DFT-Validated adsorption mechanisms of metronidazole on CO₂-activated almond shell biochar for antibiotic wastewater treatment

This study addresses the challenge of antibiotic wastewater pollution by developing highly efficient adsorbents derived from CO₂-activated and polymer-modified almond shell biochar—namely PRC@HB, PHC@HB, and PIC@HB—for the removal of metronidazole (MNZ) from aqueous solutions. These composite materials were synthesized by incorporating phenolic resin, potassium humate, and polyimide, respectively. The maximum MNZ removal efficiencies achieved were 97.44 % for PRC@HB, 92.42 % for PHC@HB, and 28.24 % for PIC@HB. Adsorption kinetics adhered to the pseudo-second-order model, suggesting that chemical adsorption is the dominant mechanism. Thermodynamic analysis indicated that the adsorption process for PRC@HB is exothermic and spontaneous (ΔH = −10.79 kJ/mol), whereas for PHC@HB and PIC@HB, it is endothermic and spontaneous (ΔH = 48.33 kJ/mol and 44.95 kJ/mol, respectively). Characterization via scanning electron microscopy (SEM), Fourier-transform infrared spectroscopy (FTIR), and Raman spectroscopy revealed that the modifications significantly enhanced pore structure and introduced surface functional groups, such as hydroxyl (-OH) and carbonyl (C O), thereby increasing the availability of adsorption sites. Density functional theory (DFT) calculations elucidated that PRC@HB and PHC@HB achieve superior adsorption through π-π interactions and hydrogen bonding, while PIC@HB exhibits reduced efficiency due to lower hydrophilicity and weaker electron transfer. By integrating experimental and theoretical approaches, this study clarifies the adsorption mechanisms of modified biochar for MNZ removal, providing a scientific foundation for designing efficient, cost-effective materials for antibiotic wastewater treatment. ● Development of High-Performance Biochar Adsorbents Almond shell biochars (PRC@HB, PHC@HB, PIC@HB) were synthesized via CO₂ activation and modification with phenolic resin, potassium humate, and polyimide for metronidazole (MNZ) removal. ● Superior Adsorption Efficiency and Mechanisms Maximum MNZ removal reached 97.44% (PRC@HB), 92.42% (PHC@HB), and 28.24% (PIC@HB). Kinetics followed a pseudo-second-order model, indicating chemical adsorption. ● Thermodynamic Insights PRC@HB adsorption was exothermic and spontaneous (ΔH = −10.79 kJ/mol), while PHC@HB and PIC@HB showed endothermic yet spontaneous adsorption (ΔH = 48.33 and 44.95 kJ/mol). ● Enhanced Structure and Functional Groups SEM, FTIR, and Raman analyses confirmed improved pore structures. Hydroxyl (-OH) and carbonyl (C=O) groups played key roles in boosting adsorption. ● DFT Validation of Mechanisms DFT analysis revealed dominant π-π interactions and hydrogen bonding for PRC@HB and PHC@HB. PIC@HB’s weaker efficiency was linked to low hydrophilicity and poor electron transfer.