Catalytic ozonation for metoprolol and ibuprofen removal over different MnO2 nanocrystals: Efficiency, transformation and mechanism

Manganese dioxide has been widely recognized as catalyst in catalytic ozonation for organic pollutants removal from wastewater in recent decades. However, few studies focus on the structure-activity relationship of MnO 2 and catalytic ozonation mechanism in water. In the present study, the oxidative reactivity of three different crystal phases of MnO 2 corresponding to α-MnO 2 , β-MnO 2 and γ-MnO 2 towards metoprolol (MET) and ibuprofen (IBU) were evaluated. α-MnO 2 was found to contain the most abundant oxygen vacancy and readily reducible surface adsorbed oxygen (O 2− , O − , OH − ), which facilitated an increase of ozone utilization and the highest catalytic performance with 99% degradation efficiency for IBU and MET. α-MnO 2 was then selected to investigate the optimum key operating parameters with a result of catalyst dosage 0.1 g/L, ozone dosage 1 mg/min and an initial pH 7. The introduction of α-MnO 2 promoted reactive oxygen species (O 2− , O − , OH − ) generation which played significant roles in IBU degradation. Probable degradation pathways of MET and IBU were proposed according to the organic intermediates identified and the reaction sites based on density function theory (DFT) calculations. The present study deepened our understanding on the MnO 2 catalyzed ozonation and provided reference to enhance the process efficiency.