Migration and decomplexation of metal-chelate complexes causing metal accumulation phenomenon after chelate-enhanced electrokinetic remediation

This study investigates the migration and decomplexation effects of metal-ethylenediaminetetraacetic acid (EDTA) complexes during an electrokinetic (ER) remediation process and the resulting metal accumulation phenomena. Six EK tests with control of the electrolyte pH and using ion-exchange membranes were performed to treat Pb-EDTA and Cd-EDTA co-contaminated red soil. The obtained results showed that a portion of free metal cations could be decomplexed from the metal-EDTA complexes due to the low pH and electrochemical degradation at the anode. These cations went back into the soil by electromigration and accumulated in separate locations according to their hydrolysis ability and the distribution of soil pH in different sections. Totals of 61% Cd and 83% Pb were removed from the soil after a 7-day treatment under the condition of controlling the electrolyte pH at 10. The removal efficiencies of metals under the anion-exchange membrane-assisted treatment were higher than those of the cation-exchange membrane-assisted treatment. Based on the mechanisms of metal accumulation phenomena, the migration of decomplexed free metal cations back to the soil is limited by using an anion-exchange membrane or pre-precipitation with alkaline conditions was confirmed to effectively reduce the effect of metal accumulation.