Study On Degradation And Mechanism For Typical Gaseous Toluene And O-chlorotoluene Based On Ultraviolet Coupling With Advanced Oxidation Processes

Volatile organic compounds(VOCs)are important precursors of PM2.5 formation and cause serious environmental problems,such as haze and photochemical smog.Gaseous toluene and o-chlorotoluene are typical pollutants among the known VOCs,having high biological toxicity,respectively.There are some disadvantages involved with conventional treatment technologies,such as incompleted purification.And then they will bring about secondary contaminants.In this paper,gaseous toluene and ochlorotoluene were selected as model pollutants.The advanced oxidation processes based on hydroxyl radical(·OH)or sulfate radical(SO4·-)were performed using conventional Fenton reagent or persulfate coupling with ultraviolet(UV)in a selfdesigned bubble bed reactor,respectively.The effects of key parameters on the reaction systems were systematically studied based on gas chromatography,electron paramagnetic resonance,free radical quenching tests,GC-MS and so on.The removal efficiency and degradation mechanism of gaseous toluene and o-chlorotoluene in the UV/Fenton and UV/Na2S2O8(UV/PS)processes were elucidated.The main conclusions of this paper are as follows:(1)The degradation performance and reaction mechanisms of gaseous toluene in the UV/Fenton reaction system were clarified.The results demonstrated that the removal efficiency of gaseous toluene in the traditional Fenton process and the UV254/Fenton process reached 55.3%and 63.8%,respectively.On the contrary,the removal efficiency of gaseous toluene in the UV365/Fenton reaction system reached 85.3%in 60 min,and the mineralization rate reached 100%.The kinetic study showed that gaseous toluene degradation in the UV365/Fenton process fitted well with the pseudo first-order kinetic model.The coupled process of UV365 irradiation and Fenton reaction had significant synergistic effect.Under continuous running of 120 min,the removal efficiency of gaseous toluene maintained at 85%,indicating that the reaction system performed excellent operation stability.Besides,UV365 irradiation promoted the photo reduction of Fe3+to Fe2+,and ensured the chain reactions onging to solve the cycling problem of iron ion in the traditional Fenton reaction system.It was proved that·OH played the key role in the degradation of gaseous toluene by the detection and quenching tests of free radicals.The intermediates of gaseous toluene degradation were identified by GC-MS,implying the possible reaction mechanisms,such as H-atom abstraction,·OH addition,ring opening and oxidation.(2)The degradation performance and reaction mechanisms of gaseous toluene in the UV/PS reaction system were clarified.It was shown that the removal efficiency of gaseous toluene reached 86.0%in 30 min,and gas absorption rate was 1.49×10-4 mol·m-2·s-1.Under continuous running of 120 min,the removal efficiency of gaseous toluene was kept above 50%.The intermediates of gaseous toluene degradation were identified by GC-MS and implied the possible reaction mechanisms,such as H-atom abstraction,electron transfer,·OH addition,ring opening and oxidation.(3)The degradation performance and reaction mechanisms of gaseous ochlorotoluene in the UV365/Fenton reaction system were clarified.It was shown that the removal efficiency of gaseous toluene reached 83.5%,and gas absorption rate 3.61×10-4 mol·m-2·s-1.Under continuous running of 240 min,the removal efficiency of gaseous o-chlorotoluene was maintained at 86%.The intermediates of gaseous ochlorotoluene degradation were identified by GC-MS,implying the possible reaction mechanisms,such as dechlorination,Cl-atom transfer,H-atom abstraction,·OH addition,electron transfer,ring opening and oxidation.(4)The degradation performance and reaction mechanisms of gaseous ochlorotoluene in the UV365/PS reaction system were clarified.It was shown that the removal efficiency of gaseous o-chlorotoluene reached 94.0%in 30 min,and gas absorption rate 2.44×10-4 mol·m-2·s-1.The UV365/PS reaction system had better purification performance than UV365/Fenton.Under continuous running of 180 min,the removal efficiency of gaseous o-chlorotoluene was maintained at 60%.Under continuous running of 180 min,the removal efficiency of gaseous o-chlorotoluene was maintained at above 60%.The SO4·-and ·OH were detected together by EPR in the UV365/PS reaction system.The results of free radical quenching tests indicated that SO4·-generated from the UV365/PS reaction system took dominant part in the degradation of gaseous o-chlorotoluene.The intermediates of gaseous o-chlorotoluene degradation were analyzed by GC-MS,implying the possible reaction mechanisms,such as dechlorination,electron transfer,ring opening,·OH addition,and oxidation.