Enhanced Antibiotic Removal from Water by Alkali-Modified Porous Biochar: Performance and Underlying Mechanisms

This study prepared alkali-modified porous biochar (KABC-1h) from straw via KOH impregnation and pyrolysis and systematically investigated its adsorption performance toward tetracycline (TC) in water. The optimal modification condition was determined as 2 mol/L KOH impregnation for 1 h followed by pyrolysis at 600 °C. Characterization results showed that alkali modification significantly increased the BET-specific surface area to 132.56 m2/g, enriched hierarchical micro-mesoporous structure, and introduced abundant oxygen-containing functional groups. The maximum adsorption capacity of KABC-1h for TC reached 21.60 mg/g, much higher than 14.11 mg/g of unmodified biochar (BC). Adsorption kinetics followed the pseudo-second-order model well, revealing that chemisorption dominated the rate-controlling step. Adsorption isotherms fitted the Freundlich model better, indicating multilayer heterogeneous adsorption on the biochar surface. Mechanism studies confirmed that TC adsorption was realized mainly through hydrogen bonding, π-π interaction, and surface complexation, whereas electrostatic interaction played a minor role. Moreover, KABC-1h exhibited stable adsorption efficiency over a wide pH range (4.5–11.5) and strong anti-interference ability against coexisting ions, with 59.0% removal efficiency retained after three regeneration cycles. This work demonstrates that KOH-modified porous biochar is a promising adsorbent for efficient TC removal from aqueous solution, providing a feasible strategy for agricultural waste valorization and antibiotic wastewater treatment.