Metal-Dependent Electron Transfer in Acidic UV/Peroxymonosulfate: Inner-Sphere Proton-Coupled Two-Electron Transfer Drives Ni(III)-Oxyl Formation Distinct from Cu Pathways

Abstract

Abstract Nickel–ethylenediaminetetraacetic acid (Ni-EDTA) persists in electroplating wastewater due to strong metal–ligand coordination and is widely assumed to degrade via radical pathways during advanced oxidation processes (AOPs). Here, we show that its decomplexation in an acidic UV/peroxymonosulfate (UV/PMS) system is governed by a metal-centered electron-transfer mechanism. Ni removal exceeded 95% within 15 min, while radical scavengers did not completely suppress Ni-EDTA degradation, indicating the involvement of an additional nonradical pathway. Probe-based experiments and density functional theory (DFT) support the formation of a high-valent Ni(III)-oxyl intermediate following UV activation of the coordinated PMS. This species enables ligand cleavage through inner-sphere proton-coupled two-electron transfer (IS-PC2ET), involving concerted hydrogen abstraction and ligand-to-metal electron flow within the coordination sphere. In contrast to Cu systems, Ni follows an electronically distinct oxyl-driven pathway dictated by its d8 configuration rather than a Ni(IV)-oxo route. The process remains effective in the presence of background ions and in real wastewater, demonstrating strong matrix tolerance and selectivity under complex conditions. These results establish metal-dependent electron-transfer pathways as a governing principle for advanced oxidation of metal–organic complexes.