Trace Mn(II)-Mediated Peroxone Process Boosts Oxidative Capacity for Electron-Deficient Pollutant Removal: Key Role of Transient Mn(III) and Overcoming pH Limitations

The ozone (O₃)-hydrogen peroxide (H₂O₂) process (peroxone) is a promising advanced oxidation technology for refractory organic wastewater but is strongly pH-dependent, operating efficiently mainly under alkaline conditions, which hinders its use in acidic matrices. Here, we report a trace Mn(II)-mediated peroxone that sustains fast pollutant degradation and oxidant activation across acidic to alkaline pH conditions. The core is a transient Mn(III) intermediate maintained by rapid Mn(II)/Mn(III) redox cycling, with H₂O₂ serving both as the HO• precursor and as a redox mediator. This dynamic cycle continuously generates HO• while suppressing unproductive oxidant consumption. Kinetic tests show that Mn(II) increases the pseudo-first-order rate constant (kobs) for electron-poor atrazine to 1.21 min⁻¹ even at a pH value of 3.0, which is 22.9-fold higher than the classical peroxone. Spectroscopic and electrochemical analyses further indicate that H₂O₂ inhibits the buildup of catalytically inactive Mn(IV)/Mn(V), stabilizing active Mn(III). The terminal Mn(IV) is captured as readily separable MnO₂ solids, ensuring catalytic turnover and minimal dissolved Mn. Overall, the Mn(II)-mediated peroxone achieves >90% atrazine removal from pH 3.0 to 11.0. In a continuous-flow reactor, the constructed system sustains high efficiency while tolerating complex water matrices. Ecotoxicity assays (plant germination, zebrafish embryo development, and bacterial proliferation) and life cycle assessment confirmed lower environmental risks than O₃ and Fenton processes. These results establish transient Mn(III) as a pivotal HO•-generating species and provide a robust, pH-resilient, and environmentally compatible framework for next-generation AOPs targeting persistent organic pollutants in diverse water matrices.