Electrochemical Removal of Metal-Organic Complexes in Metal Plating Wastewater: A Comparative Study of Cu-EDTA and Ni-EDTA Removal Mechanisms

Cu and Ni complexes with ethylenediaminetetraacetic acid(Cu/Ni-EDTA),which are commonly present in metal plating industry wastewaters,pose a serious threat to both the environment and human health dueto their high toxicity and low biodegradability. In this study, thetreatment of solutions containing either or both Cu-EDTA and Ni-EDTAusing an electrochemical process is investigated under both oxidizingand reducing electrolysis conditions. Our results indicate that Cu-EDTAis decomplexed as a result of the cathodic reduction of Cu(II) withsubsequent electrodeposition of Cu(0) at the cathode when the cathodepotential is more negative than the reduction potential of Cu-EDTAto Cu(0). In contrast, the very negative reduction potential of Ni-EDTAto Ni(0) renders the direct reduction of EDTA-complexed Ni(II) atthe cathode unimportant. The removal of Ni during the electrolysisprocess mainly occurs via anodic oxidation of EDTA in Ni-EDTA, withthe resulting formation of low-molecular-weight organic acids andthe release of Ni2+, which is subsequently deposited asNi(0) on the cathode. A kinetic model incorporating the keyreactions occurring in the electrolysis process has been developed,which satisfactorily describes EDTA, Cu, Ni, and TOC removal. Overall,this study improves our understanding of the mechanism of removalof heavy metals from solution during the electrochemical advancedoxidation of metal plating wastewaters. Theremoval mechanism of Cu and Ni from solutions containingstrong complexes of these metals is examined by employing oxidizingand reducing electrolysis in this study.