Revealing the Role of Biochar-Derived Dissolved Organic Matter during the Degradation of Sulfamethoxazole by Zero-Valent Iron-Activated Persulfate

Biochar (BC) is widely used in persulfate (PS)-based advanced oxidation processes (AOPs), yet the environmental role of biochar-derived dissolved organic matter (BDOM) remains poorly understood. Here, we revealed that BDOM released from BC pyrolyzed at 300–700 °C regulated the oxidation degradation of sulfamethoxazole (SMX) in a ball-milled N-doped BC-ZVI (NBC-ZVIbm)/PS system. Results showed that higher pyrolysis temperatures produced a lower-molecular-weight BDOM with more oxidized species. Although pyridinic and graphitic N in NBC-ZVIbm enhanced PS activation and electron transfer, BDOM significantly suppressed SMX degradation, with stronger inhibition observed at higher pyrolysis temperatures. Quenching experiments combined with Fourier infrared cyclotron resonance mass spectrometry (FT-ICR-MS) analyses identified aliphatic polyphenols and lignans as the dominant inhibitory components. Key molecular features including O/C and H/C ratios and the abundance of −OH groups and unsaturated bonds were strongly correlated with reduced SMX removal. Further mechanistic analysis revealed that BDOM hindered degradation through multiple pathways: consuming oxidants, scavenging reactive species, chelating iron ions, and altering transformation routes. This work highlights the underappreciated inhibitory role of BDOM and suggests the avoidance of excessive pyrolysis temperatures to improve the practical efficacy of BC-mediated AOPs for environmental remediation.