Removal of methylene blue from water by γ-MnO 2

Methylene blue (MB) is a cationic dyestuff, which is particularly resistant to biodegradation. The molecular sieved γ-MnO 2 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-MnO 2 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-MnO 2. However, the TOC in solution after γ-MnO 2 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-MnO 2 but also partially oxidatively degraded to other organic compounds which were colourless for UV-Vis.