Degradation of azo dye direct sky blue 5B by sonication combined with zero-valent iron

The degradation of azo dye direct sky blue 5B by sonication combined with zero-valent iron (US-Fe0)was investigated and an evident synergistic effect was observed. The synergetic effect is mainly due to the increase of OH radical concentration from Fenton’s reaction. The OH radical concentrations in sole sonication and US-Fe0 process were detected by using terephthalic acid as a fluorescent probe and found that OH radicals were generated continuously during sonication and the production of OH radicals in US-Fe0 process was much higher than that in sole sonication. The degradation of direct sky blue 5B followed a pseudo-first-order kinetics and the degradation rate constants were found to be 0.0206 and 0.169 min−1 with sole sonication and US-Fe0 process respectively. It was also found that the degradation ratio of direct sky blue 5B increased with the increase of zero-valent iron dosage and decrease of pH value of the dye aqueous solution. The degradation mechanism of direct sky blue 5B with US-Fe0 process was discussed by the changes of UV–Vis spectrogram of the dye during degradation. The dramatic changes of UV spectra showed a disappearance of both azo and aromatic groups during the degradation.

► The rate constant of the ultrasonic degradation of direct sky blue 5B was able to be increased more than 8 times by adding 2.0 g/L Fe0 into the p-NP solution.
► The OH radicals, Fe2+ and Fe3+ in the US-Fe0 process were determined.
► The degradation mechanism of direct sky blue 5B was discussed.