Simultaneous photodegradation of two textile dyes using TiO2 as a catalyst

The simultaneous photocatalytic degradation of two dyes: methylene blue (MB), a cationic dye, and arylazo pyridone dye (SD), an anionic dye was investigated using sol–gel prepared TiO2 particles. The paper shows the main difference in degradation kinetics between single dye solution and binary dye solutions. The effects of competitive adsorption and the concentration of the dyes on degradation rate were analyzed. The preferential adsorption capacity of TiO2 toward MB was attributed to electrostatic interactions between the cationic dye and negatively charged TiO2 surface, while repulsion between the negatively charged surface of the catalyst and SD dye results in its lower adsorption capacity. The TiO2 surface charge and accordingly adsorption affinity of investigated dyes were found to be the main factors affecting degradation rate of the dyes. In the single dye system, the catalyst showed considerably higher degradation activity toward MB than to SD. In binary dye systems, the presence of SD did not affect the degradation of MB. On the other hand, the degradation efficiency of SD was found to be highly influenced by initial MB concentration and absorption ability of MB. In addition, higher MB concentration induces the production of a higher amount of reaction products, which also determine the SD degradation rate. Three kinetic regimes for the photodegradation of SD in binary mixtures were observed, used as a starting point to elucidate most influencing parameters for SD reaction kinetics.