pH-dependent self-enhanced decomplexation mechanism of Cu(II)-EDTA in UV/peroxydisulfate system: Reinterpreting the traditional autocatalysis hypothesis

The peroxodisulfate (PDS)-based oxidation systems are promising for Cu(II) complex decomplexation owing to its capacity to harness the self-enhanced decomplexation (SED) effect of Cu(II) complexes. This phenomenon was conventionally attributed to liquid-phase autocatalysis via Fenton-like reactions between daughter Cu(II) species and PDS. However, this frequently adopted hypothesis is inconsistent with the actual catalytic activity of Cu(II) species and has been challenged by our preliminary results. Herein, we revisit the decomplexation of copper(II)-ethylenediaminetetraacetate (Cu(II)-EDTA) in UV/PDS system, with particular emphasis on correcting the catalytic behaviors of Cu species and clarifying the indispensable role of UV-irradiation in SED effect. Both acidic and alkaline conditions exhibit significant SED effect, but we reveal a pH-dependent SED mechanism that is fundamentally different from the prevailing liquid-phase autocatalysis mechanism. Under acidic conditions, Cu(II) complexes exhibit enhanced catalytic capacity with decreasing ligand coordination strength, primarily stemming from the activation of PDS by Cu(I) species produced via the irradiation-induced intramolecular charge transfer; In alkaline environments, only liberated Cu(II) substantially contribute to SED effect via spontaneous hydrolysis, forming nanostructured CuO after aging, which activates PDS through heterogeneous Fenton-like processes. While acidic conditions demonstrate faster decomplexation, alkaline-generated solid products facilitate multi-cycle treatment efficiency and alkaline conditions also exhibits stronger resistance to chloride. This work corrects the misinterpretation of SED mechanism in UV/PDS/Cu(II)-EDTA system, and provides a fundamental and accurate mechanistic basis for the efficient treatment of copper-containing organic wastewater by PDS-based processes.