Decolorization of methylene blue by δ-MnO2-coated montmorillonite complexes: Emphasizing redox reactivity of Mn-oxide coatings

delta-MnO2 coatings on clay substrates tend to be poorer in crystallinity as compared with their discrete counterparts, which may be of environmental significance for adsorption and oxidation of contaminants. Discrete delta-MnO2 particles and three delta-MnO2-coated montmorillonite complexes with varying MnO2 loadings (4.8-34.9%) were synthesized, and oxidative decolorization of methylene blue (MB) by the synthetic materials was investigated in batch systems. Results showed that oxidative decolorization of MB increased with increasing loading of Mn-oxide coatings, whereas oxidation capacity of the coatings, on the basis of unit mass of MnO2, tended to decrease. Initial reaction rate of MB oxidation by both delta-MnO2 coatings and their discrete counterpart increased linearly with increasing Mn-oxide loadings, but the rate of the former was higher than that of the latter. An increase in humic acid concentration displayed a progressively enhanced promotive effect on MB decolorization, whereas the promotive effect was greatly suppressed at lower pH. (C) 2010 Elsevier B.V. All rights reserved.