Zero-Valent Manganese Promoted Fe2+/H2O2 Oxidation for Acid Orange 7 Degradation: Performance, Mechanism and Water Matrix Effects

Fenton oxidation is effective for degrading refractory organic contaminants, but slow Fe3+-to-Fe2+ regeneration under acidic conditions limits continuous H2O2 activation. Here, Acid Orange 7 (AO7) was used as a model pollutant to evaluate a coupled Mn0/Fe/H2O2 system designed to promote iron redox cycling and enhance H2O2 activation. AO7 degradation was examined under different operating conditions, and reactive species and mechanisms were identified using electron spin resonance spectroscopy, chemical probes, and quenching experiments. Under optimized conditions (pH 3.0, 10 μM AO7, 2 mM H2O2, 9 μM Fe2+, and 0.05 g/L Mn0), 88% of AO7 was degraded within 30 min. Hydroxyl radicals (•OH) and FeO2+ were identified as the main reactive species, accounting for 88.6% and 11.4% of AO7 degradation, respectively. Mn0 promoted AO7 degradation by directly activating H2O2 to form •OH and by reducing Fe3+ to Fe2+, thereby sustaining Fe2+/Fe3+ cycling and facilitating •OH and FeO2+ generation. Sulfate and carbonate had negligible effects, whereas fulvic acid, nitrate, and chloride inhibited degradation. AO7 degradation decreased to 40% in lake water and 60% in river water. These results demonstrate that Mn0 addition can enhance Fenton-like oxidation by accelerating iron redox cycling, suggesting its potential in treating dye-contaminated wastewater under controlled operational conditions.