Decolourization of direct blue 15 by Fenton/ultrasonic process using a zero-valent iron aggregate catalyst

Decolourization of direct azo dye, direct blue 15 (DB15), by an advanced Fenton process coupled with ultrasonic irradiation (Fenton/US) was investigated. Zero-valent iron (ZVI) aggregates were used as the catalyst. A positive synergistic effect occurred when Fenton’s reagent was combined with ultrasonic irradiation. Experimental results showed that the optimum conditions for decolourization were pH 3.0, Fe(0) 1 g/L, H₂O₂ 5.15 × 10^?3 mol/L with ultrasound density of 120 W/L at 60 kHz. These conditions yielded 99% decolouration of 4.7 × 10^?5 M DB15 (4130 ADMI) solution within 10 min. DB15 decolouration follows the first-order decolouration kinetics. Although the solutions containing H₂CO₃, Cl^?, ${\text{ClO}}_4^{-}$, ${\text{NO}}_3^{-}$ and ${\text{SO}}_4^{2-}$ ions did not have a significant effect on the decolouration, the ${\text{H}}_2{\text{PO}}_4^{-}$ ion did decrease the decolouration rate. High ultrasonic input power accelerated the reaction and increased decolourization efficiency. The cost effectiveness of this process at high ultrasound density could be controlled despite the high electricity costs incurred by the process. ZVI aggregates were reusable; however, an increase in the number of times ZVI was recycled decreased the decolourization rate. This study demonstrates that a Fenton/US process can effectively decolour the direct azo dye DB15 in wastewater.