Reaction kinetics and radical-mediated mechanism of catalytic ozonation using waste-derived magnesia brick powder

Waste-derived magnesia brick powder (WMBP), enriched in magnesium oxide (MgO) and abundant Lewis acid–base surface sites, was employed as a robust heterogeneous catalyst for the catalytic ozonation of methylene blue (MB). Non-linear kinetic fitting demonstrated that the WMBP/O₃ system followed an apparent first-order degradation pathway with a rate constant of 0.2563 ± 0.011 min⁻ 1 , approximately 15-fold higher than that of sole ozonation (0.0165 ± 0.002 min⁻ 1 ). Mineralization was similarly enhanced, with total organic carbon (TOC) decay accelerating from 0.0035 ± 0.0011 min⁻ 1 (single ozonation process) to 0.0693 ± 0.0102 min⁻ 1 (catalytic ozonation process). Electron spin resonance analysis and selective quenching experiments confirmed the simultaneous generation of •OH, O₂•⁻, and 1 O₂, indicating a hybrid radical/non-radical oxidation regime. Ultra-high-performance liquid chromatography coupled with tandem mass spectrometry characterization revealed a sequential degradation pathway involving N-demethylation, S-oxidation, hydroxylation, and aromatic ring-opening. The WMBP catalyst exhibited excellent long-term stability over ten reuse cycles, maintaining > 90% degradation efficiency through the fourth cycle and > 78% even after the tenth cycle, highlighting its structural resilience and sustained ozone-activation capability. These findings provide new mechanistic insights into MgO-mediated catalytic ozonation and underscore the circular-economy potential of valorizing refractory industrial waste into a high-performance reusable catalyst.