The two-phase leaching of Pb, Zn and Cd contaminated soil using EDTA and electrochemical treatment of the washing solution

The feasibility of a novel two-phase method for remediation of Pb (1374 mg kg−1), Zn (1007 mg kg−1), and Cd (9.1 mg kg−1) contaminated soil was evaluated. In the first phase we used EDTA for leaching heavy metals from the soil. In the second phase we used an electrochemical advanced oxidation process (EAOP) for the treatment and reuse of washing solution for soil rinsing (removal of the soil-retained, chelant-mobilized metallic species). In EAOP, a boron-doped diamond anode was used for the generation of hydroxyl radicals and oxidative decomposition of EDTA–metal complexes at a constant current density (15 mA cm−2). The released metals were removed from the solution by filtration as insoluble participate and by electro-deposition on the cathode. Four consecutive additions of 5.0 mmol kg−1 EDTA (total 20 mmol kg−1) removed 44% Pb, 14% Zn and 35% Cd from the soil. The mobility of the Pb, Zn and Cd (Toxicity Characteristic Leaching Procedure) left in the soil after remediation was reduced by 1.6, 3.4 and 1.5 times, respectively. The Pb oral availability (Physiologically Based Extraction Test) in the simulated stomach phase was reduced by 2.4 and in the intestinal phase by 1.7 times. The discharge solution was clear, almost colorless, with pH 7.73 and 0.47 mg L−1 Pb, 1.03 mg L−1 Zn, bellow the limits of quantification of Cd and 0.023 mM EDTA. The novel method enables soil leaching with small water requirements and no wastewater generation or other emissions into the environment.