Tailored co-doped biochar by varying the N:B ratio as peroxymonosulfate activator for sulfamethoxazole degradation

Biochar doped with heteroatoms showed superior performance in activating peroxymonosulfate (PMS) for contaminant degradation. The incorporation of nitrogen (N) enhanced surface functional groups and defects, while boron (B) improved pore structure; however, uncertainties remain regarding the identification of key active sites and reaction mechanisms under different N:B ratios. To address this issue, a series of co-doped biochar derived from waste sawdust was investigated in this work. Decreasing the N:B ratio (from 4 to 0.25) led to a significant enhancement of sulfamethoxazole (SMX) degradation (from 0.20 to 0.47 min −1 ) in the PMS system. Specifically, the contribution of catalysis was limited compared to that of adsorption at higher N:B ratios (> 1). Linear fitting analysis indicated that catalytic efficiency was primarily regulated by specific surface area. Additionally, C–OH groups acted as electron donors for generating radicals, while C = O groups were active sites for singlet oxygen ( 1 O 2 ). In contrast, the catalytic rate increased substantially at lower N:B ratios (0.25–1), suggesting that catalytic sites prevalent under boron-rich conditions became the dominant active centers. Correlation analysis revealed that decreasing N:B ratio (from 2 to 0.25) enhanced SMX degradation by elevating defect density (I D /I G from 1.40 to 1.46) and graphitic N content, thereby enhancing electron transfer and 1 O 2 formation for faster SMX degradation. The promotion of nonradical 1 O 2 pathway was further validated by theoretical calculations. Collectively, this study provides new insights into the mechanisms of PMS activation by biochar with varying N:B ratios, underscoring the potential of tailored biochar for efficient pollutant degradation.