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

Abstract

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 (¹O₂). 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 ¹O₂ formation for faster SMX degradation. The promotion of nonradical ¹O₂ 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.

Keywords

Sulfamethoxazole degradation / Peroxymonosulfate activation / Defects / N and B co-doped biochar

Highlight

•	A low N:B ratio increases SSA, defect sites, and graphitic N, markedly improving SMX removal.
•	Defects and graphitic N boost 1O2 generation, and DFT confirms 1O2 route.
•	Optimized ratio shifts SMX degradation from adsorption- to catalysis-dominated.

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Funding

the National Key Research and Development Program of China(2023YFC3207701)

the Agricultural Science and Technology Innovation Program of China(CAAS-CFSGLCA-IEDA-202302)

Central Public-interest Scientific Institution Basal Research Fund(No. Y2025YC30)

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