Photocatalytic oxidation and adsorption of methylene blue on thin films of near-ultraviolet-illuminated TiO2

Abstract

Aqueous solutions of methylene blue are totally mineralized when recirculated over thin films of titanium dioxide illuminated with near-u.v. light. The rate of destruction obeys first-order kinetics with reasonable precision but the apparent first-order rate constant, k′, decreases with increasing initial concentration of solute. 1 dm³ of 10 µmol dm^–3 methylene blue solution illuminated with a 20 W lamp, is decreased to 5 µmol dm^–3 in 11.8 min. Sunlight from a 1 m² parabolic trough is capable of destroying the methylene blue at 6.4 times this rate. The decrease in k′ values with increasing concentration is consistent with curves calculated using the integrated form of the Langmuir expression. The adsorption parameter determined in the analysis of the kinetics of the photocatalytic data agreed with the adsorption affinity parameter determined using the classical Langmuir adsorption isotherm for the dark equilibrium data; this indicated the key role played by adsorption in photocatalytic oxidation with titanium dioxide. The maximum quantum yield for methylene blue destruction at high flow rates with a 10 µmol dm^–3 initial concentration was calculated to be 0.0092.