Boosting Sulfamethoxazole Degradation via Peroxymonosulfate Activation: Critical Comparison of Intrinsic and External Nitrogen-Doped Biochar Modulated by Pore Former

Biomass containing inherent nitrogen (N) is emerging as a promising and sustainable feedstock to produce N-doped biochar (BC) for peroxymonosulfate (PMS) activation and sulfamethoxazole (SMX) removal. However, it remains unclear whether adding external N is beneficial when intrinsic N is already sufficient. Here, protein-rich soybean meal was used as an N-containing precursor to systematically compare the distinct roles of intrinsic and external N sources. The results demonstrate that external N (urea) doping alone provides only marginal enhancement (4%). In contrast, co-introducing urea with a pore former (potassium acetate) leads to a 39.6-fold increase in reaction rate constant and an 80% improvement in SMX removal, highlighting a strong synergy between external N doping and pore development. Collectively, these results indicate that pore structure is the key physicochemical prerequisite for external N to translate into substantial catalytic enhancement in N-rich systems. In situ ATR-FTIR, XPS, EPR, and DFT analyses reveal distinct PMS activation mechanisms for external and intrinsic N doping. While singlet oxygen ( 1 O 2 ) predominates in both systems, the intrinsic N-doped BC/PMS system relies on superoxide radical (O 2 • ⁻) as a critical precursor, whereas the external N-doped BC/PMS system generates 1 O 2 with less involvement of O 2 • ⁻. This difference stems from variations in surface N functionalities and C=O. Optimization of the urea-to-biomass ratio yields a BC catalyst with high stability across a broad pH range (3-9) and strong resistance to coexisting ions (up to 20 mM). • Synergy of external N doping with pore structure engineering was demonstrated. • External N-doped biochar (BC) showed a 39-fold higher k obs than intrinsic N-doped BC. • Distinct PMS activation mechanisms for external and intrinsic N-doped BC were revealed. • Pyridinic N, graphitic N and C=O were identified as active sites by DFT calculations. • Possible degradation pathway of SMX was proposed by q-TOF MS analysis.