•An on-site application-oriented continuous electrocoagulation system.•pH auto-adjusting simplifies the operation and maintenance of the system.•Adjust pHs of reduction region and precipitation region with endogenous products.•The treated SFS complies with environmental legislation for discharges.•Treated SFS demonstrated better flushing effectiveness than fresh tap water.Soil flushing solution (SFS) from Cr(VI)-contaminated soil flushing remediation is a kind of non-industry-sourcing Cr(VI)-containing wastewater. The requirement of on-site treatment of SFS proposes high level requirements for the operational simplicity and transportability. This paper exhibits an on-site application-oriented continuous electrocoagulation (EC) system for SFS treatment. By a novel design, the products of a series of electrochemical, redox and precipitation reactions were used to create an acidic region for Cr(VI) reduction by Fe2+ and a basic region for CrxFey(OH)z precipitation. The concentrations of Cr(VI), total Cr and total suspended solids in the treated SFS were less than 0.03 mg·L−1, 0.16 mg·L−1 and 39 mg·L−1 respectively, complying with the typical environmental legislation for discharges. Experiments of stirring intensity effect shows that stirring is necessary to decrease Cr(VI)/TCr in sediments. Comparison of desorption kinetics between treated SFS and tap water shows that the Cr(VI) desorption efficiency with treated SFS is slightly higher than with fresh tap water, meeting the requirement for the reuse of SFS. Compared with other continuous EC systems, this continuous EC system eliminates the requirement of pH monitoring and adjusting and improves operational simplicity and transportability.Download high-res image (145KB)Download full-size image

•Characteristics of electrolytes significantly affect soil current.•Proton-transfer reaction between H+ and Ac− ions induced potential gradient peak.•Potential gradient peak induced lead-focusing effect in soil.•Fe2+ ions do not enhance the desorption of lead from soil.•Na/HAc is not an optimal catholyte in EKR of Pb-contaminated soil.Characteristics of electrolytes as boundary conditions of soil affect the soil ionic composition and conductivity during electrokinetic remediation (EKR), which directly affect the effectiveness, energy efficiency and duration of EKR. This experiment studied how the characteristics of catholytes and anolytes affect the soil conductivity. The results showed that H+ ions of higher ionic conductivity induced the increase in soil conductivity; by contrast, Fe2+ ions of lower ionic conductivity induced the decrease in soil conductivity. This experiment also found that when H+ ions from anolyte met Ac− ions from catholyte in soil, they associated into neutral HAc molecules resulting in the decrease in the soil conductivity, which reached the lowest at pH ≈ 3.7. This proton-transfer reaction induced a potential gradient peak which caused a lead-focusing in the soil. The pH value for lead desorption from soil is no more than 4.0, and therefore HAc is not an optimal candidate for catholytes in the electrokinetic remediation of lead-contaminated soil. Results also showed that Fe2+ ions did not enhance desorption of lead from soil. This study provided a methodology to explain the change pattern of soil conductivity and current, and guide the selection of electrolytes.