Degradation of tetracycline by nitrogen-doped biochar as a peroxydisulfate activator: Nitrogen doping pattern and non-radical mechanism

Recently, nitrogen-doped biochar (NBC) has attracted a great deal of attention in the field of advanced oxidation processes (AOPs) for the treatment of various pollutants. However, the effect of nitrogen doping method on the catalyst efficiency of NBC has not been fully understood. In this study, NBC was prepared using two different nitrogen doping methods: carbonization of urea mixed with maize straw (NBC1) and carbonization of urea mixed with pristine biochar obtained from maize straw (NBC2). Catalytic degradation of tetracycline (TC) in the presence of peroxydisulfate (PDS) and biochar catalyst was examined, and process factors and reaction mechanism were fully explored. Characterizations revealed that NBC1 had higher nitrogen content (6.86 %) and carbon defects compared to NBC2. Under the optimum conditions of NBC1 0.4 g L⁻¹, PDS 3 mM, TC 15 mg L⁻¹, and solution pH 4.6, the TC degradation efficiency reached over 86.55 % within 120 min, which was higher than that of NBC2 (57.10 %). Moreover, the catalytic system showed resistance to anions and dissolved organic matter, and the degradation efficiency of TC by NBC1 remained 82.47 % after secondary carbonization. Radical quenching and electron spin resonance spectroscopy verified the co-existence of ·OH, SO₄·⁻, O₂·⁻, and ¹O₂ in the system, and ¹O₂ made significant contribution to TC degradation. More importantly, quantitative structure-activity relationships (QSAR) and density functional theory (DFT) calculations displayed that graphitic N, as the primary active site, played an important role in PDS activation for TC degradation. This work provides evidence for the preparation of efficient nitrogen-doped biochar, elucidation of the catalytic mechanism, and its feasibility in the field of wastewater treatment.