Preparation And Purification Of G-C₃N₄-Based Composites For Polluted Water Of Underground Coal Gasfication

Underground coal gasification is a green method for clean utilization of coal,however it has the risk of pollution to groundwater.Graphite carbon nitride（g-C₃N₄）,as a new type of photocatalytic material,has been widely used in the field of water treatment due to the multiple advantages of simple preparation,visible light response,excellent photothermal stability and easy band gap regulation.However,the high photo-generated electron-hole pairs recombination,low specific surface area and visible light response of the pure g-C₃N₄seriously restrict the photocatalytic efficiency.Therefore,in order to improve the photocatalytic activity of pure g-C₃N₄,the methods of introducing nitrogen defects,constructing heterojunction and auxiliary load were used to regulate the band structure in this paper,so as to optimize its photocatalytic performance,and the purification effect of g-C₃N₄composite on Xinjiang bituminous coal gas washing water was investigated.Furthermore,the mechanism of enhanced photocatalytic activity was further discussed by studying the factors affecting the photocatalytic performance of g-C₃N₄composite.The specific research contents are as follows:（1）g-C₃N₄（CNx）photocatalyst modified with nitrogen defects was prepared by one-step calcination with melamine and ammonium persulfate as raw materials.The results showed that the nitrogen defects exist on the N-（C）₃bond of g-C₃N₄structure.The existence of nitrogen defects not only reduced the band gap of g-C₃N₄,but also effectively improved the separation efficiency of electron-hole pairs.Under the irradiation of visible light for 180 min,the degradation rate ofβ-naphthol by CNx-2 was75.3%,which was about 2.08 times that of g-C₃N₄.（2）Binary Z-type Ag₃PO₄/g-C₃N_4-x（APO/CNx）composite photocatalyst was prepared by in-situ growth method.The results displayed that the photoabsorption edge of APO/CNx composite was redshifted obviously by construction of heterojunction,which enhanced the optical adsorption capacity and improved the migration rate of photocarriers.Under visible light irradiation for 180 min,the degradation rate ofβ-naphthol by APO/CNx-4 was 90.5%,which was 1.76 and 2.27 times that of CNx and Ag₃PO₄,respectively.The results of free radical capture experiment and ESR test displayed that·O₂^-and·OH were the main active species in the photocatalytic degradation process.In addition,the Z-type photocatalytic reaction mechanism of APO/CNx composites was illustrated according to the energy band structure.（3）In order to further improve the photocatalytic activity of APO/CNx composites,the Ag₃PO₄/g-C₃N_4-x/GO（APO/CNx/GO）ternary composite photocatalyst was prepared with graphene oxide（GO）as the carrier.The results indicated that the loading of GO could effectively broaden the light adsorption range of the APO/CNx/GO composite photocatalyst,promote the separation efficiency of photo-generated carriers,and enhance the photocatalytic activity.The ternary photocatalyst with 1.0 wt%GO（APO/CNx/GO-2）showed the highest photocatalytic activity,and the degradation rate ofβ-naphthol reached 95.6%within 120 min,which was 1.33 times and 2.53 times that of APO/CNx and CNx,respectively.The improvement of photocatalytic activity of APO/CNx/GO composites could be attributed to the synergistic effect of GO loading,the construction of Z-type heterojunction and the LSPR effect of Ag.In addition,the purification study of simulated gas washing water showed that APO/CNx/GO composite material could make TOC removal rate of gas washing water reach 45.2%in 8 h.GC-MS results showed that APO/CNx/GO had better degradation effect on phenolic compounds in gas washing water.