Understanding the nonradical activation of peroxymonosulfate by different crystallographic MnO2: The pivotal role of MnIII 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 MnO2 with various crystallographic structures (alpha-, beta-, gamma-, and delta-MnO2) towards peroxymonosulfate (PMS) activation was investigated. PMS activated by tunnel structured alpha-, beta-, and gamma-MnO2 showed higher acetaminophen (ACE) removal than layer structured delta-MnO2 with the removal efficiency following an order of alpha-MnO2 (85%) approximate to gamma-MnO2 (84%) > beta-MnO2 (65%) > delta-MnO2 (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(IV, III)(s)) were disclosed as the dominant oxidation mechanism for ACE degradation in alpha-, beta-, and gamma-MnO2/PMS, which were rarely observed in previous reports. Moreover, the catalytic activity of alpha-, beta-, and gamma-MnO2 was positively correlated to the MnIII(s) content on the catalyst surface. Higher content of MnIII(s) 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 MnO2.

Automated summary

This study investigated the mechanism of manganese oxide with different crystallographic structures in activating persulfate, revealing that surface-bound PMS complexes and direct oxidation by surface manganese species are the dominant oxidation mechanisms for ACE degradation. Furthermore, the catalytic activity of MnO2 was positively correlated with the content of MnIII(s) on the catalyst surface, which stimulated the generation of more oxygen vacancies beneficial for PMS adsorption and the formation of reactive complexes.