Photocatalytic degradation of reactive black 5 in aqueous solutions: Effect of operating conditions and coupling with ultrasound irradiation

The degradation of reactive black 5 (RB 5), a representative diazo dye found in textile effluents, by means of ultraviolet irradiation (9 W UVA) over TiO2 suspensions, ultrasound irradiation (80 kHz, 135 W) and their combined application was investigated. Several commercial TiO2 catalysts were screened and an anatase Hombicat UV 100 sample exhibited considerable activity in terms of solution decoloration, COD and ecotoxicity reduction. Photocatalytic degradation increased with increasing TiO2 loading (in the range 0.05–1 g/L) and decreasing dye concentration (in the range 120–20 mg/L) and solution pH (in the range 9–2.6). At the typical conditions employed in this study (60 mg/L dye, 0.25 mg/L catalyst, ambient pH=5.8, oxygen sparging), complete decoloration was achieved after 60 min of reaction. Addition of H2O2 up to 0.01 M hindered degradation, scavenging the photogenerated holes and hydroxyl radicals. Ultrasound irradiation resulted in low decoloration, e.g. less than 10% after 60 min at 60 mg/L dye and oxygen sparging and slightly improved under an argon atmosphere. The simultaneous application of ultraviolet and ultrasound irradiation resulted in increased decoloration compared to that achieved by photocatalysis and sonolysis operating separately; moreover, the overall sonophotocatalytic effect was greater than the additive effect of the two processes, implying possible synergy.