Porosity optimization and Fe₂O₃ functionalization synergistically enhance nitrate adsorption on bamboo biochar

The escalating issue of nitrate-nitrogen (NO₃^--N) pollution in water bodies demands advanced remediation solutions. This study developed engineered iron-modified biochar (FeB) via a sequential acid-washing and FeCl₃-impregnation strategy to enhance NO₃^--N adsorption. Comprehensive characterization (SEM, FTIR, XRD, and XPS) confirmed that the modification successfully optimized the pore structure, enriched oxygen-containing functional groups, and loaded active Fe₂O₃ particles. FeB exhibited a reversed positive surface charge (zeta potential =  + 7.58 mV), favoring anion capture. Adsorption isotherms were well-described by the Langmuir model, revealing a maximum adsorption capacity of 6829.74 mg/kg, which is more than three times that of pristine biochar. Kinetic studies indicated that the process followed the pseudo-second-order model, suggesting a chemisorption-dominated mechanism. Detailed mechanistic analysis demonstrated that nitrate removal was synergistically governed by electrostatic attraction, ligand exchange (forming inner-sphere complexes), and physical pore filling. Crucially, FeB achieved high adsorption efficiency under neutral pH (7.0) with a low dosage (1.0 g/L) and exhibited excellent structural stability after adsorption. This work provides a robust, scalable adsorbent for mitigating aquatic NO₃^--N contamination, showing significant potential for practical applications in wastewater treatment and constructed wetlands.