Enhanced Electrokinetic Remediation Of Chromium-contaminated Soil By Double Permeable Reactive Layers

In this study,taking the actual chromium-contaminated soil obtained from an abandoned chemical plant in Chongqing as the research object,the enhancement of Cr（Ⅵ）removal from the soil by coupling EKR and double permeable reactive layers was discussed.Firstly,the physical and chemical characterization as well as pollution property of soil contaminated samples were discussed.The soil samples mainly contained O,Ca,Si,Cr,Fe,Al,Mg,S and other minor elements,and the chromium content was 9.17%,which showed that the major pollutant of the contaminated site was chromium.The total chromium and Cr（VI）concentration in soil were 31318.90mg/kg and 338.84mg/kg,respectively.This indicated that the contaminated site posed a serious threat on natural environment and human health.In addition,the Cr（VI）leaching concentration was14.54 mg/L by sulphuric acid and nitric acid method.Based on the traditional EKR,the invigorating effect of three different anodic permeable reactive layer materials（activated carbon,ZVI,a mixture of ZVI and activated carbon）on EKR was studied when adding the 10%phosphoric acid treated activated carbon as cathodic permeable reactive layer.The results showed that introducing double permeable reactive layers could control the soil pH to some extent and improve the Cr（VI）remedial efficiency.Especially,when the a mixture of activated carbon and ZVI was used as the anodic permeable reactive layer,the Cr（VI）remedial efficiency increased more than 4 times.In single-factor experiments,the highest Cr（VI）remedial efficiency（58.62%）was obtained when a mixture of ZVI and activated carbon with the mass ratio of 1:1 was selected as the anodic permeable reactive layer.When adding the mass ratio of 1:1,the adsorption of activated carbon and the reduction of ZVI can be maximized.The Cr（VI）remedial efficiency of three different soil areas（namely cathodic sampling area S1,middle sampling area S2,and anodic sampling area S3）followed the order of S1﹥S3﹥S2,which indicated that the remediation of Cr（VI）might be achieved in the following two ways:（1）Removing Cr（VI）from soil by electric field,（2）Reducing Cr（VI）by Fe²⁺to relatively less toxic and less soluble Cr（III）in soil.The control of OH^-produced at cathode（namely,the phosphoric acid concentration of acidified activated carbon）,voltage gradient,and remediation time were designed as the variables,and nine experiments were carried out under the three levels of three factors.The maximum Cr（VI）remedial rate（75.01%）was obtained with phosphoric acid concentration of 20%,voltage gradient of 2.0V/cm and remediation time of 8 d.After EKR,the Cr（VI）concentration in soil was reduced from 338.84mg/kg to84.68mg/kg.Moreover,the Cr（VI）extration toxicity and unstable chromium（acid soluble form,reducible form and oxidizable form）were greatly decreased.Therefore,the contamination degree and environmental hazard of the actual chromium-contaminated soil was lowered down significantly after EKR coupled with double permeable reactive layers.