Study On Remediation Of Phenanthrene And 2,4,6-trichlorophenol Contaminated Soil Using Electrokinetics Coupled With Permeable Reactive Barrier

Polycyclic aromatic hydrocarbons and chlorophenols are typical persistent organic pollutants of strong carcinogenesis,teratogenesis and mutagenesis for human beings and animals.Soil is a sink of pollutants where hydrophobic organic compounds tend to be firmly sorbed on the surface and the interior of soil particles,and thus are hardly degraded naturally.As an emerging soil remediation technique,electrokinetics coupled with permealble reactive barrier（EK-PRB）receives increasing concerns because of short remediation period,high efficiency,little secondary pollution,low cost,environmental friendliness and so on.The principle of this technique is that organic contaminants are migrated towards the cathode with the electroosmotic flow,and get degraded once they reach the PRB.In this paper,EK-PRB technique is used to repair soil artificially amended with phenanthrene（PHE）and 2,4,6-trichlorophenol（TCP）,using zero valent iron and activated carbon（Fe/C）with acid-washed quartz sand as the PRB packing materials（organic compounds are expected to be degraded via iron carbon micro-electrolysis）.The main objectives of the present study are:to figure out an optimal EK-PRB operation scheme for organic compound contaminated soil;to investigated the enhancement effect of biosurfactants and Fenton reactions on EK-PRB based on determining the optimal operation parameters;to establish a mathematical model for qualitative and quantitative description of the remediation processes of organic compounds contaminated soil by the EK technique.The main results are as follows:（1）A clay soil with low permeability was evaluated in this study since the EK technique is suitable for this type of soil.The organic compounds PHE and TCP in the soil were migrated towards the cathode with the electroosmotic flow,hence,the PRB was installed in the cathode area.Under the optimum conditions（Fe/C mass ratio was 6:1,pH of catholyte was 4,voltage gradient was 1 V/cm and remediation period was 15 days）,the removal efficiencies of PHE and TCP in the soil column without PRB were only 15.40%and 24.80%,respectively.The removal efficiencies of PHE and TCP in the soil column installed with PRB were 17.0%and 26.4%,respectively.However,up to 80.5%of PHE and 100%of TCP that were migrated out of the soil column were degraded by PRB.（2）Biosurfactants of low concentrations had an inhibitory effect on organic compound desorption from the soil,while desorption increased sharply when the biosurfactant concentration reached 400 mg/L.The removal efficiencies of PHE and TCP in the treatment of1000 mg/L rhamnolipid increased to 49.89%and 64.60%,respectively.The removal efficiencies of PHE and TCP in the treatment of 1000 mg/L saponin were 55.41%and 69.16%,respectively.Moreover,high viscosity of biosurfactants indirectly affected the amount of electroosmotic flow and the soil pH.（3）The addition of H₂O₂ improved the degradation of organic compounds to some extent in both the soil column and the PRB.When 7%H₂O₂ was added to the anolyte,the removal efficiencies of PHE and TCP in the soil column were 24.30%and 33.80%,respectively,and both PHE and TCP migrated out of the soil were completely degraded.When 1000 mg/L saponin and 7%H₂O₂ were added simultaneously to the anolyte,the removal efficiencies of PHE and TCP in the soil column were up to 59.40%and 73%,respectively.Moreover,the percentages of PHE and TCP migrated out of the soil and then degraded by PRB were as high as 98.68%and 99.34%,respectively.（4）In terms of qualitative description of EK remediation of organic compounds contaminated soil,the established theoretical model could clearly explain the observed phenomena and the mechanisms in the process of experiment.As for the quantitative description of EK remediation technique,PHE was used as a representative highly hydrophobic organic compound whose molecule would not change with pH to verify the simulation results.The verification results showed that the model could fit the experimental data accurately.As a result,the model can serve as an effective prediction and guidance tool for the application of EK technique to the remediation of soil contaminated with PHE and the other similar compounds.