Investigating the Combined Effect of Ultrasound and an Iron Oxide Catalyst for the Degradation of Congo Red Dye

Industrial dyes remain a major source of water pollution and pose environmental and human health risks because of their carcinogenic and mutagenic properties. Of these, Congo Red dye shows high toxicity and stability in aquatic environments; therefore, removing it from water bodies is desirable. Among numerous methods, ultrasound-assisted catalysis for dye degradation remains promising. This work investigates the combined effect of ultrasound and an iron oxide (Fe2O3) catalyst for degrading Congo Red (CR) dye (a model industrial dye). The characterization of Fe2O3 particles was performed through SEM, XRD, and FTIR analyses. CR degradation was performed at 100–600 W ultrasound power, 0–120 min, and with 5–20% (w/v) Fe2O3. The ultrasound-driven CR degradation was compared with and without the Fe2O3 catalyst. Using ultrasound alone, maximum CR degradation reached 39.30%, while adding Fe2O3 (20% w/v) during ultrasonication enabled 93.20% CR degradation in 120 min. The optimized conditions for maximum CR degradation (93.21%) were 600 W, 30 °C, 120 min, and 20% w/v Fe2O3. The enhancement was attributed to acoustic cavitation, heterogeneous bubble nucleation on Fe2O3 surfaces, improved mass transfer, Fe3+/Fe2+ redox cycling, and reactive oxygen species generation. Further, the kinetic analysis indicated that the CR degradation followed a pseudo-second-order kinetic model, showing strong agreement with experimental data (R2 = 0.96). A detailed mechanism was proposed involving CR adsorption, azo-bond cleavage, aromatic ring hydroxylation, fragmentation, ring opening, and progressive oxidation into smaller intermediates. Overall, the present study demonstrates that ultrasound combined with Fe2O3 effectively enhanced Congo Red removal/decolorization under the investigated laboratory-scale conditions.