energy demand,uranium has been widely developed as a strategic resource.Due to the inherent toxicity and radioactivity of uranium,a large amount of uranium-containing wastewater produced in the mining process will adversely affect the ecosystem and public safety.Uranium is usually available in quadrivalent（U（IV））and hexavalent（U（VI））forms,and U（VI）has higher fluidity,solubility,and toxicity compared to U（IV）.Therefore,exploring effective methods to remove U（VI）has become a key issue in the research.The adsorption method is the simplest method for uranium immobilization,which can capture uranium through the coordination of active groups.The composites can solve the shortcomings of poor selectivity and insufficient affinity of a single material.In addition,photocatalytic reduction of U（VI）to insoluble U（IV）is the most promising method for uranium removal,and composites can solve the shortcomings of narrow optical absorption range,easy electron-hole recombination and insufficient density of active sites of single semiconductor catalysts.Therefore,it is of great significance to develop new U（VI）immobilized materials and methods to solve the problem of uranium pollution in water.In this paper,organic molecules or covalent organic frameworks（COFs）were used to modify or compound single materials,so as to change the direction of electron flow and modulate band gap width to promote adsorption and photocatalytic reaction efficiency.A variety of composites with stable structure,large specific surface area,regular pore distribution and many active groups were prepared.It was used for U（VI）adsorption,photocatalytic reduction and plasmonic photocatalytic reduction in complex water environment,realizing effective removal of U（VI）in real water environment.The main research contents are as follows:1.Carbon nanotubes/covalent organic frameworks composites were used to remove U（VI）.In view of the insufficient adsorption performance of single carbon nanotubes（CNTs）,covalent organic framework COF-OH was grown in-situ on the surface of CNTs,then oxidized reduction composite CNT/COF-OH with high crystallinities,stable structure,and specific U（VI）binding site was synthesized,which was used to remove uranium from rare earth tailings wastewater.The introduction of COF-OH increased the active adsorption site of U（VI）,and the good electron transport capacity andπ-conjugation property of CNTs increased the electron density of adsorption functional groups,ensuring the reduction reaction.The U（VI）immobilization of CNT/COF-OH was realized through three synergies of electrostatic attraction,chemical complexation,and chemical reduction.The U（VI）removal capacity of CNT/COF-OH was 390.7%,54.6%,and 84.5%higher than those of CNT,COF-OH,and mixed CNT+COF-OH,respectively.Meanwhile,the selective separation coefficient of CNT/COF-OH was 1.6 times that of COF-OH.The uranium removal experiment of rare earth tailings wastewater was carried out by using CNT/COF-OH,and the removal rate of U（VI）was up to 96.7%.Therefore,the synergistic effect of carbon nanotubes modified by covalent organic frameworks has important theoretical and practical significance to solve the safety problems caused by harmful ions in wastewater.2.Photocatalytic reduction of U（VI）was enhanced by D-π-A structure of Bi₄Ti₃O₁₂-triazine-aldehyde benzene skeleton.The D-π-A structure was reconstructed and optimized by grafting electron-absorbing organic molecules onto the surface of Bi₄Ti₃O₁₂ due to its lack of adsorption sites and large band gap width.In this paper,Bi₄Ti₃O₁₂（B1）particles were synthesized first,and then B1 was crosslinked with6-chloro-1,3,5-triazine-diamine to obtain B2.Finally,B3 with D-π-A array structure was synthesized by Schiff base reaction of B2 and 4-formylbenzaldehyde to investigate the effect of D-π-A array structure on photocatalytic removal of U（VI）.B3,Bi₄Ti₃O₁₂（donor）-triazine unit（π-electron bridge）-aldehyde benzene（acceptor）crosslinking,can effectively extend the D-π-A structure,and by the end of the benzene ring ofπ-πinteraction to form[-A-π-D-π-A-（π-π）-A-π-A-]_n array.As a result,multiple polarized electric fields were formed inside B3,shortening the single transmission distance of electrons.Moreover,the electron push and pull effects increased the driving force of carrier transfer,which further reduced the band gap.Therefore,through the band matching effect,U（VI）was more likely to obtain electrons at the adsorption site of B3 and be reduced to U（IV）,which fully improved the photocatalytic efficiency.Under simulated sunlight,the U（VI）removal capacity by B3 reached 684.9 mg g^-1,which was 2.5 times greater than B1 and 1.8 times greater than B2.The structure of D-π-A array provides a design idea for modulating band gap width and improving the performance of photocatalytic U（VI）reduction.3.Sn S₂-covalent organic framework Z-scheme van der Waals heterojunction enhanced photocatalytic reduction of U（VI）.In order to solve the problems of low light-energy utilization efficiency of single-component catalysts,and insufficient active sites and easy recombination of photoelectrons-holes of all-inorganic heterojunction catalysts,a covalent organic framework with triazine structure and Sn S₂ closely packed composite Sn S₂COF were synthesized for the photocatalytic reduction of U（VI）in rare earth tailings wastewater.Sn S₂COF was a Z-scheme van der Waals heterojunction photocatalyst,and van der Waals interaction formed high speed electron transport channel.Due to the difference of Fermi energy level between the two components,the electrons of the composite spontaneously diffused at the interface,forming a built-in electric field.Under photoexcitation,the flow direction of electrons in the built electric field would be reversed,which further promoted the separation of photogenerated electron-hole,maintained the high reducibility of the conduction band,and avoided the photocorrosion of Sn S₂.Sn S₂COF had a high U（VI）reduction and removal capacity of 1123.3 mg g^-1,which far exceeded the U（VI）reduction capacity of Sn S₂ and COF.In rare earth tailings wastewater,the removal rate of U（VI）by Sn S₂COF was up to 98.5%,and Sn S₂COF had good selectivity and recycling.The design concept of organic-inorganic heterojunction composites provides a new strategy for improving the performance of photocatalytic U（VI）reduction.4.The double Schottky well and oxygen vacancy had a synergistic effect on plasmonic photocatalytic reduction of U（VI）.Plasmonic photocatalysis is an effective strategy to increase electron density due to the shortage of electron supply in single photocatalysis.Plasmonic photocatalyst Bi/Bi₂O_3-x@COFs was prepared by in-situ growing covalent organic framework on Bi/Bi₂O_3-x surface for U（VI）adsorption and photocatalytic reduction in rare earth tailings wastewater.The oxygen vacancy in Bi/Bi₂O_3-x and the Schottky potential well formed at the Bi/Bi₂O_3-x interface increased the number of free electrons,resulting in localized surface plasmon resonance（LSPR）,which enhanced the light absorption performance of Bi/Bi₂O_3-x@COFs.In addition,the oxygen vacancy increased the Fermi level of Bi/Bi₂O_3-x,making the Schottky barrier between Bi₂O_3-x and COFs interface became another potential well,and the electron transfer direction was reversed,thus increasing the electron density of the COFs layer.The outer COFs was N-type semiconductor with appropriate band structure and specific binding U（VI）groups,which could be used as active reaction sites.Due to the synergistic action of photogenerated charge and thermoelectric charge,the removal rate of U（VI）by Bi/Bi₂O_3-x@COFs was as high as 1411.5 mg g^-1.The U（VI）removal performance from rare earth tailings wastewater by composite was investigated through cyclic experiments,and removal rate of U（VI）by regenerated Bi/Bi₂O_3-x@COFs was still higher than 93.9%.Therefore,the design scheme of LSPR and Schottky potential well provides an effective way to increase electron density and improve photocatalytic performance.5.Dual-function MOF525@BDMTp had a synergistic effect of photocatalytic U（VI）reduction and chlorpyrifos（CP）degradation.Because radioactive ion U（VI）and organophosphorus pollutants contained in sewage will cause great harm to the ecological environment and human health,the one-to-two method of photocatalytic reduction of U（VI）and degradation of chlorpyrifos（CP）can effectively solve this problem.In this paper,a bifunctional photocatalyst MOF525@BDMTp with MOF525 as core and BDMTp as shell was synthesized by covalent bond bridging in-situ.In the dark condition,electrons flowed spontaneously from BDMTp to MOF525 due to the difference of Fermi energy level,resulting in the positive charge of BDMTp and the formation of internal electric field at the interface.The negatively charged surface of CP was more likely to interact with the positively charged part of the composite material,which improved the adsorption affinity.Under light conditions,electrons transferred from MOF525 to BDMTp due to the action of the internal electric field,which resulted in the increase of the electron density of BDMTp and activated the U（VI）binding site on the pore wall.The type-II heterojunction formed at the interface improved the efficiency of electron-hole separation and ensured the simultaneous electron reduction of U（VI）on BDMTp and the oxidation of CP on MOF525.Moreover,the presence of U（VI）and CP improved the photocatalytic performance of MOF525@BDMTp to the corresponding species.MOF525@BDMTp had a removal capacity of 625.0 mg g^-1 for U（VI）and a removal rate of 99.8%for CP.Therefore,the design of functional integrated composites is an effective means to improve the photocatalytic performance in complex environments.