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 to biodegradation. Its decomposition was examined in aqueous TiO2 dispersions under UV illumination to assess the influence of temperature, pH, concentration of dissolved oxygen (DOC), initial concentration of MB, and light intensity on the kinetics of decomposition. Hypsochromic effects (i.e. blue shifts of spectral bands) resulting from N-demethylation of the dimethylamino 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. The rate of degradation of MB remains fairly constant regardless of whether the dispersion was purged with oxygen prior to irradiation or with air during the light irradiation period. The photocatalytic process depends on light intensity, but not on the total light energy absorbed. The photoreaction followed pseudo-first-order kinetics even at high MB concentrations (0.3 mM). The temperature dependence of the photodegradation kinetics was assessed (Ea=8.9kJ/mol), as well as the relative photonic efficiency, ξr, relative to phenol (0.48).