Enhanced visible light photodegradation of water pollutants over N-, S-doped titanium dioxide and n-titanium dioxide in the presence of inorganic anions

The potential application of waste water treatment by photocatalysis is very likely to find its place in the near future. We have studied the photocatalytic degradation of three dyes (Eosin B, Rhodamine 6G, Rhodamine B) in the presence of doped n-TiO₂ in water and found that anchoring groups are favorable to the photodegradation of the pollutants. Taking Rhodamine B as a model pollutant, this study points out an alternative route to enhance photodegradation in invisible light, which consumes energy to synthesize, but addition of 0.1 mM of I⁻ or S₂O₃²⁻ increases the discoloration by up to three folds. For example, KI increased degradation to 36% while Na₂S₂O₃ enhanced it by 61%, which was higher than that of pure n-TiO₂ after sun light irradiation of 40 min. The enhancement of degradation by I⁻ and S₂O₃²⁻ may be linked to the scavenging of the holes by the inorganic anions, thus inhibiting recombination between h⁺/e⁻ after excitation of the semiconductor. The degradation is more effective in the presence of S₂O₃²⁻. In the presence of 0.1 mM KI, the rate constant increased from 0.0231 s⁻¹ to 0.0325 s⁻¹.Peroxodisulphate increases degradation, however, this is attributed to the sulfate radicals.