Reductive photocatalytic decolourization of an azo dye Reactive Black 5 using TiO2 : mechanism and role of reductive species

Photocatalytic oxidation of dyes using TiO2 is amply studied; however, there are no reports demonstrating reductive photocatalytic decolourization of dyes and underlying mechanisms. We investigated photocatalytic reduction of Reactive Black 5 (RB5) dye using TiO2 and UV light in the presence of various hole scavengers (HSs). Around 94% of 50 mg L-1 initial RB5 was decolourized in the presence of 2 mM oxalic acid (OA) and 0.5 g L-1 TiO2, after 1 h irradiation at initial pH 2.8. OA yielded highest rate and extent of RB5 decolourization among all HS, probably due to superior hole-scavenging activity and capacity to produce reductive radicals. LC-MS analysis of decolourized RB5 sample revealed the presence of 2-((4-aminophenyl) sulphonyl) ethyl hydrogen sulphate, and 3,4,6-triamino-5-hydroxynaphthalene- 2,7-disulphonic acid as end-products; indicating cleavage of azo bonds as a main mechanism of decolourization. The contribution of reducing species for reductive RB5 decolourization was found to be in the order of e(cb)(-) > C2O4 center dot- radical > CO2 center dot- radical. The rates of RB5 decolourization, appearance and time point of highest accumulation of end-products, were greatly influenced by the initial pH, type and concentration of HS. TiO2 exhibited excellent reusability and regenerability. Around 65% decolourization was obtained even after the fifth reuse. The regenerated TiO2 exhibited photocatalytic activity almost similar to the fresh TiO2.