Hydrodynamic cavitation–H2O2 systems for azo dye degradation: Per-pass kinetics, synergy coefficients, and scale-up insights using methyl orange as a model pollutant

Hydrodynamic cavitation (HC) combined with hydrogen peroxide (H2O2) is a scalable and energy-efficient hybrid advanced oxidation process (AOP) for treating azo dye wastewater. This review critically analyses HC–H2O2 systems for the degradation of methyl orange (MO), selected as a structurally representative sulfonated monoazo model pollutant. This review adopts a per-pass kinetic framework. All synergy coefficients are derived from per-pass rate constants, kpp (pass−1), rather than time-based rate constants, kt (min−1), which depend on volume of holding tank. The review provides the first systematic compilation of synergy coefficients S for HC–H2O2 systems (calculated S values from published datasets where it was not reported). Three regimes based on H2O2 concentration have been identified: under-dosed, optimal, scavenging. The values of S were found to be in the range of 2–4.8 for the optimal H2O2 regime. Reactor design, degradation pathways, ecotoxicological implications, and scale-up are analysed using the standardised metrics throughout. The review concludes with a proposed minimum reporting standard and recommends future research priorities aimed at closing the current gap between laboratory findings and industrial implementation.