Degradation of Sulfamethoxazole in Soil by Peroxydisulfate Activated with Biochar-Supported Sulfidated Nanoscale Zero-Valent Iron: Effect of Soil Organic Matter

To improve the removal efficiency of sulfamethoxazole (SMX) in soil and to elucidate the role of soil organic matter (SOM) in peroxydisulfate (PDS)-based in situ chemical oxidation, a biochar-supported sulfidated nanoscale zero-valent iron (BC@S-nZVI)-activated PDS system was constructed in this study. The removal behavior and removal mechanisms of SMX were systematically compared between aqueous and soil systems, and the regulatory role of SOM was further clarified. Characterization results showed that BC@S-nZVI was successfully constructed with a composite interface consisting of a biochar support framework, an Fe0 core, and surface Fe-S structures. Under the optimized conditions, the BC@S-nZVI/PDS system achieved 92.9% removal of SMX within 120 min in the aqueous system, which was significantly higher than that of the nZVI/PDS and BC/PDS systems. In the soil system, the removal efficiency of SMX reached 74.4% within 120 min, and further increased to 91.3% after targeted removal of SOM. Results from radical quenching experiments, electron paramagnetic resonance (EPR) spectroscopy, and chemical probe tests demonstrated that •OH and SO4•− were the dominant reactive species driving SMX degradation in the aqueous system, while 1O2 played an auxiliary role. In contrast, in the soil system, SOM, acting as a natural reductive component, competitively consumed •OH and SO4•−, thereby markedly suppressing the radical oxidation pathway. Compared with these radical species, 1O2 exhibited stronger resistance to background interference and became the key reactive species responsible for the sustained transformation of SMX in soil. These findings demonstrate that the BC@S-nZVI/PDS system has considerable potential for the remediation of antibiotic-contaminated soils and reveal a mechanistic shift from radical-dominated to non-radical-dominated pathways under the interference of soil organic components.