| Polyvinyl alcohol?PVA?has been widely used in human production and life due to its excellent performance,but PVA is difficult to be biodegraded.If it is not properly treated,it will cause pollution problems and affect the effective reuse of PVA-containing wastewater.Advanced oxidation based on ozone?O3?and hydrogen peroxide?H2O2?has strong oxidizing properties and is considered to be a promising technology for oxidative degradation of PVA,but there are still some shortcomings.The oxidation of O3 alone has the disadvantages of low utilization rate and high cost of occurrence;while the oxidation of H2O2 alone has the disadvantages of the slow oxidation process and low PVA removal rate.Studies have shown that homogeneous catalysis combined with advanced oxidation techniques such as O3 oxidation and H2O2 oxidation can promote the formation of active free radicals,thereby improving their oxidizing ability and increasing PVA removal rate.Commonly used homogeneous catalysts are mainly transition metal ions.Heteropolyacid?HPA?is a kind of solid multi-nuclear acid,which has the structural characteristics of complexes and metal oxides.HPA is a common chemical reaction catalyst,and is applied to various types of acid catalysis,catalytic oxidation and photochemical catalysis.However,there are few reports on the degradation of organic pollutants in wastewater by HPA-catalyzed ozone and hydrogen peroxide.In this paper,the catalysis of typical HPA on the oxidation of O3,H2O2 and O3/H2O2 was investigated by using PVA as the representative of biodegradable organic matter.The effects of different experimental factors on PVA removal rate and homogeneous catalysis in three systems were studied,and the optimal reaction conditions were screened.At the same time,this paper compares the removal effect of PVA under different combinations of O3,H2O2 and HPA,and speculates on the possible degradation pathway of PVA by quenching free radical experiment and Fourier transform infrared spectroscopy?FTIR?.In this paper,the preliminary research on supported heteropolyacid catalysts is also carried out,which lays a foundation for further research on the catalytic effect of HPA on O3 and H2O2oxidation in wastewater treatment.In the ozone/silicotungstic acid?O3/HSiW?system,the catalytic action of silicotungstic acid is related to reaction time,O3 concentration,system pH,silicotungstic acid dosage and reaction temperature.When the reaction time is shorter,the O3 concentration is lower,the pH value is lower,the amount of silicotungstic acid is larger,and the reaction temperature is lower,the catalytic effect is more significant.In the study of the degree of influence of factors,the pH of the system had the greatest influence on the removal rate of PVA,followed by the reaction temperature and O3concentration,while the influence of the amount of silicotungstic acid was relatively small.The optimum conditions for the removal of PVA in the O3/HSiW systemdetermined by orthogonal experiment are:O3 concentration 25 mg·L-1,reaction temperature 30°C,system pH 8.3,silicotungstic acid dosage 250 mg·L-1.Under the condition of degradation reaction for 5 min,the PVA removal rate can reach 98.3%,which effectively shortens the treatment time and reduces energy consumption.The addition of silicotungstic acid does not change the basic route of O3 oxidation degradation of PVA;the presence of silicotungstic acid can promote the decomposition of O3 to generate more hydroxyl radicals?HO·?and can also catalyze the direct reaction between O3 and PVA.In the hydrogen peroxide/phosphoric acid?H2O2/HPW?system,the longer the reaction time,the higher the concentration of H2O2,the higher the amount of phosphotungstic acid added,and the lower the pH,the higher the PVA removal rate in the system,the more obvious the catalytic effect of phosphotungstic acid,the effect of adding O3 on the removal rate of PVA was not obvious.Under acidic conditions for 5min,the removal rates of PVA in the six systems were as follows:O3/H2O2/HPW?54.6%?>H2O2/HPW?47.8%?>O3/HPW?21.2%?>O3?15.5%?>O3/H2O2?13.5%?>H2O2?9.9%?.It is indicated that the addition of phosphotungstic acid to the O3/H2O2 system can significantly increase the PVA removal rate.When catalyzing the oxidation of PVA with O3 and H2O2,H2O2 acts as the main oxidant to oxidize.In the H2O2,O3/H2O2 and O3/H2O2/HPW systems,the number of HO·is small and does not play a significant role,which may be caused by excessive H2O2quenching the HO·generated in the system.In the O3,O3/HPW and H2O2/HPW systems,HO·participates in the oxidative degradation process of PVA.Supported silicotungstic acid catalyst and supported phosphotungstic acid catalyst prepared with ZSM-5 molecular sieve,?-Al2O3,Fe-?-Al2O3(?-Al2O3modified by Fe3+)and Mn-?-Al2O3(?-Al2O3 modified by Mn2+)by solution impregnation method,ultrasonic impregnation method and mechanical grinding method did not significantly improve the removal rate of PVA in O3 system and O3/H2O2 system. |
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