Experimental Study On Electrode Capture Potential In Cr-contaminated Soil Electrokinetic Remediation

Electrokinetic remediation?EKR?is a promising technology for removing heavy metals from low-permeability soils.Previous studies have shown that the removal efficiencies of heavy metals are directly related to its chemical fractionation in the soil,but the role of heavy metal fractionation in the EKR and its transformation and migration process are unclear.At the initial stage of this study,EKR tests with different types of soils and electrolytes were conducted.This paper investigated the electrode capture potential?ECP?of Cr in artificially and real contaminated soils in the EKR with different electrolytes from a view of chemical fractionation to better understand the migration and transformation of Cr for a more efficient remediation process.The results indicated that ECP includes the transformation potential?TP?among Cr fractions,the exchange potential?EP?and the migration potential?MP?of Cr.The ECP of Exp.1,Exp.2,Exp.3and Exp.4 are 386.2,0,1412.76 and 347.73 mg·cm v^-1,respectively.The ECP for the real contaminated soil is limited by the transformation process among Cr fractions in the solid phase.For the artificially contaminated soil,the ECP mainly depended on the migration process of exchangeable Cr in the liquid phase.Citric acid as an electrolyte was a better choice for the real contaminated soil compared with KCl due to higher TP of exchangeable Cr and lower ecotoxicity risk.The ecotoxicity risk index?ERI?showed that the ecotoxicity risks of Cr in soils after experiments followed the sequence as Exp.4>Exp.2>Exp.1>Exp.3.The ecotoxicity risk of remaining Cr in Exp.1,Exp.3 and Exp.4 reduced after EKR,while that in Exp.2 increased due to high TP to the bound to Fe–Mn oxides?FM?and organic matter?OC?fractions and very low ECP.Therefore,High ECP may potentially increase the ecotoxicity risk with higher TP than MP.Based on the results of the initial stage,in the advanced stage of this study,enhanced EKR experiments were performed to enhance the transformation and migration of Cr chemical fractionation in the soil,such as increasing citric acid electrolyte concentration,soil modification pretreatment,and anolyte circulation.The results showed that high concentration of citric acid electrolyte can effectively increase TP among Cr fractionations in the solid phase,resulting in higher ECP.On the basis of the test with high-concentration electrolyte,soil modification pretreatment did not further increase the ECP of Cr.Anolyte cycling technology can significantly reduce the"focusing"effect of Cr in artificially contaminated soils,resulting in higher migration removal of Cr.The Cr removal efficiencies of EKR experiments with low-concentration citric acid,high-concentration citric acid,soil modification treatment,and anolyte cycle were 21.6%,44.86%,44.90%,and 70.89%,respectively.According to results of ERI,high-concentration electrolyte and soil modification pretreatment significantly increased the ecotoxicity risk of residual Cr in the soil.