Photocatalytic degradation of a heteropolyaromatic dye (methylene blue) in aqueous ZnO suspension was investigated under UV and visible light irradiation. Adsorption behavior was found to be well fitted with the Langmuir and Freundlich isotherms. Free energy of adsorption was found to be negative which indicates that adsorption was spontaneous. Under UV light irradiation for 6 hours, the catalytic process produced a net decolorization and mineralization (50 µM MB) of about 98% and 89% respectively. Meanwhile, under visible light irradiation (10 µM MB) for 8 hours these resulted ca. 95% and 77% decolorization and mineralization, respectively. Degradation efficiency was found to be highest at basic pH (8.6). Catalyst loading of 1.0 gl-1 was found to be optimum for the degradation of 100 µM MB solution. In all conditions the UV irradiation showed much better photo-assistance than the visible light irradiation.
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