Degradation Performance and Mechanism of Methyl Orange by Iron-Modified Sludge Biochar-Activated Peroxymonosulfate

Azo dye wastewater poses a persistent environmental challenge because of its high toxicity, poor biodegradability, and widespread occurrence in textile and dyeing effluents, highlighting the need for efficient and sustainable treatment technologies. In this study, iron-modified sludge biochar (Fe@SBC) was prepared via impregnation–pyrolysis and employed to activate peroxymonosulfate (PMS) for methyl orange (MO) degradation, providing a dual benefit of sludge valorization and pollutant removal. Iron modification increased the specific surface area from 50.93 to 66.54 m2·g−1 and total pore volume from 0.085 to 0.309 cm3·g−1, while introducing Fe3O4, Fe2+/Fe3+ redox pairs, and oxygen-containing functional groups that significantly enhanced electron transfer. The Fe@SBC/PMS system achieved 95.2 ± 0.2% MO removal within 25 min under optimal conditions, notably outperforming unmodified biochar (86.0 ± 0.6%). The catalyst exhibited excellent pH adaptability (3–11) and maintained 88.0 ± 0.8% removal after five cycles. Quenching experiments and EPR analysis identified singlet oxygen (1O2) as the dominant reactive species (~80% contribution), with sulfate and hydroxyl radicals playing auxiliary roles. LC-MS analysis proposed three parallel degradation pathways involving azo bond cleavage, hydroxylation, and desulfonation. These findings demonstrate that Fe@SBC is an effective PMS activator and provide mechanistic insight into the design of sludge-derived carbon catalysts for advanced oxidation processes, offering a promising approach for simultaneous sludge resource recovery and azo dye wastewater treatment.