Photodegradation of dyes with poor solubility in an aqueous surfactant TiO2 dispersion under visible light irradiation

The photodegradation of a cationic dye, Malachite Green (MG), is examined both in an anionic surfactant sodium dodecylbenzene sulfonate (DBS)/TiO2 dispersion and in a cationic surfactant hexadecytrimethylammonium bromide (HTAB)/TiO2 dispersion under visible irradiation (lambda > 470 nm). In the absence of surfactants, MG is difficult to degrade in aqueous TiO2 dispersion, owing to its poor solubility in water and low adsorption on the surface of TiO2 particles. Addition of surfactants enhances the solubility and adsorption ability of MG and hence accelerates significantly the MG degradation rate in aqueous TiO2 dispersions. The MG degradation rate reaches a maximum when the concentration of DBS added is at its c.m.c. According to the adsorption characteristics of the ionic surfactant molecules, the degradation rate of MG decreases with increases in the pH of DBS/TiO2 dispersions, while the degradation of MG is more rapid at higher pH ranges in HTAB/TiO2 dispersions. These results confirm and reinforce that the adsorption of dyes molecules upon the TiO2 particle surface is a prerequisite for TiO2-assisted photodegradation under visible irradiation and the degradation takes place at the TiO, particle surface, rather than in the bulk solution. The photodegradation kinetics of the dyes in surfactant/TiO2 dispersions under visible irradiation obeys the Langmuir-Hinshelwood model. Evidence for the generation of active (OH)-O-. radicals is obtained using spin-trapping (DMPO) EPR spectroscopy.