Synergistic adsorption mechanisms of self-N-doped biochar for levofloxacin elimination: integrated experimental and DFT studies

The efficient removal of fluoroquinolone antibiotics from water is critical for water pollution control. In this study, a novel self-nitrogen doped porous biochar (KBC) was synthesized using waste peanut meal as a precursor, combined with high-temperature pyrolysis (900 °C) and an activating agent (C 2 H 2 K 2 O 5 ) at an optimal mass ratio of 1:2. The KBC exhibited excellent adsorption capacity for levofloxacin (LEV) with the maximum capacity of 717.87 mg g −1 (at 0.15 g L −1 ). The adsorbents demonstrated rapid adsorption kinetics, achieving 83.36 % of total adsorption within 2 h, and exhibited pH-dependent adsorption behavior, with optimal performance at pH = 9. Thermodynamic studies indicated that the adsorption process was spontaneous and endothermic. The adsorption isotherm followed the Langmuir model, suggesting monolayer chemisorption as the dominant mechanism. Additionally, the pseudo-second-order kinetic model further confirmed the involvement of chemical interactions. Material characterization and density functional theory (DFT) simulations were employed to reveal the underlying adsorption mechanisms: (1) micropore filling-dominated physical adsorption (BET surface area = 2119.96 m 2  g −1 ); (2) hydrogen bonding and π-π electron donor-acceptor interactions between oxygen-containing functional groups on the KBC surface and LEV molecules; (3) DFT calculations revealed that the adsorption energy of anionic LEV (−321.72 kJ mol −1 ) was significantly higher than that of neutral and cationic forms, consistent with the experimental pH-dependent trend; and (4) DFT-optimized configurations demonstrated strong interactions between LEV and the biochar with a bond distance of 2.0 Å, providing atomic-level evidence for adsorption stability. These findings offer practical insights into the valorization of agricultural waste and advanced strategies for water remediation.