Low PH Catalytic Ozonation Of Polyvinyl Alcohol Based On Metal Oxides And Carbon Nitride

polyvinyl alcohol（PVA）is a water-soluble polymer with special physical properties,which is widely used in various industrial production,so there are a lot of industrial wastewater containing PVA.When the membrane technology is used to treat the wastewater containing PVA,the PVA in the water will form a film on the membrane and cause membrane blockage.In addition,PVA is difficult to biodegrade,and foam is easy to form when it enters the water environment,which affects the water reoxygenation and causes the migration of heavy metals into the water,thus affecting the living environment of microorganisms in the water.Therefore,it is necessary to pretreat wastewater containing PVA before membrane treatment or biological treatment.Ozonation method is widely used because of its low selectivity,strong oxidation and environmental friendliness,and the addition of catalysts to form ozonation catalytic oxidation method also greatly improves the efficiency of oxidative degradation of organic matter.Nowadays,there are many low p H industrial wastewater,but the catalytic oxidation of ozone is not effective in the treatment of low p H wastewater because the decomposition of ozone is inhibited under acidic conditions.In order to solve this problem,a catalytic ozonation method based on Cu-O-Mn/γ-Al₂O₃metal oxide（CMA）catalyst was established in this research group,and low concentration hydrogen peroxide was used to treat PVA wastewater under low p H conditions,and a good degradation effect was achieved.Carbon nitride as a good photocatalyst can produce trace hydrogen peroxide and has excellent catalytic performance.Therefore,in this paper,carbon nitride is used to replace hydrogen peroxide on the original basis,and the ozone catalytic system established by interaction with CMA can show good degradation performance of PVA under low p H conditions,and expand the range of p H application.According to the above ideas,firstly,the modification method of carbon nitride and the optimum preparation conditions of modified carbon nitride were studied.Secondly,the optimum degradation conditions of catalytic ozonization system based on carbon nitride and metal oxides were determined.Finally,the effects of each component of the system,the reaction mechanism and the repeated use of carbon nitride were discussed.The results are as follows:By degrading PVA wastewater with p H=3 and comparing different modification methods of carbon nitride,the nitrogen gap modification method is determined.The optimal modification conditions of carbon nitride are as follows:melamine is ultrasonic for 2 h,put into a tube furnace and calcined at 500℃in air atmosphere for 5 h,and then mixed with 1:5 sodium hydrogen borate and calcined at 400℃in nitrogen atmosphere for 1 h.The characterization results show that the modified carbon nitride has mesoporous structure.In the reduction process,the opening of the triazine ring of carbon nitride leads to the destruction of pyridine nitride,resulting in the structure of C≡N and CN=C and N-C=N.Tertiary nitride forms nitrogen vacancy for the reduction site,resulting in the increase of the C/N atomic ratio.After the catalyst was added into the CMA/O₃system,the degradation rate increased from 48%to 99%within 25 min.After 5 times of repeated use,the catalyst can still maintain good catalytic performance and stability.A catalytic system was established based on the modified carbon nitride and CMA to degrade PVA with p H=3 and initial concentration of 14mg/L.The optimized reaction conditions are as follows:adding 100 mg/L of carbon nitride,100 mg/L of CMA,9.2 mg/L of O₃,and dark reaction for 30 min of carbon nitride adsorption.Under indoor lighting conditions,the removal rate of PVA by catalytic ozonation system can reach 99%within 25 min.The results show that the catalytic ozonation system follows the hydroxyl radical（·OH）oxidation mechanism,and hydrogen peroxide plays a key role in the system.The removal rates of CMA and carbon nitride catalysts were 1.8%and 8%,respectively.Secondly,the effect of ozone alone is limited at low p H,and the removal rate is only23%.After adding CMA catalyst,the degradation rate increased to 48%,indicating that CMA has catalytic effect.The degradation rate of CMA system can reach 99%after the introduction of trace hydrogen peroxide.When carbon nitride is added into the CMA/O₃system,the degradation rate also increases from 48%to 99%,which proves that the modified carbon nitride can replace hydrogen peroxide.This is because the modified carbon nitride and CMA have a synergistic effect to promote the decomposition of O₃.The active sites on the surface of the two catalysts absorb ozone and decompose it into active free radicals such as·OH.The generation of·OH triggers a chain reaction to promote the degradation of PVA,while providing a site for the degradation of PVA.O₃decomposes to form O₂^-and·OH at the active site on the catalyst surface,and then electron transfer occurs with H⁺in the solution to form HO₂^-,which further produces trace H₂O₂.Trace H₂O₂is absorbed on the surface of the catalyst and decomposed into HO₂^-and·OH,thus promoting the decomposition of O₃and producing a cycle in which active substances such as·OH are generated constantly,thus promoting the degradation of PVA.