Mechanism investigation of visible light-induced degradation in a heterogeneous TiO₂/eosin Y/rhodamine B system

Visible light-induced degradation of rhodamine B (RhB) and eosin Y (EO) in a heterogeneous TiO₂ P-25/EO/RhB system was investigated in the present work. The results showed that the photodegradation of RhB is enhanced significantly when EO is introduced into the P-25/RhB system. Under optimal conditions (50 mg P-25, 20 mg L^-1 EO), RhB (4 mg L^-1) almost decomposed completely after 35 min of visible light irradiation, though EO was photodegraded simultaneously. The possible photodegradation mechanism was studied by the examination of active species HO•, O₂ ^•- anions, or dye radical cations through adding their scavengers such as methanol, t-butanol, benzoquinone, EDTA, and the I ^- anion. In addition, the electron paramagnetic resonance (EPR) spin trapping technique was also used to monitor the active oxygen species formed in the photocatalytic process. Combined with the contrastive experiments under different atmospheres (N₂-purged or air) and in different systems, it can be deduced that dissolved O₂ plays a crucial role in dye photodegradation and the O₂ ^•- anion is possibly the major active oxygen species. The low degradation rate with the introduction of EDTA or I- indicated that dye radical cations also play a part in photodegradation. Furthermore, except for the dye-sensitized photodegradation on the P-25 surface, reaction in bulk solution also occurs in this system, leading to effective photodegradation of RhB.