Understanding the nonradical activation of peroxymonosulfate by different crystallographic MnO: The pivotal role of Mn content on the surface.

Manganese oxide-activated persulfate plays a critical role in water purification and in situ chemical oxidation processes, but the underlying mechanism needs to be further revealed. Herein, the detailed mechanism of MnO with various crystallographic structures (α-, β-, γ-, and δ-MnO) towards peroxymonosulfate (PMS) activation was investigated. PMS activated by tunnel structured α-, β-, and γ-MnO showed higher acetaminophen (ACE) removal than layer structured δ-MnO with the removal efficiency following an order of α-MnO (85%) ≈ γ-MnO (84%) > β-MnO (65%) > δ-MnO (31%). Integrated with chemical quenching experiments, electron paramagnetic resonance, Raman spectra, X-ray photoelectron spectroscopy, and Langmuir-Hinshelwood model on kinetic data, both surface-bound PMS complexes and direct oxidation by surface manganese species (Mn) were disclosed as the dominant oxidation mechanism for ACE degradation in α-, β-, and γ-MnO/PMS, which were rarely observed in previous reports. Moreover, the catalytic activity of α-, β-, and γ-MnO was positively correlated to the Mn content on the catalyst surface. Higher content of Mn would stimulate the generation of more oxygen vacancies, which was conducive to the adsorption of PMS and the formation of reactive complexes. Overall, this study might provide deeper insight into the nonradical activation mechanism of PMS over different crystallographic MnO.