Mechanisms of Methylene Blue Degradation by Nano-Sized β-MnO2 Particles

Degradation of a dye by 1-D nano-sized beta-MnO2 particles was investigated to understand the mechanisms involved. Batch experiments were performed in a system of beta-MnO2, hydrogen peroxide (H2O2), methylene blue (MB), and radical scavengers. The prepared beta-MnO2 had a 1-dimensional nano-sized structure and showed a negative zeta potential at pH higher than 3.7. In the presence of H2O2, MB was rapidly degraded by beta-MnO2. The rate of MB degradation was not significantly inhibited by the initial pH values from 6.05 to 10.02, but notably decreased at pH 3.04, at which point the beta-MnO2 particles dissolved. The effect mechanisms of operational parameters (i.e., H2O2, catalyst dose) in beta-MnO2/MB/H2O2 system were investigated. The removal rate of MB was remarkably reduced in the existence of p-quinone and was even more affected when sodium azide was added, indicating that the singlet oxygen radical (O-1(2)) plays an important role. The obtained results point out that MB degradation by nano-sized beta-MnO2 is due to the catalytic oxidation of H2O2 on beta-MnO(2)surface reactive sites, rather than surface oxidation or adsorption. The reusability of nano-sized beta-MnO2 was also examined after five times of recycling to evaluate the applicability of beta-MnO2 for wastewater treatment.