Switching peroxymonosulfate activation pathway from free radical to surface-bound radical over MnFe2O4 for enhanced degradation of ofloxacin: Key role of size effect and surface hydroxyl group

The modulation of peroxymonosulfate (PMS) activation pathway to achieve effective degradation of pollutants is significant, but still challenging. Herein, a series of hydroxyl- and size-controlled MnFe₂O₄ catalysts were synthesized through an alkaline microenvironment regulation strategy. The variable-sized MnFe₂O₄ (submicron, nanoscale and microscale) exhibited size-dependent catalytic behavior, while the changes in surface hydroxyl content altered the activation pathway from dissolved radicals to surface-bound radicals. The quenching experiments, electron spin resonance spectroscopy, electrochemical studies, in-situ Raman spectra and density functional theory calculations were conducted to reveal the evolution of reactive oxygen species. Due to strong binding energy, PMS was stabilized by the rich surface hydroxyl to form surface complexed MnFe₂O₄-HOOSO₃⁻ and simultaneously activated by the active bimetallic components, resulting in oriented-production of surface-bonded radicals. Benefiting from smaller particle size and rich hydroxyl groups, the optimal nano-MnFe₂O₄-OH/PMS system could massively generate surface-bound SO₄^•−, which achieved a highly efficient removal efficiency (88.6 %) for ofloxacin (10 mg/L) degradation under wide pH ranges from 3.0 to 9.0. The evaluation of ecotoxicity, reusability, pH applicability, universality and anti-interference property confirmed the practical application prospect of nano-MnFe₂O₄-OH/PMS system.