Biochar supported zero valent iron activates peroxymonosulfate to degrade sulfamethoxazole through radical and nonradical pathways

Sulfamethoxazole (SMX) is poorly removed by conventional biological treatment, highlighting the need for more effective treatment strategies. In this study, a ball-milled biochar-supported zero-valent iron composite (BC-ZVI) was evaluated as an activator of peroxymonosulfate (PMS) for SMX degradation. Among BC mass ratios of 1:2, 1:5, 1:10, and 1:20, the 1:10 composite exhibited the highest catalytic performance. Under the baseline conditions, 98.0% of SMX was removed within 60 min, with an observed pseudo-first-order rate constant ( k obs ) of 0.0628 min −1 , which was 3.69 times higher than that of the ZVI/PMS system. Arrhenius analysis over 15–35 °C yielded an apparent activation energy of 47.5 kJ mol −1 (R 2 = 0.982), whereas k obs decreased by approximately 30-fold as the initial pH increased from 3 to 9, indicating strong pH dependence. Quenching experiments, electron paramagnetic resonance analysis, and solvent-isotope tests collectively indicated the coexistence of radical-mediated and 1 O 2 -associated pathways. Five major transformation-product ions were identified, and a TOC removal of 62.7% indicated that SMX degradation was accompanied by only partial mineralization. The SMX removal efficiency decreased to approximately 69.0% after five reuse cycles, while dissolved-iron release measured during the first cycle suggested incomplete stabilization of the iron phase within the composite. Overall, ball milling promoted the formation of an effective biochar–iron interface for PMS activation and substantially enhanced SMX degradation, although strong acid dependence, matrix inhibition, catalyst deactivation, and iron release remain important limitations for application in complex aqueous systems.