Heterogeneous oxidation of naproxen in the presence of α-MnO2 nanostructures with different morphologies

α-MnO2 nanostructures with different morphologies including nanoparticles, flower-like nanostructures and nanorods have been successfully prepared and used in the heterogeneous oxidation of naproxen in water. It has been found out that the heterogeneous oxidation process is highly pH dependent, with higher efficiency at lower pH values. The oxidation kinetics of naproxen were modeled by Langmuir–Hinshelwood equations. Based on the kinetic constants (k), the oxidation efficiency follows the order of commercial particles < nanorods < flower-like nanostructures < nanoparticles. The mechanism for the oxidation of naproxen has been studied in depth, which shows that the high efficiency can be ascribed to the specific adsorption, electron transfer, and byproducts release. Dissolved anions (Cl−, CO32−, SO42−, PO43−) and cations (Mn2+) could remarkably decrease the removal rate by competitively adsorbing and reacting with MnO2, respectively. In addition, a total of 7 byproducts were identified by LC–MS from which a tentative pathway was proposed.