Removal of methylene blue from water by γ-MnO₂

Methylene blue (MB) is a cationic dyestuff, which is particularly resistant to biodegradation. The molecular sieved γ-MnO₂ was used as an adsorbent/oxidant to remove the MB at room temperature and in visible light. The removal efficiency was mainly evaluated by X-ray diffractometer (XRD), UV–Vis spectrometer (UV–Vis), total organic carbon (TOC). The results revealed that the mechanisms of MB removal by γ-MnO₂ are significantly influenced by the pH. In acidic conditions, hypsochromic effects (i.e. blue shifts of UV–Vis spectra) resulting from N-demethylation of the dimethylamino group in MB may occur concomitantly with oxidative degradation by γ-MnO₂. However, the TOC in solution after γ-MnO₂ treatment in acidic conditions did not show a dramatic decrease. At near neutral pH conditions, there was almost no UV–Vis absorption for the MB solution, however, only 50% TOC removal was observed. It indicated that MB was not only adsorbed onto γ-MnO₂ but also partially oxidatively degraded to other organic compounds which were colourless for UV–Vis.