Study On Remediation Of Soil Polluted By Organic Pollutants Using Non-thermal Plasma Technology

Because of the acceleration of industrialization and urbanization in China,a large number of pollutants are discharged,resulting in serious soil pollution,among which organic-polluted soil is another main form of soil pollution besides heavy metal pollution.Traditional soil remediation methods have some limitations,such as secondary pollution,addition of chemicals or long treatmet time and limited types of treated pollutants.Therefore,it is urgent to develop a green,efficient and rapid soil remediation technology.Non-thermal plasma is a new advanced oxidation technology,which is gradually applied in the field of environmental pollution remediation.During the plasma discharge,the gas or liquid molecules in discharge area collide and ionize with high speed electrons,generating active species,ultraviolet light,ultrasonic wave,microwave radiation and thermal effects.This technology ia a rapid,effective and environmentally friendly technique to remediate soil pollution without chemical injections.In this study,dielectric barrier discharge plasma and pulse corona discharge plasma were utilized for the treatment of soils contaminated with gasoline and PFOA.Effects of discharge typies,electrical parameters and soil characteristics on degrading efficiency were discussed,and the mechanisms of pollutant degradation were analyzed.Influences of structure,composition,feitility and safety on soil after the treatment were investigated by comparing soil properties and planting lettuce in the soil before and after plasma treatment.The main research conclusions were shown as follows:(1)Gasoline was chosen as the target contaminant to investigate the effect of reactor parameters,soil characteristics and mechanism on gasoline degradation using dielectric barrier discharge and pulsed corona discharge plasma.The degradation efficiency of gasoline was 90% and the energy efficiency was 0.0772 mg/k J after 60 min treatment using dielectric barrier discharge plasma,under the conditions of input power 81 W,electrode gap 15 mm,initial concentration of gasoline 5000 mg/kg,soil p H 7.2,soil moisture 10%,and air flow rate 0.6 L/min,.With initial concentration of5000 mg/kg,30 k V of discharge voltage,50 Hz of discharge frequency,38 W of power input,20 mm of discharge gap,7.2 of p H value,7.9% of soil moisture,0.6L/min of air flow rate,the degradation efficiency of gasoline was 90% using pulse corona discharge plasma.(2)The main active radicals for gasoline degradation during non-thermal plasma discharge were studied by controlling soil moisture content and discharge atmosphere.A series of byproducts including 2-methylpentane,3-methylpentane and hexane were detected by GC-MS.The analysis result of the total carbon in the soil revealed that 26%of gasoline was mineralized,14% of gasoline was volatilized and 33% of gasoline was broken into small molecule byproducts.Finally,nitrogen in soil before and after plasma treatment was investigated.It was found that after treatment,the ammonia nitrogen in was obviously increased,and the nitrate nitrogen in soil was also increased by recycling exhaust gas.It indicates that plasma technology can degrade organic pollutants simultaneously with increasing nitrogen,promoting nitogen cycle process in soil.Compared with polluted soil,lettuce planted in the soil after discharge showed better germination rate,dry weight and lettuce growth.Scanning electron microscope(SEM),energy dispersive X-ray spectroscopy(EDS)and X-ray diffraction(XRD)were used to analyze the soil before and after the treatment,the results illustrated that there were no obvious differences for the soil before and after plasma treatment,except hydrophilicity of soil was improved after discharge.Plasma technology can be used as a rapid,efficient and friendly remediation method for organic pollutants in soil with simultaneous enhancement of soil fertility and safety.(3)PFOA was chosen as the target contaminant to investigate the effect of reactor parameters,soil characteristics and mechanism on PFOA degradation using dielectric barrier discharge and pulsed corona discharge plasma.Results showed that the removal rate of PFOA was as high as 87% using dielectric barrier discharge plasma,under the condition of input power 81 w,the discharge gap was mm,the initial concentration 300 mg/kg,the soil p H 6.3,the soil moisture 2%,and the air flow rate 0.6 L/min.78% of the PFOA was degraded using pulse corona discharge plasma,with 30 k V of voltage,50 Hz of discharge frequency,38 W of input power,20 mm of electrode gap,5% of soil moisture,300 mg/kg of PFOA concentration,6.3 of soil p H and 0.6 L/min of gas flow rate.(4)The dominant active agents for PFOA degradation were investigated by changing discharge atmosphere and adding electron scavengers.The by-products detected by LC-MS and the bond length and angle of PFOA calculated based on DFT illustrated the degradation pathway,by which the C-C bonds in PFOA were broken and PFOA decomposed into smaller by-products including perfluoroheptanoic acid(PFHp A),perfluorohexanoic acid(PFHx A),perfluoropentanoic acid(PFPe A),perfluorobutyric acid(PFBA),pentafluoropropionic acid(PFPr A)and trifluoroacetic acid(TFA).The PFOA degradation pathway was also inferred.51%of PFOA was decomposed into short chain by-products,19% mineralized,and 1%volatilized by TC analysis.The efficiency and safety of the two discharge technologies was proved by planting lettuce in soil before and after plasma treatment.(5)Through the comparative study of dielectric barrier discharge plasma and pulse corona discharge plasma in treating gasoline and PFOA contaminated soil,it can be seen that the two different plasma technologies could act as a rapid and efficicent remediation on gasoline and PFOA contaminated soil.However,there are some differences in the remediation of gasoline contaminated soil,such as the discharge waveform,input power and energy efficiency.In the same condition,the input power of the pulsed corona discharge plasma is 38 W,and the energy efficiency can reach 0.0563 mg/k J.However,the input power of the dielectric barrier discharge plasma can reach 81 W,the energy efficiency is only 0.0312 mg/k J.