Performance And Mechanism Of Photocatalytic Reduction Of U（Ⅵ） By Metal Sulfide@g-C₃N₄

Nuclear energy is a kind of clean energy with good comprehensive economic benefits,which can replace the use of fossil fuels to reduce greenhouse gas emissions and play an important role in mitigating climate disasters and other environmental problems.However,with the development of uranium mining and utilization,a large number of waste liquid containing a large number of radionuclides is produced,and the radioactive substances in the environment are continuously accumulated,which has caused great harm to human health and caused increasingly serious environmental and public health problems.In order to solve and prevent the pollution of uranium containing wastewater,the reduction of soluble U（Ⅵ）to insoluble U（IV）oxide has been recognized as one of the effective ways to reduce the toxicity and migration of uranium.Among them,photocatalytic materials have attracted much attention due to their low preparation cost,mild reaction conditions and strong reduction ability.Graphite carbonitride（g-C₃N₄）has many advantages of new polymer semiconductor materials,such as suitable band gap（2.7e V）,absorption of visible light,simple preparation method and environmental friendliness.It can be used in many fields,such as organic matter degradation,decomposition of H₂O,hydrogen production,CO₂ and heavy metal ion reduction.However,due to its high electron hole ratio,small specific surface area and low photogenerated electron yield,its practical application is affected.Therefore,the photocatalytic activity of the original g-C₃N₄ under light irradiation still needs to be improved.In order to overcome these shortcomings,the binary composite photocatalyst was synthesized by in-situ thermal polycondensation.The synergistic effect between metal sulfide and g-C₃N₄ makes the photocatalyst have a wide range of light response,which improves the photocatalytic activity and finally reduces U（Ⅵ）.The specific research contents of this paper are as follows:（1）In this paper,we build a binary structure to adjust the band gap and absorption intensity of the material to improve the photocatalytic activity of U（Ⅵ）.XRD,SEM,TEM,XPS,UV and PL were used to characterize the in-situ thermal polycondensation.The microstructure and physicochemical properties of the ZnS@g-C₃N₄composite photocatalyst were characterized.The results showed that under light irradiation,when pH=5.0,The reaction rate constant of the ZnS@g-C₃N₄-5 sample is 0.01824 min^-1,which is 4.34 times of the original g-C₃N₄.Due to the narrowing of the band gap,the charge transfer efficiency increases and the carrier recombination efficiency decreases,the photocatalytic reduction of U（Ⅵ）is obviously enhanced.ZnS@g-C₃N₄-5 catalytic elution experiment:after five times of photocatalytic reaction,the photocatalytic reduction performance of the material for UO₂²⁺did not decrease significantly,indicating that the composite photocatalyst can be reused.Through the exploration of photoelectric performance,compared with g-C₃N₄,it has wider optical absorption range,lower fluorescence intensity,higher photocurrent response intensity and lower impedance.These results strongly indicate that the synthesis of composite photocatalyst is an effective way to solve and repair the reduction of radioactive pollutants.（2）In this paper,a series of characterizations such as XRD,SEM,TEM,XPS,UV and PL were used to characterize the binary materials synthesized by in-situ thermal polycondensation.The physical and chemical properties of the WS₂@g-C₃N₄photocatalyst were characterized.The WS₂@g-C₃N₄ composite photocatalyst was used for photoreduction of U（Ⅵ）.The results show that the material has high photocatalytic reduction efficiency of U（Ⅵ）.At pH=5.0,The WS₂@g-C₃N₄-5 photocatalytic performance of the sample is the best,which is 3.7 times of that of the original g-C₃N₄.The remarkable performance is due to the narrowing of the band gap,the increase of charge transfer efficiency and the decrease of carrier recombination efficiency.Through the exploration of photoelectric performance,compared with g-C₃N₄,the WS₂@g-C₃N₄ has wider optical absorption range,lower fluorescence intensity,higher photocurrent response intensity and lower impedance.（3）In this paper,a series of characterizations,such as XRD,SEM,TEM,XPS,UV and photoluminescence,were used to characterize the in-situ thermal polycondensation of 2H type MoS₂@g-C₃N₄.The physical and chemical properties of the catalyst were characterized.The characterization results showed that under light irradiation,when pH=5.0,the photocatalytic activity of the 2H-MoS₂@g-C₃N₄-10 sample is the best,which is 12 times of that of g-C₃N₄.The remarkable optical performance of the proposed method is attributed to the narrowing of the band gap,the increase of the charge transfer efficiency and the decrease of the carrier recombination efficiency.Finally,the photocatalytic mechanism of the composite was studied.