SonoFenton degradation of an azo dye, Direct Red

• A decolorisation efficiency of 99% for DR81 (4.7 × 10−5 M) was achieved within 75 min under SonoFenton treatment process.
• Degradation depends on pH, [Fe2+], [H2O2] and ultrasonic frequency.
• Degradation products were identified and the dye degradation mechanism proposed.
• Enhanced extent of degradation in the combined processes due to synergistic effects.

The degradation of a reactive azo dye, Direct Red 81 (DR81), by Fenton process and in conjunction with sonolysis (SonoFenton) was studied. The synergistic effect of Fenton process and sonolysis enhanced the degradation of Direct Red 81 in aqueous solutions and the reaction followed the mechanism of hydroxyl radical (HO) oxidation. The influence of the initial substrate concentration, pH and catalyst loading on the rate of decolorisation were studied. The dye decolorisation followed apparent first order kinetics. The optimum conditions for decolorisation were pH = 3.0, [Fe2+] = 0.2 g/l, [H2O2] = 5.1 × 10−3 mol/l and ultrasonic frequency = 120 kHz, 60 W. These conditions yielded 99% decolorisation of DR81 within 75 min. The sonolytic degradation products of DR81 were identified using Electrospray Ionization-Mass Spectrometry (ESI-MS). The presence of CO32−, HCO3−, Cl−, NO3−, and SO42− ions in the dye solution did not have a considerable effect on the decolorisation efficiency. This study demonstrates that Fenton and SonoFenton methods can effectively decolorize DR81 dye in waste water. The dye concentration used in this study is higher compared to earlier studies illustrating the effective mineralization by the SonoFenton process. The mechanism of dye degradation is also proposed.