Enhanced removal and recovery of heavy metals from contaminated soils by combining citrate extraction and electrochemical adsorption.

Soil washing with biodegradable organic acids is an effective strategy for remediation of contaminated soils. However, this process generates metal-laden leachate, which limits its large-scale application. This study developed an integrated citrate extraction-electrochemical recovery system that simultaneously achieves soil decontamination, leachate purification, and metal recovery, thereby providing a leachate-centered closed-loop remediation strategy for heavy metal-contaminated soils. Washing of heavy metal-contaminated soil samples (2.5 kg) with 0.1 mol L-1 sodium citrate followed by electrochemical precipitation at 0.50 A decreased the soil Cd, Zn, As, and Pb contents from 105, 1504, 388 and 366 mg kg-1 to 46, 1055, 306 and 307 mg kg-1, respectively, with Cd, Zn, and Pb meeting the Chinese risk-control standard for development land. Importantly, the post-washing leachate containing high concentrations of dissolved metals was subjected to in situ electrochemical recovery, which reduced Cd, Zn, As, and Pb concentrations to 1.41, 21.6, 8.1, and 4.7 mg L-1, respectively. Cd, Zn, and Pb were mainly removed via cathodic hydrolysis/precipitation, whereas As removal was closely associated with Fe-mediated adsorption and precipitation. The recovered metal hydroxides could be stripped and recycled, and the system maintained a high removal efficiency and electrode stability over multiple cycles. These results highlight the novelty of coupling soil decontamination, leachate purification, and metal recovery within one integrated process, and demonstrate the practical potential of the proposed system as a scalable and sustainable strategy for contaminated soil management.