Photocatalyzed N-demethylation and degradation of methylene blue in titania dispersions exposed to concentrated sunlight

Simulated wastewaters that contain methylene blue (MB) were bleached in a photocatalytic aqueous TiO2 dispersion illuminated by concentrated sunlight using a parabolic round concentrator reactor (PRCR). The kinetic analysis was carried out well when the temporal concentration variation was a function of the concentrated light energy irradiated. The photocatalyzed N-demethylation of NIB takes place concurrently with photocatalytic decomposition of MB by pseudo-first-order kinetics. The dependence of the photo-decomposition kinetics on the initial concentration of MB is consistent with the Langmuir-Hinshelwood model. Elimination of TOC (total organic carbon) also occurs by pseudo-first-order kinetics prior to full bleaching of the aqueous TiO2 dispersion, after which the TOC level decreases only slightly. Also, compared with the open to air, the photodegradation of MB is not influenced by molecular oxygen bubbling continuously through the reactant suspensions during illumination. TiO2 loadings and flow rates markedly affect the degradation of MB. Under concentrated sunlight, the relative photonic efficiency of MB photodegradation is zeta(rel) = 0.49 (relative to phenol). The efficiency zeta(rel) for the degradation of MB is independent of photoreactor geometry (cylindrical bottle reactor versus round-bottomed flask), of light sources (solar light concentrator versus a Hg lamp) and of the operating mode used (flow versus batch operation). (C) 2002 Elsevier Science B.V. All rights reserved.