Upcycling iron slag into a monolithic catalyst has led to low‐cost heterogeneous catalytic ozonation (HCO) strategies for water treatment. The primary industrial challenges are the low chemical oxygen demand (COD) reduction rate in treating authentic wastewater and difficulties sustaining close‐loop reaction cycles. In this study, we present a monolithic catalyst transformed from low‐cost iron oxide scale slag (IOSM) for highly efficient, sustainable ozone catalysis. The IOSM, constituted of 59.70% Fe2O3 and 40.30% Fe3O4, effectively decomposes ozone into superoxide radicals (·O2⁻) through a close‐loop cyclic reaction facilitated by interfacial charge transfer. This mechanism enables super‐fast degradation of 100 mg·L⁻¹ tetracycline at ozone concentrations below 1.00 mg·L⁻¹ in just four minutes, with the iron leaching merely at 59 μg·L‐1 after 10 cycles. In tests with authentic antibiotic wastewater, the IOSM‐based HCO system achieves a record‐breaking 81.70% COD reduction within two hours. This research underscores the potential of upcycling industrial waste into highly effective ozone catalysts for sustainable practices in wastewater treatment.

Upcycling iron oxide scale slag into a high‐performance, sustainable ozone catalyst