Photoassisted Degradation of Dye Pollutants. 3. Degradation of the Cationic Dye Rhodamine B in Aqueous Anionic Surfactant/TiO₂ Dispersions under Visible Light Irradiation:  Evidence for the Need of Substrate Adsorption on TiO₂ Particles

The TiO2 photoassisted degradation of the cationic dye rhodamine B (RhB) has been examined in aqueous dispersions under visible light irradiation at wavelengths longer than 470 nm in the presence and absence of the anionic surfactant sodium dodecylbenzenesulfonate (DBS). RhB degrades slowly via a pH-independent process in TiO2 dispersions containing no DBS. The surfactant DBS adsorbs strongly on the TiO2 particles and significantly accelerates RhB degradation with initial rates reaching maximal values at the critical micelle concentration of DBS (cmc = 1.2 mM). In the presence of DBS, rates decrease with increase in pH, an effect directly attributable to variations in the extent of adsorption of RhB with changes in the surface charge of TiO2 particles. The zeta (ζ)-potentials of TiO2 particles in RhB/DBS/TiO2 dispersions (pH 2.1) show that DBS significantly enhances RhB adsorption and correlates with an enhancement in the rate of photodegradation of RhB. The results confirm the heretofore presumed but valid notion that preadsorption on the surface of TiO2 particles is prerequisite for efficient photodegradation of RhB under visible light irradiation; moreover, the data infer that degradation occurs at the particle surface and not in the solution bulk. Present observations are consistent with a pathway in which excited RhB* injects an electron onto TiO2 (an electron-transfer mediator) that is subsequently scavenged by O2 to form the O2•- radical anion and ultimately the OH• radical, as evidenced by DMPO spin-trapping ESR experiments carried out under conditions otherwise similar to those in photodegradation which, we infer, participates in the RhB photodegradation.