Study On Non-thermal Plasma For The Remediation Of Organic Polluted Soils And Its Mechanisms

China is recently facing serious soil contamination problems because of the rapid development of industrialization and urbanization which lead to abandunt pollutant emission.Organic pollutants are the main soil contaminants besides heavy metals.Traditional remediation methods can no longer achieve green,effective and fast remdiation purpose for heavily organic polluted soils due to their limitations.Physical methods have secondary pollution problem;chemical methods need the addition of chemicals;bioremediation methods can only deal with limited types of pollutants and have long remediation cycle.NTP is more and more used for environmental applications and is considered a promising remediation technique.NTP is also called non-equilibrated plasma.It represents the plasma system with electrons,positive heavy ions,UV,O3,electric field,shock wave,active radicals(·0,· O2,· OH,etc.).In this system,the density of high-temperature electrons is relatively low and the opportunity of colliding with heavy radicals is rare.Thus,the kinetic energy of heavy radicals which directly affect the system temperature varies significantly with high-temperature electrons.This leads to the low temperature of the system.This method is called "green remediation method" due to the fact that it does not need chemical additions and can generate in-situ active species.As a result,this technique is drawing massive attention in the remediation filed.In this study,DBD and PCD plasma were utilized for the treatment of soils contaminated by PNP and PHE.The effects of soil characteristics on degrading efficiency were discussed.The degrading pathways of PNP and PHE were analyzed.DBD and PCD plasma were compared to discuss the feasibility of non-thermal plasma for the remediation of refractory and heavily polluted soils.The main research topics and conclusions were as follows:(1)PNP was selected the target pollutant.The effects of discharging voltage,initial concentration,soil pH,air flow rate on removal efficiency were studied to check its feasibility on treating soils contaminated by PNP.The results were as follows.DBD plasma could remove 63.6%of PNP in 50 min and the energy efficiency was 0.0067 mg/kJ.DBD plasma could mineralize 20%of PNP.Ozone accounted for 25%in the degradation processes by comparing plasma treatment and ozone treatment.In addition,GC-MS,HPLC,IC and FTIR spectra before and after the plasma treatment detected products such as catechol,hydroquinone,fumaric acid,maleic acid,oxalic acid,formic acid,acetic acid,nitrate.The denitrification,hydroxylation,substitution processes and ring opening took place while PNP was degraded.Possible degrading pathways were proposed.(2)The target pollutant was shifted to PHE,a typical PAHs,to investigate the degradation efficiency and mechanisms by DBD plasma.When the frequency was 150 Hz,the voltage was 110 V,the gap was 1.5 cm and the duty cycle was 20%,the input power was 64 W and the removal rate of PHE was 98%.DBD plasma technology showed satisfactory results for soil remediation and it proved to be effective for different kinds of organic pollutants.The main intermediates were 9,10-phenanthraquinone and 2,2'-diphenic acid from GC-MS,IC and FTIR spectra analyses.PHE molecule was optimized by DFT calculation.The degradation pathway was proposed by instrumental detections and theoretical calculations.OES was applied to identify active species and their formation pathways were deduced.(3)PCD plasma was utilized to remediate PHE polluted soil.The data manifested that the PCD plasma system was able to remove 93%of PHE from soil under the following conditions:initial concentration was 160 mg/kg;the peak to peak voltage was 30 kV;the frequency was 50Hz;the gap was 20cm;the atmosphere was air.The degrading reaction abided by first-order kinetic reaction and the reaction coefficient was 0.032 min-1.Positive,negative and double PCD plasma were compared by waveforms,degradation efficiency and degrading mechanisms.The FTIR spectra showed that PHE decomposed and produced by-products with C=O?N-H and hydroxide radicals as a result of the constant attack by reactive species.By comparing the results of DBD and PCD plasma technology,we could see that these two kinds of plasma both had satisfactory degrading efficiencies.However,there were differences in discharging waveforms,energy efficiencies,co-efficient of kinetics and degrading products.DBD plasma was fast and had higher removal rate.PCD plasma had higher energy efficiency.