Autocatalytic Decomplexation of Cu(II)–EDTA and Simultaneous Removal of Aqueous Cu(II) by UV/Chlorine

Traditional processes usually cannot enable efficient water decontamination from toxic heavy metals complexed with organic ligands. Herein, we first reported the removal of Cu(II)–EDTA by a UV/chlorine process, where the Cu(II)–EDTA degradation obeyed autocatalytic two-stage kinetics, and Cu(II) was simultaneously removed as CuO precipitate. The scavenging experiments and EPR analysis indicated that Cl• accounted for the Cu(II)–EDTA degradation at diffusion-controlled rate (∼1010 M–1 s–1). Mechanism study with mass spectrometry evidence of 11 key intermediates revealed that the Cu(II)–EDTA degradation by UV/chlorine was an autocatalytic successive decarboxylation process mediated by the Cu(II)/Cu(I) redox cycle. Under UV irradiation, Cu(I) was generated during the photolysis of the Cl•-attacked complexed Cu(II) via ligand-to-metal charge transfer (LMCT). Both free and organic ligand-complexed Cu(I) could form binary/ternary complexes with ClO–, which were oxidized back to Cu(II) via metal-to-ligand charge transfer (MLCT) with simultaneous production of Cl•, resulting in the autocatalytic effect on Cu(II)–EDTA removal. Effects of chlorine dosage and pH were examined, and the technological practicability was validated with authentic electroplating wastewater and other Cu(II)–organic complexes. This study shed light on a new mechanism of decomplexation by Cl• and broadened the applicability of the promising UV/chlorine process in water treatment.