Overriding Surface Area Limitation: Mesopore-Driven Norfloxacin Adsorption on High-Temperature P-Doped Biochar

Antibiotic wastewater pollution is a serious environmental problem. This study prepared three types of P-doped biochar (P-CB, P-WB, and P-BB) from corn straw (CB), wheat straw (WB), and bamboo (BB) using phosphoric acid activation at 800 °C. Characterization by SEM, XRD, FTIR, and N2 adsorption confirmed that high-temperature H3PO4 activation drastically reduced the specific surface area but transformed the original microporous framework into a mesopore-dominated structure (average pore diameter 12–15 nm). Adsorption experiments showed that all P-doped biochars enhanced norfloxacin (NOR) removal, with P-CB performing best: 87.14% removal within 30 min at C0 = 50 mg·L−1 and a maximum adsorption capacity of 305.5 mg·g−1 at higher concentration. Kinetics followed a pseudo-second-order model (R2 > 0.9982), and isotherms fitted the Freundlich model (R2 = 0.9764–0.9855). Thermodynamics indicated a spontaneous and exothermic process, suggesting that the macroscopic driving force is dominated by physisorption. Mechanism analysis revealed that the synergistic effect of mesopore-dominated diffusion, graphitic π-π sites (from enhanced aromatization at 800 °C), and P-anchored chemisorption overrides the loss of specific surface area, enabling rapid and high-capacity adsorption. Optimal adsorption occurred at pH 5–9, while coexisting anions had a mild inhibitory effect. These findings demonstrate that high-temperature P-doping offers a strategy to rebalance pore architecture and surface functionality rather than simply maximizing specific surface area for efficient antibiotic removal from wastewater.