Degradation Of Typical Organic Pollutants In Offgas From Thermal Desorption Treated Soils By Non-thermal Plasma

Organic pollutants contaminated sites,caused by the relocation of industial enterprises in Chinese urban area,are a class of high-risk contaminated sites.If not well remediated,They would be a threat to the health of surrounding residents and also affect the redevelopment of land resources in urban.Thermal desorption is a technology that repair sites contaminated by volatile or semi-volatile organic compounds.Due to high speed and high efficiency,it has been widely used in China and abroad.However,thermal desorption is a physical separation technology,the pollutants can only be evaporated from soil into the offgas.Therefore,the offgas should be treated before being discharged into the air.The cost of offgas treatment would account for nearly half of the thermal desorption technology cost.Therefore,it is imperative to develop an efficient and low-cost offgas treatment technology.In this paper,two commonly detected pollutants in contaminated sites,DDTs and PAHs,were choosed as representative orgainc pollutants.Degradation of these two organic pollutants in offgas from thermal desorption treated soils were studied by pulsed discharge plasma technology.Effect of process parameters?pulse voltage,pulse frequency,residence time?and offgas characteristic parameters?temperature,initial concentration,water content?on the degradation of organic pollutants were investigated.At the same time,the removal mechanisms of DDTs and PAHs in pulsed discharge plasma treatment were discussed by means of degradation products analysis,active substances analysis,and molecular orbital theory simulation under various inlet gas.These results would help the engineering application of thermal desorption offgas treatment by non-thermal plasma.The main work and the results are as follows:1.Study on the degradation characteristics of DDTs in offgas from thermal desorption treated soils by pulsed discharge plasma.The effect of pulsed discharge plasma on the removal of DDTs in soil thermal desorption offgas was investigated from the process parameters of pulsed discharge plasma and the offgas characteristic parameters.The degradation of DDTs was fast and high efficient in pulsed discharge plasma,the degradation rate of DDTs was up to 83.5%.The degradation rate of DDTs increased with the increase of pulse voltage,pulse frequency and the residence time,but decreased with the increase of initial concentration of DDTs.However,the absolute degradation amounts increased with the increase of initial concentration of DDTs.The degradation rate of DDTs decreased with the enhancement of the water content in the offgas.However,the degradation rate of DDTs could still maintain at a high level when the pulse voltage was relatively high,the high energy induced by the high pulsed voltage in the plasma reactor was probably the reason.2.Study on degradation mechanism of DDTs in offgas from thermal desorption treated soils by pulsed discharge plasma.The degradation rate of DDTs was highest in argon,followed by oxygen,air and nitrogen.DM.BH.DBP and DDMU with low contents were detected as the degradation products of DDTs.As the pulse voltage increased,the concentration of CO and HCl increased,the concentration of CO₂ decreased,meanwhile,NO and NO₂ were not detected.The content of H₂O₂ was very low under high temperature?150??.The concentration of O₃ and Cl^- were in the order of O₂>Air>Ar>N₂.The molecular configuration and energy of DDTs were calculated by molecular orbital simulation,and the possible degradation pathways of DDTs in offgas from thermal desorption treated soils were discussed.DDTs in the offgas were mainly degraded by the attack of high energy electrons and the oxidation of the active particles,that was,p,p'-DDT was dechlorinatied to produce p,p'-DDD and p,p'-DDE,p,p'-DDD was further dechlorinatied to produce DDMU.DDMU was directly oxidized by O₃ to produce DBH.DBH was further dechlorinatied to produce BH,and then-OH was removed by high-energy particles to generate DM.DM reactes with Cl₂ to produce DPM.DPM was further degraded to CO,CO₂,and H₂O.3.Study on the degradation characteristics and mechanism of PAHs in offgas from thermal desorption treated soils by pulsed discharge plasma.The differences of the degradation rate of naphthalene and DDTs under various inlet gas were compared and analyzed.The effects of temperature and inlet gas type on the degradation efficiency of PAHs were studied.The treatment of mixed PAHs in offgas including acenaphthene,phenanthrene,pyrene and benzo?a?pyrene were also investigated.Temperature of the plasma reactor had a significant effect on the degradation of naphthalene.At 50?,the degradation rate of naphthalene was 98.6%,at 100?,the degradation rate of naphthalene reached up to 99.5%,while the degradation rate of naphthalene was only 81.6%at 180?.These results indicates that the pulsed discharge plasma could efficiently degrade PAHs in offgas from thermal desorption treated soils.The degradation rates of the four PAHs?acenaphthene,pyrene,phenanthrene,benzo?a?pyrene?are more than 80%when the pulse voltage was 30.0 kV.The degradation rates of naphthalene at various inlet gas were in the order of O₂>Air>Ar>N₂.Through the comparison of the difference of molecular structures of naphthalene and DDT,It was found that the bond length of naphthalene was shorter than DDT,which means that the molecular bond length of naphthalene tended to average,therefore the whole molecule becomes stable.The binding force between molecules was stronger.Therefore,the fracture of naphthalene molecular requires higher energy.