Ultrasound and heat enhanced persulfate oxidation activated with Fe0 aggregate for the decolorization of C.I. Direct Red 23

• Heat and ultrasound promoted DR23 decolorization in PS/Fe0 system.
• Key operating factors affecting PS/Fe0 was investigated.
• Mineralization of DR23 occurred quickly within 10 min.
• Ultrasound in PS/Fe0 system did not incur a substantial increase in electricity.

Effluents from the paper printing and textile industries are often heavily contaminated with azo dyes. Azo dyes are difficult to oxidize biologically. This work investigated the decolorization of an azo dye, C.I. Direct Red 23 (DR23), by persulfate (PS) activated with Fe0 aggregates (PS/Fe0). Ultrasound (US) and heat were used as enhancement tools in the PS oxidation system. Neither US-activated PS nor thermally activated PS was effective in oxidizing DR23. However, the decolorization was significantly enhanced by PS/Fe0 combined with US (PS/Fe0/US) or heat (PS/Fe0/55 °C). Approximately 95% decolorization of 1 × 10−4 M DR23 was achieved within 15 min in the PS/Fe0/US system at an initial pH of 6.0, PS of 5 × 10−3 M, Fe0 of 0.5 g/L and US irradiation of 106 W/cm2 (60 kHz). Complete decolorization was achieved within 10 min in the Fe0/PS/55 °C system. The rate of decolorization doubled when US was introduced in the PS/Fe0 system during the treatment of different initial dye concentrations. The dependence of dye and true color (ADMI) depletion on PS concentration has been discussed. DR23 was completely degraded based on the disappearance of aromatic groups of UV–vis spectra and the variation of TOC mineralization. The observed pseudo-first-order decolorization rate was substantially enhanced by increasing temperature. The Arrhenius activation energy for the PS activated with Fe0 was estimated as 8.98 kcal/mol, implying that higher temperature is beneficial for the DR23 decolorization. The addition of US into the PS/Fe0 system did not incur a substantial increase in electricity, whereas the mineralization of DR23 occurred quickly. Thus, both PS/Fe0/US and heated PS/Fe0 systems are practically feasible for the effective degradation of the direct azo dye in textile wastewater.