| The demand for uranium resources is increasing with the development of nuclear power industry and the needs of nuclear strategy,produce a large amount of wastewater with uranium radionuclides.These sewage through the natural infiltration,water migration into the natural water source,will cause serious harmful for to human and environment.It is urgent to develop a new technology which efficient and sustainable technology for the treatment of wastewater with radionuclide uranium.In recent years,the photocatalytic reduction of U(?)with graphitic carbon nitride(g-C3N4)has become the focus of researchers.However,due to the limitations of the preparation method and the characteristics of the polymer material,the bulk g-C3N4faces the deficiencies of low specific surface area,high photogenerated carrier recombination rate,and low utilization of visible light,which seriously limits its practical application in photocatalytic reduction of U(?).In order to solve the above shortcomings,this thesis mainly focuses on research on morphology regulation(e.g.,acid etching),energy band regulation(e.g.,metal doping,thiophene monomer copolymerization),surface modification(e.g.,co-catalyst loading,three-dimensional carbon composite).The contents of this research are as follows:(1)Acid etching for the microstructure regulation of g-C3N4.The bulk phase g-C3N4was prepared by simple thermal polymerization with urea as precursor,the modified g-C3N4was obtained by"thermal polymerization-acidification-annealing"method.The results showed that the g-C3N4-NA7.6 showed a coral-like porous structure with few layers,which had the advantages of large specific surface area and narrow band gap.The reduction rate of U(?)reaches 94%in 100 min under visible light,and the reduction activity was 2.5 times than that of bulk g-C3N4.After 5 cycles of experiments,the reduction rate was still maintained at about 90%with good stability and repeatability.(2)Optimizing the energy band structure of g-C3N4 by alkali metal doping to improve the activity of photocatalytic reduction of U(?).The PNa-g-C3N4 was prepared by simple thermal polymerization with urea and sodium compounds(sodium nitrate,sodium hydroxide,sodium bicarbonate)as precursor.The photocatalytic reduction of U(?)by PNa-g-C3N4 was studied.The results showed that sodium doping improved the structure and chemical composition of g-C3N4 and perfected the delocalized conjugated system of g-C3N4.The Na atom coordinated with N atoms in tri-s-triazine units to form the Na-N bond which act as"electron bridge"that promote the separation and transfer of photo-induced electrons,optimize the energy band structure and electron configuration,broaden the visible light adsorption range and improve the separation and transport efficiency of photogenerated carriers of g-C3N4.The effects of p H value and initial U(?)concentration on the photocatalytic reduction performance of PNa-g-C3N4 catalyst were investigated.The PNa1-g-C3N4 prepared with sodium nitrate exhibits the best photocatalytic reduction activity of U(?).Under visible light irradiation,the reduction rate of U(?)reaches 96%in 20 min,the apparent rate value is 0.071min-1,which is 7.1 times than that of bulk g-C3N4.The reduction removal rate reached 90%after 5 cycles,indicating that PNa1-g-C3N4 has a high stability for photocatalytic removal of U(?)pollutants.(3)Thiophene copolymerization regulates g-C3N4?-conjugated system to enhance the photocatalytic reduction activity of U(?).Thiophene group grafted g-C3N4(CNTA)was synthesized by nucleophilic copolymerization with urea and 3-thiophenoic acid(3-Th A)as a copolymer.The results show that the introduction of thiophene can regulate the?-conjugated system and electron configuration of g-C3N4 at the molecular level,promote the transport and separation efficiency of photogenerated carriers,and thus enhance the activity of g-C3N4 photocatalytic reduction of U(?).Through the study of the reaction system environmental factors on thiophene functionalized CNTA photocatalytic reduction of U(?)performance,96%U(?)in the solution was reduced by CNTA0.1in 80 minutes under visible light.(4)Surface modification g-C3N4 by Ni(OH)2 for enhanced photocatalytic activity of reduction of U(?).Simple precipitation method was used to synthesize Ni(OH)2/g-C3N4.And Ni(OH)2 as co-catalyst was deposited on the surface of g-C3N4.The physical and chemical properties and photoelectric properties of the catalyst were characterized and analyzed.The results show that the surface modification with Ni(OH)2 can regulate g-C3N4at the molecular level,optimize the electron configuration structure,chemical composition and light absorption properties of g-C3N4,and improve the transfer rate of photoelectric charge as well as the utilization of visible light.The activity of photocatalytic reduction of U(?)by 0.5%Ni(OH)2/g-C3N4 under different environmental factors was studied.It was found that the optimal reduction rate of U(?)in solution was 89%under illumination in 80min and the reduction rate was twice that of bulk g-C3N4.(5)3D carbon materials/g-C3N4 composites for enhanced photocatalytic reduction of U(?).The porous carbon materials with 3D structure were composite with g-C3N4 by chemical impregnation and thermal polymerization.The 3D PC/g-C3N4 with strong light collection and good internal coordination was obtained with the advantages of curved three-dimensional cage structure,large specific surface area and low recombination rate of photo-induced electrons and holes.The results showed that 3D PC was introduced with excellent conductive properties and became the transfer center of photogenerated electrons in the photocatalytic reaction,effectively inhibiting the recombination of photogenerated electron holes,thus greatly improving its activity for photocatalytic reduction of U(?).The photocatalytic reduction of U(?)by 3D PC/g-C3N4under the influence of different environmental factors was studied.It was found that 20%3D PC/g-C3N4 can reduce 90.6%U(?)in solution under visible in 100 min,and the reaction rate is about 4.4 times than that of bulk g-C3N4 with good repeatability and stability.Finally,the effects of the above five catalysts in photocatalytic reduction of U(?)were compared comprehensively,and the application prospect of g-C3N4-based photocatalytic materials in the treatment of Uranium-containing wastewater was proposed. |
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