Study On Heterogeneous Catalytic Oxidation With Composite Metal Oxides For PVA Degradation In Water

Polyvinyl alcohol?PVA?is widely used in industries and exists in corresponding effluents due to its water solubility.The poor biodegradability of PVA results in the low biological treatment efficiency.PVA is a high molecular polymer,which is likely to cause membrane blockage during the membrane treatment.PVA may promote the release of heavy metals from sediments into water,and the adverse effect on aerobic microbial activities due to the generation of foam,when PVA is discharged into water environment,resulting in water pollution and ecological damage.Therefore,it is necessary to pretreat this wastewater before the biological or membrane treatment.In this dissertation,efficient and stable catalysts were prepared,and the heterogeneous catalytic oxidation systems were established according to oxidant types,to degrade PVA.In addition,the influence factors,the catalytic activity and stability of the catalyst,the generated path of the main oxidative species and the mechanism of PVA degradation were investigated in details.The Cu-O-Mn/?-Al₂O₃ catalyst with Cu/Mn molar ratio of 1:2 and loading rate of 4wt.%was determined by screening the catalyst components and optimizing the preparation conditions.Based on the prepared catalyst,a catalytic ozonation system was established.After optimization,under ozone?O3?dosage of 5.5mg/?L·min?,catalyst dosage of 100 mg/L and reaction temperature of 25?,the simulated wastewater with initial PVA concentration of 20mg/L was degraded.The PVA removal rate in the system reached above 97%after 15 min and 10 min reaction at pH=7 and 10,respectively.The reaction accorded with the pseudo first-order kinetic model,and reaction rate constants in catalytic ozonation system were 0.232 min^-1 and 0.298 min^-1 when pH=7 and 10,respectively.The results of free radical scavenging experiments show that the catalytic ozonation systemfollowed the hydroxyl radicals?·OH?oxidation mechanism,and the catalyst surfacehydroxyl may be the key to decompose O₃ to·OH.·OH made the long chain of PVA molecules break randomly,resulting in a part of PVA mineralization,and some of them were oxidized into compounds with lower molecular weight,such as carbonyl compounds.However,the removal rate at pH=4 was only 60.5%after 20 min.Therefore,the system of H2O2 assisted catalytic ozonation was established by adding trace H2O2 under acid conditions.After optimization,PVA removal in the system could reach 93.2%after 10min under pH=3,O₃ dosage of 5.5 mg/?L·min?,catalyst dosage of 150 mg/L,H₂O₂ dosage of 1.4mg/l and reaction temperature of 25?.The reaction accorded with the pseudo first-order kinetic model,and reaction rate constant in the system was 0.246 min^-1.And the system followed the hydroxyl radicals?·OH?oxidation mechanism,trace H₂O₂ was adsorbed by the catalyst and-OOH formed at the active site was used to decompose O₃.The degradation pathway of PVA was similar to the catalytic ozonation without H2O2.In order to expand the application of the prepared catalyst,the catalyst was applied to catalyze and activate persulfate oxidation.The heterogeneous catalytic peroxymonosulfate?PMS?oxidation system was established by the prepared catalyst doping Co component.After optimization,under pH=7,PMS dosage of 2.5mmol/L and catalyst dosage of 700mg/L,PVA removal in the system could reach95.4%after 25 min.The reaction accorded with the pseudo first-order kinetic model,and the reaction rate constant was 0.128 min^-1.The system followed the sulfate radical(SO₄^·-)oxidation mechanism,and Cu-O-Mn/?-Al₂O₃ provided a large number of surface hydroxyl groups to interact with Co?II?to form the key Co-OH.The degradation pathway of PVA was similar to the catalytic ozonation system.The suitable catalytic ozonation systems were selected under different pH conditions to treat PVC centrifugal mother liquor wastewater,and the dosages of O₃,catalyst and H₂O₂ were 5 times of that under the optimum conditions according to COD of centrifugal mother liquor?about 5 times of simulated wastewater?.The results were as follows:at pH=3 and 5.3,the system of H2O2 assisted catalytic ozonation with better performance in acidic was used,and the PVA removal rates were 98.4%and 98.9%after 60 min,the COD removal rates were 68.4%and 67.0%at 120min,respectively.And at pH=7 and 10,the catalytic ozonation with better performance in neutral and alkaline was used,and the PVA removal rates were 97.5%and 98.1%after 60 min,the COD removal rates were 69.5%and 74.6%at 120 min,respectively.After the reaction,the leaching amount of metals was low,and the catalyst could still maintain high catalytic performance after 5 times of reuse,indicating that the catalyst had a good stability.The heterogeneous catalytic ozonation system and its improved system established in this study could realize the efficient and stable degradation of PVA in water in a wider pH range.The system with the small amount of catalyst and H₂O₂,the high utilization rate of O₃ and the convenient operation may provide a certain reference for the related research of catalytic oxidation technology and the pretreatment prior to the biological or membrane treatment of PVA wastewater.