Enhanced degradation of micropollutants by UV/freshly formed colloidal MnO₂: Reactive species, kinetics and pathways

This study reports that freshly formed manganese dioxide (MnO2) is an effective photocatalyst for the abatement of micropollutants, such as nalidixic acid (NDA) and gemfibrozil (GFRZ). The freshly formed MnO2 was flower-like beta-MnO2 and showed better efficiency in micropollutant abatement owing to its better adsorption capability than commercial MnO2. Electron paramagnetic resonance (EPR) spectra confirmed that reactive oxygen species (ROS), including hydroxyl radicals (HO center dot), superoxide radicals (O-2(center dot-)) and singlet oxygen (O-1(2)), were formed in the UV/MnO2 system. Valence band X-ray photoelectron spectroscopy (VB-XPS) spectra and UV-vis adsorption curves verified the generation of ROS via the reaction between e(-) and oxygen as the sole pathway. Holes (h(+)) dominated the degradation of NDA, while both ROS and h(+) were important for GFRZ abatement. Side chain cleavage, kentonization and hydroxylation took place in NDA and GFRZ degradation, while demethylation and decarboxylation also occurred during NDA degradation.