Sulfur-doped pine-needle biochar activates peroxymonosulfate for singlet-oxygen-dominated norfloxacin degradation: Structure-activity relationship and mechanistic insights.

Norfloxacin (NOR), a widely used fluoroquinolone antibiotic, is increasingly detected in aquatic environments. Herein, pine-needle biomass was valorized into B-, N-, and S-doped biochars for adsorption-oxidation coupled NOR removal via peroxymonosulfate (PMS) activation. Systematic comparison showed that sulfur-doped biochar (SBC) exhibited the best overall performance for NOR removal and PMS activation. Structure-activity analysis revealed that the enhanced performance of SBC was closely associated with defect-rich structures and thiophene-S species. Kinetic analysis demonstrated a clear synergy between adsorption and catalytic oxidation. Under the optimal conditions, the SBC/PMS system achieved 93.2% NOR removal at an initial NOR concentration of 50 mg L-1. Quenching experiments, electron paramagnetic resonance spectroscopy, DPBF probing, and electrochemical analyses consistently indicated that singlet oxygen (1O2) was the dominant reactive species. Liquid chromatography-mass spectrometry suggested that NOR degradation mainly proceeded through piperazine-ring opening, defluorination, and quinolone-ring cleavage. In addition, TOC analysis and toxicity prediction indicated the enhanced mineralization capability and reduced ecological risk of the SBC/PMS system. Density functional theory calculations further supported the key roles of defect-rich structures and thiophene-S in PMS activation. This work highlights the potential of pine-needle waste as a sustainable biochar precursor and provides insight into the heteroatom-regulated design of biochar-based PMS activators for antibiotic-contaminated water treatment.