The pH Dependence of O₃/Mn(II) Synergy for the Reactive Species Evolution and Micropollutant Degradation

The combination of ozonation and Mn(II) (O3/Mn(II)) is expected to synergistically eliminate recalcitrant organic micropollutants in water. However, the decay of O3, formation of •OH and reactive manganese species (RMnSs), and synergism in micropollutant degradation have not been comprehensively understood. Herein, the synergism of O3 and Mn(II) was observed to be pH-dependent. Compared to ozonation alone, O3/Mn(II) preferentially formed Mn(V and VI) and slightly increased •OH yield (from 0.22 to 0.30) at acidic pH 4–5, while it preferentially formed Mn(VII) and significantly increased OH yield (from 0.52 to 0.74) at circumneutral pH 6–7. Thereafter, O3/Mn(II) prominently increased RMnS exposure at acidic pH, while it dramatically diminished O3-exposurse to increase •OH exposure at circumneutral pH. As a result, O3/Mn(II) outperformed ozonation alone for the degradation of various O3-resistant micropollutants (3.7–7.9 times faster). For example, the RMnSs (82.7% contribution) and •OH (84.2% contribution) were the dominant contributors to the degradation of the isothiazolinone biocide (5-chloro-2-methyl-4-isothiazolin-3-one) at acidic and circumneutral pH, respectively. Mn(II) can enhance the removal of O3-resistant micropollutants in relatively clean waters (e.g., tap water), but its effectiveness was limited in more contaminated waters (e.g., WWTP effluent). These findings support the practice and pH optimization of the O3/Mn(II) in various water types.