Remediation Of Organic Pollutants Contaminated Soil By Pulse Discharge Plasma

Recently,the gradually increased soil pollution has seriously threatened human health and sustainable development of agriculture,therefore,soil remediation has become an urgent task of environmental protection.For the remediation of organic pollutants contaminated soil,the traditional remediation methods such as physical methods,chemical methods and biological methods are difficult to meet the fast,efficient and clean remediation requirements.However,the low-temperature plasma technology which is efficient,fast and without secondary pollution,provides a new direction for remediation of organic pollutants contaminated soil.In this paper,polycyclic aromatic hydrocarbon fluorene and pesticide atrazine were studied as two target contaminants with high-voltage pulsed power source and needle-plate reactor.The main research includes the optimization of the detection conditions of fluorene,the discharge characteristics of the reaction system,the influence of the system parameters on the fluorene degradation rate,and the degradation mechanism of fluorene and atrazine.The main results are as follows:(1)The optimum HPLC conditions for analyzing fluorene in acetic acid,acetone,ethanol and methanol were obtained,simultaneously,a good correlation standard curve could be gained in the concentration range of 0 ~ 60mg/L.Besides,the best vibration time of the sample in the shaker was 10 h when the soil samples were prepared,while fluorenen was extracted,only 1h vibration was enough.In addition,the result of fluorene extraction by centrifugation was the same as oscillation,however,centrifugation process was more complicated but time-saving while oscillation process was relatively simple and time-consuming.Moreover,the order of the extraction rate of fluorene by four solvents was as follows: acetic acid> acetone> ethanol> methanol,after comprehensive consideration of physical and chemical properties of four solvents,the detection time of HPLC and the extraction rate of fluorene,methanol was chosen as the solvent of fluorene.(2)The input power of the negative electrode was greater than that of the positive electrode,and the input power increased when the discharge voltage and the discharge frequency were raised,however,it decreased when the electrode spacing was changed from 20 mm to 50 mm.For the same electrode space,the input power increased with the increase of discharge voltage.However,when the electrode gap was kept 10 mm,the input power decreased with the increased discharge voltage.Additionally,the increase of soil thickness would resulted in the decrease of input power.The air flow has no significant effect on the input power.(3)The energy efficiency of positive high-voltage pulse was 0.035mg/k J higher than that of negative pulse;besides,the degradation rate of fluorene in the soil increased with the increase of the discharge voltage and discharge frequency.The optimal voltage and optimal frequency were 30 k V and 50 Hz considering the degradation effect of pollutant and energy consumption.The optimum electrode spacing was 20mm;but the air flow rate at the range of 0L/min ~ 6L/min has no obvious effect on the degradation rate of fluorene,thus natural ventilation was applied during experiment.Moreover,the order of the actual degradation amount of fluorene with different initial concentration of plasma in the soil was as follows: 1000 mg / kg> 200 mg / kg> 100 mg / kg;and the order of the degradation rate of fluorene in soils with soil moisture of 0%、10% and 20% was 0%> 10%> 20%,in addition,the order of degradation rate of fluorene in soils with p H values of 3.0,6.8 and 9.0 was 9.0>6.8>3.0.The degradation rate of fluorene in soil at p H value of 9 was only 0.23% higher than that of 6.8,so this technique does not require p H adjustment when treating typical PAHs contaminated soils.Furthermore,the process of plasma degradation of fluorene was fitted by kinetic method and it was found that the process accorded with quasi-first order kinetics.(4)The effect of plasma on fluorene degradation was better than that of ozone alone,after treatment of 60 min,the role of O3 accounted for 46.96% of fluorene degradation.Besides,it was found that the degradation of fluorene could be improved by the periodic elution of the oxidized product from the soil surface.Based on the FTIR spectra of fluorene before and after discharge,the possible degradation pathway of fluorene was deduced,and the main products were phenol,alcohol and acid.Furthermore,the degradation rate of atrazine by plasma was 23%,among this degradation rate,only 2.19% was degradated by O3,which indicated that low-temperature plasma technology may be advantageous over ozone-based techniques for remediation of soil contaminated by pestisides.Also,the method of eluting soil oxide layer had the same promotion effect on atrazine degradation,and the degradation pathway of atrazine is analyzed by FTIR combined with related literatures,it showed that the final product was cyanuric acid.