Upcycling traditional Chinese medicine residue into Fe/Zr bimetallic biochar for synergistic PMS activation: A superoxide-driven route for levofloxacin degradation and toxicity mitigation

The practical application of biochar (BC) for peroxymonosulfate (PMS) activation is hindered by its limited surface area, paucity of active sites, and poor electron transfer. To address these challenges, we synthesized an iron-zirconium bimetallic-modified BC (Fe/Zr 0.8 -BC) from traditional Chinese medicine residue via impregnation-pyrolysis. Comprehensive characterization confirmed the successful loading of Fe (Fe 3 O 4 /metallic Fe) and Zr (ZrO 2 ), which dramatically increased the specific surface area from 4.10 to 269.59 m 2 /g and created a mesoporous architecture conducive to both PMS activation and pollutant adsorption. The Fe/Zr 0.8 -BC/PMS system achieved 89.54% levofloxacin (LVF) removal and 42.31% total organic carbon mineralization within 90 min under optimized conditions (catalyst 0.6 g/L, PMS 1 mM, LVF 10 mg/L, pH 7). The catalyst exhibited robust reusability, with only ∼19% activity loss after four cycles and leached Fe/Zr concentrations (0.10 and 0.02 mg/L) below regulatory limits. Mechanistic investigations, combining radical quenching and electron paramagnetic resonance, revealed synergistic contributions from O 2 • - , SO 4 • - , •OH, and 1 O 2 , with O 2 • - playing the dominant role, suggesting a hybrid radical/non-radical pathway. X-ray photoelectron spectroscopy further elucidated the interfacial catalytic mechanism driven by reversible Fe 2+ /Fe 3+ and Zr 4+ /Zr 3+ redox cycles. Ten degradation intermediates were identified via high-performance liquid chromatography-mass spectrometry, enabling the proposal of three primary pathways: decarboxylation, defluorination, and piperazine ring opening. Toxicity assessment using the T.E.S.T software indicated that most intermediates possessed lower acute toxicity, bioaccumulation potential, developmental toxicity, and mutagenicity than the parent compound. This work not only provides a sustainable strategy for upcycling traditional Chinese medicine residue into high-performance catalytic materials but also offers mechanistic insights into bimetallic synergistic PMS activation, presenting a promising and environmentally benign solution for antibiotic-contaminated water remediation.