Robust decomplexation of Cu(II) complexes in excessive ligand environments by Mn(II)/PMS process: Ligand concentration-dependent decomplexation mechanisms

The speciation compatibility of Cu-ligand complexes and catalytic oxidation systems fundamentally governs their decomplexation efficiency, exhibiting pronounced ligand concentration dependence. While contemporary methodologies predominantly focus on stoichiometric Cu(II)-ligand complexes, their efficacy in treating wastewater with ligand excess remains technologically contentious. This study demonstrates the Mn(II)/peroxymonosulfate (PMS) process as a robust solution for the elimination of Cu(II)-ligand complexes even in excessive ligand environments. Using Cu(II)-EDTA as the representative complex, 90% decomplexation was achieved within 1 and 5 min at [EDTA] 0 :[Cu] ratios of 1:1 and 2:1, respectively, surpassing reported decomplexation approaches such as UV/H 2 O 2 , UV/peroxydisulfate (PDS), and Co 2+ /PMS. Mechanistic investigations through oxidation species characterization and product analysis revealed a successive decarboxylation of EDTA by non-radical pathway, i.e., high valence metals and singlet oxygen, enhancing process selectivity while minimizing oxidant consumption ([PMS] 0 :[EDTA] 0 = 30:1). The system exhibits dual mechanistic regimes: (i) instantaneous decomplexation under ligand-deficient conditions via PMS activation by in-situ formed CuO through alkaline precipitation, and (ii) cascade decomplexation for ligand-rich systems involving Mn-Cu synergistic destruction of excessive EDTA prior to CuO-catalyzed degradation of residual ligand. Such synergism is believed to stem from the electron transfer from Cu(II) to Mn(V/VI), forming reactive Cu(III) for decomplexation through intramolecular electron transfer. The Mn(II)/PMS system also exhibited the applicability in treating various Cu(II)-ligand complexes and real wastewater. This work establishes a practical strategy for reliable treatment of industrial wastewater containing non-stoichiometric Cu-ligand complexes.