Adsorption-Oxidation Mechanism of <i>δ</i>-MnO₂ to Remove Methylene Blue

Abstract

The material, δ-MnO 2 , has exhibited superior performance on the removal of methylene blue (MB), but the process is significantly impacted by pH, and the impacting mechanism is still unclear. In this study, the effects of pH on the removal mechanism of MB using synthesized δ-MnO 2 were investigated by distinguishing the adsorption and oxidation of MB by δ-MnO 2 during the removal process in the dark. The results show that the total removal efficiency of MB by δ-MnO 2 decreased significantly with an increase in the pH. MB could be removed by δ-MnO 2 via an adsorption mechanism and oxidation mechanism, and the proportion of adsorptive removal and oxidative removal was different under different pH conditions. With an increase in the initial pH from 2.00 to 8.05, the redox potential of δ-MnO 2 decreased, and its oxidation ability for the removal of MB also gradually decreased. In contrast, the surface negative charges of δ-MnO 2 increased with an increase in the pH, and the adsorption ability towards positively charged MB also gradually increased. This indicates that the effects of pH on the removal of MB by δ-MnO 2 are primarily dominated by its influence on the oxidation ability of δ-MnO 2 . In addition, it is further proved that the pH value has a significant effect on the oxidation and adsorption of MB on δ-MnO 2 . Moreover, the significant effects of pH on the oxidation of MB by δ-MnO 2 are further demonstrated by observing the changes in Mn 2+ and the UV-Vis spectra of intermediate products during the reaction, as well as the changes in the FTIR and XPS characterizations of δ-MnO 2 after the reaction.