UV/TiO2 photocatalytic oxidation of commercial dyes in aqueous solutions

Textile industry wastewaters containing dyes and pigments were difficult to treat, due to the ineffectiveness of biological processes. The photocatalytic oxidation kinetics of Acid Blue 40, Basic Yellow 15, Direct Blue 87, Direct Blue 160 and Reactive Red 120 were studied at pH 3-11 in a titanium dioxide/UV slurry reactor. The reaction rates decreased in the order: Basic Yellow 15, Acid Blue 40, Reactive Red 120, Direct Blue 160, Direct Blue 87. The oxidation kinetics increased with increasing pH. pH had the most effect on the oxidation of Direct Blue 87 and least on that of Basic Yellow 15. The photocatalytic oxidation kinetics of the dyes were described using the Langmuir-Hinshelwood model. The oxidation kinetics of Direct Blue 160 and Reactive Red 120 followed the model at all pH values studied. The oxidation kinetics of Acid Blue 40 fitted the model at pH 3, Basic Yellow at pH 3, 5 and 11 and Direct Blue 87 at pH 7 and 9. For the azo dyes, the oxidation rates decreased with increasing number of azo linkages in a dye molecule. The effects of hydrogen peroxide and iron(II) on the photodegradation of Basic Yellow 15 were studied. Iron(II) improved the reaction rate of Basic Yellow 15 at concentrations of 0.5 and 1 mM. Hydrogen peroxide had no significant effect on reaction rate in the 0.0001-0.5 mM concentration range.