Experimental Study On DBD-wet Homogeneous Catalytic Coupling Degradation Of Toluene

Volatile organic compounds（VOCs）,which are from a wide range of sources and complex in composition,are a crucial part of current air pollution.They can cause serious damage to the ecological environment and human health.Non-thermal plasma（NTP）process as a promising VOCs treatment technology has been widely concerned.Dielectric barrier discharge（DBD）is one of the most commonly used discharge methods for plasma oxidation of VOCs,which has the advantages of stable process and uniform discharge.However,one of the bottleneck problems restricting the practical application of DBD technology is the large amount of by-product ozone（O₃）emission in this study,it was proposed for the first time that the wet homogeneous catalytic system is connected in series with DBD,the by-product O₃ of DBD was used to generate hydroxyl radicals（·OH）in the wet system for further degradation of toluene.The feasibility of DBD-wet homogeneous catalytic coupling system was verified.The suitable metal ions were screened and used for the capture and conversion of O₃,and the operating parameters were optimized.The mechanism of DBD-wet homogeneous catalytic coupling degradation of toluene was analyzed based on EPR and GC-MS technologies.The main research conclusions are as follows:（1）DBD-wet washing coupling system had the potential to further improve removal rate of toluene by using ozone.When the initial concentration of toluene was263 mg·m^-3and the discharge voltage was 16 k V,the removal rate of toluene by DBD was stable at 43%,and the yields of CO₂ and O₃ were 34 mg·m^-3 and 433 mg·m^-3,respectively;In the DBD+Fe²⁺system,the removal rate of toluene stabilized at 56%after 30 minutes,the production of CO₂ reached 81 mg·m^-3,and the O₃ removal rate reached 72%.In view of that both the removal rate of toluene and the production of CO₂of DBD+Fe²⁺system were higher than that of single DBD system,especially the production of CO₂ was increased by 2.4 times,indicating that DBD-wet homogeneous catalytic coupling technology has a synergistic effect in the degradation of VOCs.Metal ions Fe²⁺,Mn²⁺and Cu²⁺improved the removal rate of ozone.In the range of 50-2000mg·L^-1ion concentration,the removal rate of O₃ increased with the increase of Fe²⁺and Mn²⁺concentration,while increased first and then decreased with the increase of Cu²⁺concentration.（2）The degradation of gaseous toluene by DBD coupled with Fe²⁺,Mn²⁺and Cu²⁺solutions of different concentrations exhibited that 1 g·L^-1 Fe²⁺had the best performance in the DBD-wet homogeneous catalytic coupling system.The removal rate of toluene was 60%,the production of CO₂ was 106 mg·m^-3,and the removal rate of O₃was nearly 90%.In the DBD+pure water system,the removal rates of toluene and O₃was increased with the increase of the initial p H value（p H:2,4,6 and 8）.When the p H was 8,the removal rates of toluene and O₃ reached 53%and 12%,respectively.In the DBD+Fe²⁺system,the highest removal rate of toluene and O₃ were obtained at p H=4,which were 60%and 90%,respectively.When the discharge voltage of DBD increased from 14 k V to 16 k V,the removal rate of toluene,the production of CO₂ and O₃ of a single DBD increased to 43%,34 mg·m^-3,and 433 mg·m^-3,respectively.The 240 mins degradation experiment(toluene:263 mg·m^-3,Discharge voltage:16 k V;Fe²⁺concentration was set as 1 g·L^-1)showed that the removal rate of toluene was stable at about 60%,and the removal rate of O₃ was basically stable at 88%in the first 120 min and slightly decreased after then.The DBD coupled Fe²⁺catalytic system had a good stability,because the liquid phase system could continuously produce·OH for toluene degradation.（3）The degradation mechanism of toluene in DBD coupled Fe²⁺system was studied by adding tert-butyl alcohol（TBA）,a free radical quencher.The results displayed that the removal rate of toluene decreased from 60%to 46%,and the production of CO₂ decreased from 106 mg·m^-3 to 18 mg·m^-3after the addition of TBA,indicating that TBA had significant inhibitory effects on the removal of toluene and the deep oxidation of toluene or organic intermediates.The DBD coupled Fe²⁺system followed the hydroxyl radical action mechanism,and the EPR results further proved the above inference.Finally,the degradation mechanism of toluene in DBD-wet homogeneous catalytic coupling system was analyzed based the determination of free radicals and intermediate products.