Comparative study on photocatalytic decolorization of an anionic and a cationic dye using different TiO2 photocatalysts

The photocatalytic degradation of Malachite green oxalate (MG), a cationic dye, and Eosin yellow (EY), an anionic dye, was studied and compared using different TiO2 catalysts such as Degussa P25 and Millennium PC: PC500, PC105, PC100, and PC50 in slurry form under UV irradiation at natural, acidic, and alkaline mediums. The structural properties and phase composition of the TiO2 samples were determined by X-ray diffractometry. The obtained results indicated that the photocatalytic activity was correlated to the crystalline phase and the particle sizes of each catalyst; the mixed phase of anatase/rutile (Degussa P25) exhibited a higher photoactivity to degrade both dyes than pure-phase anatase (Millennium PC). In addition, TiO2 with larger particles and lower surface area (P25 and PC50) performed better in the degradation of both dyes in comparison with smaller particles and larger surface area. An enhancement of the degradation efficiency was observed in alkaline medium for the cationic dye (MG) and in acidic medium for the anionic dye (EY). In an acidic medium, the various degrees of rate degradation obtained using three different proton sources of pH (H2SO4, HCl, and HClO4) are directly related to the presence of the corresponding counter ions. The effect of various parameters like the presence of Cl-, HCO3- and H2O2 on the photocatalytic degradation of both dyes, using Degussa P25 as photocatalyst, depends on several factors such as their concentrations in solution, pH, dye structure, and the previous adsorption in the dark.