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

Abstract

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 (λ>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 TiO₂ 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 radicals is obtained using spin-trapping (DMPO) EPR spectroscopy.