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 exami ned both in an anionic surfactant sodium dodecylbenzene sulfonate (DBS )/TiO2 dispersion and in a cationic surfactant hexadecytrimethylammoni um 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 acceler ates 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 decreas es with increases in the pH of DBS/TiO2 dispersions, while the degrada tion of MG is more rapid at higher pH ranges in HTAB/TiO2 dispersions. These results confirm and reinforce that the adsorption of dyes molec ules upon the TiO2 particle surface is a prerequisite for TiO2-assiste d 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 dispersi ons under visible irradiation obeys the Langmuir-Hinshelwood model. Ev idence for the generation of active (OH)-O-. radicals is obtained usin g spin-trapping (DMPO) EPR spectroscopy.