Efficient Dye Degradation Catalyzed by Manganese Oxide Nanoparticles and the Role of Cation Valence

Abstract

The menage of 20–35 nm manganese oxide (Mn−O) nanoparticles (NPs) namely MnO, α-Mn2O3, Mn3O4 and δ-MnO2 have been employed as highly recyclable heterogeneous Fenton catalysts for organic dye degradation in wastewater treatment. Mn3O4 was found to be superior to MnO, α-Mn2O3 and δ-MnO2 in the descending order under similar reaction conditions. The availability of active sites proportionate with the specific surface area could not interpret the relative performance since the specific surface area of MnO, α-Mn2O3, Mn3O4 and δ-MnO2 NPs were 71(±4), 16(±5), 63(±2) and 165(±5) m2/g, respectively. In fact, the presence of unpaired eg electrons in both Mn2+ and Mn3+ ions in Mn−O was found to be crucial for effective electron transfer to H2O2 for the generation of intermediate radicals.The best MB degradation activity of 99.3(±2)% was observed with 100 mg/L 15–20 nm Mn3O4 particles and 0.25 mol/L H2O2 at 80°C within 60 min. The mechanism of the degradation process follows first-order kinetics and is dependent on the generation of •OH radicals at the initial stage and 1O2 radicals after 30 min.