Counterion effect of cationic surfactants on the oxidative degradation of Alizarin Red-S photocatalysed by TiO2 in aqueous dispersion

The TiO2 photocatalyzed degradation of a hazardous texile dye, 3,4-dihydroxy-9,10-dioxo-2-anthracenesulfonic acid sodium salt (Alizarin Red-S, ARS), has been assisted by various cationic surfactants and examined in air-equilibrated aqueous medium under UV light irradiation. Absorption spectral analysis showed that ARS degrades slowly via a pH-dependent process in TiO2 dispersions containing no surfactant. The photodegradation efficiency of the dye was significantly enhanced, especially in alkaline media, by the addition of surfactants such as cetyltrialkylammonium salts (CTRAX). Since both ARS and the TiO2 surface are negatively charged in basic conditions, the addition of cationic surfactants has effectively allowed the mutual interaction among the dye, CTRAX and TiO2. The cooperation between the inorganic and organic components, which is very likely realized through the formation of cationic surfactant bilayers on the TiO2 particles, permitted an improvement in the coadsorption of ARS onto the TiO2 particles and thus rendering ARS more available to the action of the photo-yielded oxidative species formed on the semiconductor surface. The crucial influence of the surfactant adsorption was clearly pointed out by the investigation of the counterion effect on the degree of ARS photodecomposition as a function of the cetyltrimethylammonium salt (CTAX) concentration. Two particularly significant aspects have emerged from this investigation: the almost complete degradation of ARS was achieved, for all surfactants used, at a concentration close to 1 mM (value well above the cmc in the experimental conditions); also, by increasing the surfactant concentration, the photocatalytic reaction became less and less efficient and significantly dependent on the counterion nature. These results have allowed to gain insight and a better understanding of the surfactant-assisted photocatalytic degradation mechanism of the dye.