Preparation Of Carbon Nitride And Titanium Oxide-based Catalysts And Their Photo/electrocatalytic Reduction Properties For Uranium

The proposal of carbon peaking and carbon neutrality strategy has brought unprecedented opportunities for the development of nuclear energy.Among them,uranium is a key resource in the nuclear fuel cycle.Therefore,the recovery of uranium resources is of great significance for the sustainable development of nuclear energy;At the same time,due to the unique chemical properties of uranium itself,it may pose a certain threat to the environment.Therefore,the development of uranium separation and enrichment technology is of great significance to environmental protection and purification.A range of solid adsorption materials for uranium extraction have been developed,such as inorganic minerals,mesoporous silica,magnetic nanomaterials,carbon-based materials and advanced porous materials.However,the practical application of these materials is still inevitably limited due to the combined properties of stability,removal kinetics,adsorption capacity,selectivity,and reusability.The photocatalytic technology can reduce the easily soluble and easy-flowing U（Ⅵ）to the more difficult to dissolve and relatively non-flowing U（IV）,thereby overcoming the above problems and realizing the reduction and fixation of uranium,and the technology only relies on inexhaustible solar energy as the energy drive,which has the advantages of green and clean.Therefore,the separation and enrichment of uranium by photocatalytic technology is of great significance for the sustainable development of nuclear energy.However,there are still many difficulties in photocatalytic reduction of uranium,such as the photogenerated electron holes generated by the catalyst have a fast recombination speed,the visible light response of a single semiconductor material is weak,and additional sacrifices and inert gases need to be introduced in the reaction.In order to solve the above problems,this paper studies the performance effect of photocatalytic reduction of U（Ⅵ）by constructing heterojunctions,introducing oxygen vacancies,applying external electric fields and other strategies to modify the catalyst,and elaborates the mechanism of photocatalytic reduction of U（Ⅵ）with the help of advanced characterization methods.The main contents and conclusions of this paper are as follows:（1）Aiming at the problem of fast recombination of photogenerated electron-hole pairs in a single semiconductor,based on the excellent electron transfer ability of Fe²⁺/Fe³⁺cycle in La Fe O₃,a carbon nitride/perovskite oxide heterojunction composite（g-C₃N₄/La Fe O₃）was designed and used to simulate the photocatalytic reduction of U（Ⅵ）in aqueous solution under sunlight.Since the rapid recombination of photogenerated electron-hole pairs is inhibited,the lifetime of photogenerated carriers is significantly extended,and the proposed g-C₃N₄/La Fe O₃heterojunction exhibits efficient removal ability at a wide range of U（Ⅵ）concentrations（460 mg/g）.Moreover,after 5 cycles,the catalytic efficiency remains at a high level.（2）Aiming at the weak visible light response of g-C₃N₄,based on the strong interaction effect of conjugatedπbond structure,carbon nitride/activated carbon composites（CN/AC）were prepared,and their reduction properties on U（Ⅵ）under visible light were studied.The photocatalytic results show that the photocatalytic removal rate of U（Ⅵ）by CN/AC composites is 70 times that of traditional bulk g-C₃N₄.The strong interaction conjugate bond structure between g-C₃N₄and AC accelerates the migration of carriersπthereby extending electron lifetime.CN/AC composites are inclusive and resilient to a wide range of p H changes and abundant competitive anions/cations.Moreover,by electron microscopy characterization showed that U（Ⅵ）was reduced by photogenerated electrons and deposited at the edge of CN/AC composites.（3）In response to the coexistence of uranium and organic matter,based on the heterojunction-defect engineering co-modification strategy,the oxygen-rich C₃N₄-Ce O_2-xheterojunction was constructed and used for visible photocatalytic removal of U（Ⅵ）in organic radioactive wastewater.Kinetic characterization and density functional theory（DFT）show that photoelectrons are transferred from g-C₃N₄to Ce O_2-xthrough the built-in electric field generated by the heterostructure,and then captured by shallow wells generated by surface vacancies,thereby achieving carrier space separation.Therefore,the separation rate of photoinduced carriers is significantly increased,the lifetime is significantly longer（about 125%longer than g-C₃N₄）,and the photocatalytic activity is significantly improved（39 times stronger than that of traditional single g-C₃N₄）.In addition,C₃N₄-Ce O_2-xexhibits good selectivity for various competing ions,so it can maintain excellent performance in uranium-spiked seawater systems and organic radioactive wastewater systems.The photocatalytic reduction mechanism was further elaborated by X-ray absorption fine structure（XAFS）analysis,and the reduction and surface complexation of the inner sphere contributed to the fixation of uranium.（4）In view of the additional introduction of sacrificial agents and inert gases in photocatalytic reduction of uranium,based on the coupling advantages of photo-electrochemistry,nano-TiO₂conical arrays（nano TiO₂cone array/titanium mesh）were grown on titanium nets with flow-through structures,and used for photocatalytic（PEC）reduction in the air to extract uranium without adding sacrifices.The unique one-dimensional cone structure and external bias voltage drive the spatial separation of charges,coupled with the rapid fluid diffusion and material transport formed by the flow-through structure based on titanium mesh,resulting in excellent PEC performance.At-0.55V_{Ag/Ag Cl}time current density up to 3 m A cm^-2,efficient uranium fixation（97.1%）is achieved.In addition,despite the PEC performance of uranium extraction in a variety of demanding water chemical systems（weak acids,real seawater,carbonates,etc.）,uranium extraction remains impressive.With the help of advanced spectroscopy and electron microscopy,it is known that U（Ⅵ）is mainly reduced to U（Ⅳ） by electrons.