Performance of titanium dioxide microcapsules as a photooxidation catalyst for decolourization of methylene blue

Abstract

The performance of anatase type titanium dioxide (TiO2) encapsulated with styrenic copolymers via the solvent evaporation or suspension polymerization process was investigated as a photocatalyst for the decolourization of methylene blue (MB, 3,4-bis(dimethylamino)-phenothiazine-5-thionium chloride) in the aqueous phase. The TiO2 microcapsules, loaded with 4–8 wt% TiO2, were dispersed in a MB aqueous solution containing an adequate amount of hydroperoxide as an oxygen source, and the mixture was exposed to 365 nm UV light. The decolourization of MB proceeded according to the first order of the MB concentration. The apparent rate constant, defined based on the unit weight of loaded TiO2, depended on the initial concentration of MB. The capsule walls, composed of cross-linked and/or uncross-linked poly(styrene-co-2-ethylhexyl acrylate), favoured the adsorption of MB on the capsule surface and promoted the decomposition. The observations of the cross-section of microcapsules by transmission electron microscopy (TEM) showed similar morphology of microcapsules regardless of preparation method; a thin layer of hydrophilic TiO2 particles being localized near the particle surface. The sustainability of the microcapsules was also proved by doing experiments successively for 7 days. Smaller size microcapsules were favoured for the decomposition of MB, and the rate constant increased with the surface area of microcapsules in unit volume of the reaction mixture. Although large size microcapsules with uncross-linked polymer wall had a disadvantage of breaking under high shear agitation, those prepared with cross-linked polymer wall by suspension polymerization still remained effective after the sustainability test for 7 days.