Photooxidative N-demethylation of methylene blue in aqueous TiO2 dispersions under UV irradiation

Methylene blue (MB) is a representative of a class of dyestuffs resistant t o biodegradation. Its decomposition was examined in aqueous TiO2 dispersion s under W illumination to assess the influence of temperature, pH, concentr ation of dissolved oxygen (DOC), initial concentration of R IB, and light i ntensity on the kinetics of decomposition. Hypsochromic effects (i.e. blue shifts of spectral bands) resulting from N-demethylation of the dimethylami no group in MB occurs concomitantly with oxidative degradation. The maximum quantity of MB adsorbed on TiO2, and the kinetics of degradation of MB and of total organic carbon (TOC) removal were also measured at constant pH 4. Photobleaching of MB solutions takes place at low DOCs and is caused by a reversible reductive process involving photogenerated electrons on TiO2. Th e rate of degradation of MB remains fairly constant regardless of whether t he dispersion was purged with oxygen prior to irradiation or with air durin g the light irradiation period, The photocatalytic process depends on light intensity, but not on the total light energy absorbed. The photoreaction f ollowed pseudo-first-order kinetics even at high MB concentrations (0.3 mM) . The temperature dependence of the photodegradation kinetics was assessed (E-a = 8.9 kJ/mol), as well as the relative photonic efficiency, xi (r), re lative to phenol (0.48). (C) 2001 Elsevier Science B.V. All rights reserved .