Artifical Generation Of Oxidative Radical For The Abatement Of Gas Pollutants And Its Mechanism

With the rapid development of economy,a large amount of gaseous pollutants (e.g.,volatile organic compounds and sulfides)have been released into the atmosphere,which results in the increasingly serious environmental pollution and threatens human health and harmonious development of human and nature.There exists a stable balance between substance and energy in the environment that human depends on,for certain amount of pollutant emission can be cyclically purified inside the environment.The interaction mechanism of oxidative free radicals with various pollutants in atmospheric circulation and the transport and transformation of pollutants in environment are important processes for maintaining this balance in ecological environment,which have already been widely studied.The aim of the thesis is to simulate the interaction process between oxidative free radicals and various pollutants in atmospheric circulation under artificial conditions and apply this technique to control air pollution so that pollutants can be removed and utilized before being emitted into the environment.Firstly,by use of electron spin resonance spectrum technology(ESR), hydroxyl radicals(HO.)formed in corona discharge process were trapped and determined;O₂ and H₂O molecule as the source of HO.formation was identified and the formation of HO.under artificial conditions was confirmed,which lays the foundation for the study on simulating the reactions between oxidative free radicals and volatile organic compounds(VOCs)in atmospheric environment in the laboratory.The degradation of aromatics by HO.formed in DC corona discharge was mainly studied and the effect of operating conditions on degradation efficiency was investigated by means of GC,HPLC,UV-VIS and FTIR,the intermediate products was determined.The results show that the increase of peak voltage(lower than sparking voltage)and residence time can lead to the yield increase of oxidative radicals,which can be beneficial to the degradation of pollutants,and that the relative humidity(RH)of gas has a large effect on degradation efficiency.With air as background gas,the highest degradation efficiency of benzene,whose initial concentration was 304 mg/m³,increased from 55%to 78%when RH was increased from 5%to 70%;while with nitrogen as background gas the highest degradation efficiency of benzene increased from 29%to 74%under the same conditions.Some soluble intermediate products were determined by analyzing intermediates collected by the liquid phase absorption solution with HPLC,the degradation course of benzene was confirmed was the continuous addition of HO.radicals on benzene ring,which led to the ring open and deep oxidation.When benzene,toluene, ethylbenzene and xylene(BTEX)co-existed,there was a competitive mechanism in their degradation,with the following sequence from easiness to hardness:o-xylene＞ethylbenzene＞toluene＞benzene,and the more branch chains there were on benzene ring,the more easily be attacked and broken.Based on that,the enhancement and mechanism of toluene degradation in aerosol containing transition metal ions was further investigated.Aerosol systems containing Fe²⁺,Co²⁺and Mn²⁺were prepared by aerosol dispersion technique,and the effect of the kind and concentration of ions in micro-droplets on toluene degradation rate was examined.It was found that three kinds of ion were similar in their enhancement,among which the enhancement of Mn²⁺was slightly stronger than that of Co²⁺and Fe²⁺,and the enhancement effect was the most obvious when the concentration of ion was 0.05 mol/L.The enhancement mechanism of toluene oxidation rate was presented as follows:(1)by reacted with free radicals,toluene was partially degraded in the gas phase to form soluble intermediates which could dissolved into mini-droplets through the gas-liquid interface.(2)oxidants such as H202 generated in the discharge process occurred Fenton-like reaction with the transition metal ions in the mini-droplets to generate strong oxidative HO·.(3)the dissolved toluene and intermediate products further occurred redox reaction with HO·radicals with the result of being degraded.These processes were confirmed by the determination of intermediates in both gas phase and liquid phase,which well corresponded with the multiphase transformation mechanism of pollutants in complex atmospheric aerosol.Hydrogen sulfide,carbon disulfide,dimethyl disulfide,thiophene,dimethyl sulfide and diethyl sulfide were typical odor gas which largely affects the climate in the atmosphere.The transference and transformation of them attracts the researchers' attention and has been widely studied.The experimental results show that the odor gas was more easily degraded by the existence of oxidative free radicals.The increase of oxygen content in background gas,discharge power or residence time was all beneficial to the removal of pollutants,and it was difficult to decompose COS because of its chemical inertia.Oxidation products of other sulfur compounds were mainly COS,SO₂ and CO₂,which accords with the corresponding sulfide transformation law in atmospheric circulation.The feasibility of oxidizing toluene with NO_x as oxygen source was preliminarily discussed.It was proved by experiments that the oxygen in NO and NO₂ occurred oxidation reaction with toluene molecules serving as a target. electronic donor,NO_x(NO and NO₂)serving as an electron acceptor and providing oxygen atom to oxidize toluene and itself reduce to N₂.The effect of oxidative free radicals generated by corona discharge on bacterial inactivation in microbe contained aerosol was observed and the results show that the reactor inter-electrode voltage and treatment time are main factors to influence bacterial inactivation efficiency.When the initial concentration of bacteria was 3×10₆ CFU/m³,inter-electrode voltage 20 kV and treatment time lower than 150s, the bacterial inactivation efficiency was 99.99%;while inter-electrode voltage changed to 10kV and treatment time lower than 35 s,the bacterial inactivation efficiency could still be up to 97.90%.It was found experimentally that strong oxidative active particles formed from corona discharge,such as HO·,have oxidative effect on cell wall,cell membrane and other parts of microorganism which resulted in bacterial inactivation.By controlling the inter-electrode voltage of the reactor,action time and other parameters(e.g.,V_p = 10 kV-12 kV,t≥35 s), enough oxidative active particles could be generated which lead to bacterial inactivation without ozone formed.The industrial scale-up design and survey were made on treating gaseous pollutants by HO·generated by corona discharge.Pilot tests for volatile waste gas from wastewater treatment in chemical plants and pharmaceutical factories and odor sulfur-gas in refineries were carried out by the pilot reaction equipment with processing ability of 5000 m³/h.The experiment results show that for hydrogen sulfide whose concentration was lower than 1000 mg/m³,its removal rate was mostly higher than 90%.The DC corona discharge reactor with pretreatment had good treatment effect on some volatile odor waste gases of two sewage treatment plants:the removal rates of odor sulfur gas with low concentration,halogenated alkanes and pyridine were mostly 70%-80%,those of aromatics and total organic compounds were all over 80%,and those of odor containing sulfur and MIBK were over 95%.The experiment shows that this technique has wide scope of application and good application prospect.