Design And Preparation Of Novel G-C₃N_4-x Photocatalyst And Study On Deep Purification Of Shale Gas Fracturing Flowback Wastewater

With the continuous development of the economy,energy shortages and environmental pollution problems have gradually threatened the further development of human society.Shale gas is a clean and efficient unconventional natural gas resource,which has begun large-scale mining in the Fuling area of China,promoted the economic development of the Yangtze River Economic Belt,and effectively guaranteed China's energy security.However,in the process of shale gas exploitation,a large amount of industrial wastewater containing refractory organic matter is generated,posing a great threat to natural ecology and human life.Photocatalytic technology is a sustainable,environmentally friendly technology that uses light energy to remove organic pollutants and has received extensive attention.Recently,the new non-metallic photocatalyst-graphite phase carbon nitride(g-C₃N₄)has a suitable bandgap(?2.7 e V),good physical and chemical stability,abundant raw materials,simple preparation process,resulting in a better application prospect.However,g-C₃N₄ has a higher recombination rate of photogenerated electrons and holes during the photocatalytic reaction,which lowers photocatalytic activity.To improve the photocatalytic activity of g-C₃N₄,three kinds of nitrogen-defective g-C₃N_4-x composite catalytic materials were developed through defect control,bandgap regulation and heterojunction formation and their properties were studied through simulation test.Finally,the prepared materials are used for the photocatalytic deep treatment of the liquid returning wastewater to reach the standard discharge or reuse,realizing the resource utilization of industrial wastewater.The main findings are as follows:(1)g-C₃N_4-x with a nitrogen-deficient structure was prepared through raising the heat polycondensation temperature to 625?.The raw materials were easy to obtain,and the preparation process was simple and easy.The composition and morphology of the samples were studied.The photocatalytic properties of g-C₃N_4-x were evaluated with Rhodamine B(Rh B)and Acid Red(AR)as target pollutants,and the reaction rate was2.40 and 1.66 times that of g-C₃N₄ respectively.Then the photocatalytic mechanism was discussed.It was believed that the introduction of nitrogen defects could improve the photogeneration,carrier migration,and separation rates,which could significantly improve photocatalytic activity.(2)Based on g-C₃N_4-x,a one-step synthesis method was adopted to synthesize Fe₂O₃/g-C₃N_4-x catalyst by compounding Fe₂O₃ on g-C₃N_4-x.The degradation rate constants of g-C₃N₄,g-C₃N_4-x,0.02Fe₂O₃/g-C₃N_4-x to Rh B were 0.0065 min^-1,0.0155min^-1 and 0.0791 min^-1,respectively,indicating that the photocatalytic performance of the composite material had been further improved based on the nitrogen defect,and the photocatalytic mechanism of the composite catalyst was discussed.(3)Mo O₃/g-C₃N_4-x composite photocatalyst with heterostructure was synthesized by stepwise synthesis.The bandgap position of the prepared Mo O₃ was 2.94?-0.04 e V.The photocatalytic degradation and bactericidal performance of Mo O₃/g-C₃N_4-xcomposites were better than g-C₃N_4-x.The main result was that the internal electric field in Z-scheme charge transfer reduced the recombination probability of photogenerated electron-hole pair,thus promoting the production of·OH,·O₂^-and h⁺,which was conducive to the photocatalytic reaction.(4)Bi₂Mo O₆/g-C₃N_4-x composite photocatalyst with the Z-scheme electron transfer mechanism was successfully synthesized by hydrothermal synthesis.Combined with the test results of XRD,TEM and XPS,the structure,morphology and element valence composition of the sample was characterized.The results showed that the z-scheme electron transfer could significantly improve the photocatalytic activity of the composite materials mainly,because the formation of the Z-scheme carrier transfer path in the composite materials reduced photogenerated electron-holes,reducing recombination rate of photogenerated electron-hole pair,and the lifetime of photogenerated carriers was prolonged,enhancing the photocatalytic activity of the catalyst.(5)The wastewater discharged from the shale gas pretreated by the flocculation process was subjected to visible light purification treatment using g-C₃N₄,g-C₃N_4-x,0.02Fe₂O₃/g-C₃N_4-x,0.50Bi₂Mo O₆/g-C₃N_4-x and 0.03Mo O₃/g-C₃N_4-x.Results showed that the introduction of nitrogen defects could improve the purification effect of graphite phase carbon nitride materials on the effluent.The photocatalytic properties of 0.02Fe₂O₃/g-C₃N_4-x,0.50Bi₂Mo O₆/g-C₃N_4-x and 0.03Mo O₃/g-C₃N_4-x composites could further improve the photocatalytic properties.The degradation process of organic pollutants in the backwater by the five photocatalytic materials began with the degradation of macromolecular organic substances into small molecular organic substances,which were finally mineralized into CO₂ and H₂O.After photocatalytic deep purification,COD and other indexes of tailwater were lower than the limits stipulated by national standards and could be discharged or reused.