Research On Electrokinetic Remediation Of Cr-contaminated Soil In A Granular Activated Carbon （GAC/Fe₃O₄） Based Three-dimensional Electrode System

The chromium-contaminated soil from industrial site poses a huge threat to the ecological environment and human health and need to be remediate economically and efficiently.In this dissertation,the Cr-contaminated soil from a chemical plant in Chongqing was taken as the research object,based on the GAC/Fe₃O₄ particles,a three-dimensional electrokinetic remediation（3D EKR）system was constructed.Effect of various operating parameters on Cr（Ⅵ）removal efficiency has been disscussed.A mathematical model of Cr removal efficiency in GAC/Fe₃O₄-based 3D EKR system was established.The mechanisms of Cr migration、transformation and removal in the GAC/Fe₃O₄-based 3D EKR system were explored.Soil particles are alkaline and ranging from 2μm to 200μm.The main constituent elements of the sample are Ca,O,Si,Al,Cr,Fe,and Mg,and no Cr-containing crystal phase was found.The Cr（Ⅵ）concentration in soil sample is 520.79 mg/kg,and the leaching toxicity of Cr（Ⅵ）is 11.79 mg/L,which pose unacceptable risks to the ecological environment and human health.During the preparation of the particle electrode by the immersion-calcination method,the single-facter experiments and the orthogonal experiment were conducted to explore the influence of the preparation conditions on the Cr removal in the soil by the 3D EKR system.The chemical composition,crystal phase structure,micro-morphology,and functional group of the particle electrode was analyzed,and the adsorption and reducibility of the particle electrode in the static adsorption experiment of the simulated soil leaching solution was explored as well.The results showed that the optimum preparation conditions of particle electrodes were:GAC:Fe SO4 ratio 1:3、calcination temperature 350℃and calcination time 2 h.The particle surface was successfully loaded with a mixture ofγ-Fe₂O₃ and Fe₃O₄ and was rich in functional groups such as-CHO,-OH,-C=O,-COOH,etc.It showed good adsorption and reduction of Cr（Ⅵ）in the simulated soil leaching solution.The reducibility of GAC/Fe₃O₄ increased with an increasing p H under acidic conditions and decreased with an increasing p H under alkaline conditions.The 3D EKR for Cr-contaminated soil based on GAC/Fe₃O₄ were conducted under different conditions.The soil physicochemical characteristics prior to and after remediation were investigated.The results showed that the existence of GAC/Fe₃O₄particles contributed to improve the removal of Cr（Ⅵ）and Cr（Ⅲ）by reducing the acid-base polarization phenomenon in the soil during EKR process,and the Cr（Ⅵ）removal efficiency in 3D EKR system is significantly higher than that in 2D EKR system.The removal efficiency of Cr（Ⅵ）in 3D EKR system reached 86.46%under voltage gradient of 1V/cm,particle electrode size of 2 mm,particle electrode dosage of 4%and remediation time of 10 d.Besides,the lifetime of GAC/Fe₃O₄ was found to be shortened and the specific surface be reduced due to the large voltage gradient and particle size,which resulted in the decrease of removal efficiency of Cr（Ⅵ）and its leaching toxicity.A high-fitting degree and significant quadratic polynomial regression model was built according to the central composite rotatable design（CCRD）from response surface methodology（RSM）,in which the operating parameters of the 3D EKR system were used as independent variables and the removal efficiency of Cr（Ⅵ）and Cr（Ⅵ）leaching toxicity as response variables.According to the model calculation,the optimal operating parameters of the 3D EKR system were:GAC/Fe₃O₄ particle size 2.54 mm,voltage gradient 1.22 V/cm,remediation time 12.87 d.The corresponding predicted removal efficiency of Cr（Ⅵ）was 88.75%,and predicted removal efficiency of Cr（Ⅵ）leaching toxicity was 96.59%.The error between verification test results and model predicted results was small,which indicates that the regression model is successful and can be used to predict and evaluate the removal efficiency of Cr（Ⅵ）in the GAC/Fe₃O₄ based 3D EKR system.A series of characterizations of the soil and corresponding particle electrodes in different soil sections was conducted before and after remediation.It was found that the3D EKR system based on GAC/Fe₃O₄ particles effectively reduced the proportion of Cr exchangeable fraction and carbonate-bound fraction in soil,lowering the migration and transformation of Cr in the environment.The particle electrode promoted the formation of stable chromium-containing silicates from Cr（Ⅲ）and soil colloids by electrosorption,reducing the environmental risk.The Cr（Ⅲ）oxidation and Cr（Ⅵ）reduction both occurred on the surface of the particle electrode,wherein the reduction of Cr（Ⅵ）was due to existence of iron oxides loaded on the surface of the particle electrode,and the oxidation of Cr（Ⅲ）was due to the existence of·OH generated in Electro-Fenton（EF）process.The mechanism of Cr removal in the 3D EKR system was the synergistic effect of electromigration,hydrolysis on both main electrode and particle electrode,the EF process as well as the adsorption/electrosorption of the particle electrode,which significantly remove the Cr（Ⅵ）and unstable Cr（Ⅲ）from soil,and thus the Cr-contaminated soil was remediated effectively.