Adsorption-coupled PMS activation over waste iron sludge-derived Fe/biochar for sulfamethoxazole degradation: A singlet oxygen-associated oxidation pathway.

Fenton-derived iron sludge and discarded calamus biomass were converted into Fe/biochar (Fe/BC) catalysts through hydrothermal pretreatment followed by KOH-assisted pyrolysis/activation. Among the prepared materials, Fe/BC-800 exhibited the highest SMX adsorption and PMS-assisted removal performance, achieving 67% adsorption within 120 min and nearly complete parent-SMX removal within 60 min after PMS addition following 20 min pre-adsorption. Phase-resolved mass-balance analysis showed that pre-adsorption mainly established a surface-associated SMX reservoir and enhanced early aqueous-phase removal rather than increasing the apparent oxidation-stage rate constant. Component-control experiments further demonstrated that the high activity of Fe/BC-800/PMS could not be explained by adsorption, the isolated carbon or iron-sludge-derived components, their physical mixture, or dissolved Fe alone, supporting a cooperative heterogeneous Fe-carbon interfacial process. Quenching, TEMP-EPR, H2O/D2O solvent-isotope kinetics, and electrochemical analyses supported a predominantly 1O2-associated oxidation pathway accompanied by auxiliary O2•-, SO4•-, and •OH processes and interfacial electron transfer. Fe0, Fe2+/Fe3+ redox cycling, iron oxides, and carbonaceous sites collectively contributed to PMS activation rather than a single dominant Fe species. Fe/BC-800 maintained high removal over 10 NaBH4-assisted regeneration cycles, while the Fe/BC/PMS system achieved 88-95% removal of five antibiotics in undiluted hospital wastewater after pH adjustment to 3.0. These results demonstrate the valorization of waste-derived Fe and carbon precursors while clarifying the coupled roles of adsorption enrichment and heterogeneous Fe-carbon PMS activation.