Lanthanum loading influences tetracycline adsorption through coupled effects of active-site generation and pore...

Antibiotic residues in aquatic environments remain an important environmental concern because of their contribution to antimicrobial resistance. Although lanthanum-modified carbons have shown promising performance for tetracycline (TC) removal, the relationship between lanthanum incorporation, pore accessibility, interfacial adsorption chemistry, and adsorption performance remains poorly understood. Longan peel was converted into lanthanum-modified activated carbon through hydrothermal pretreatment, H₃PO₄ activation, and subsequent La₂O₃ incorporation. The resulting composite, prepared with a theoretical La₂O₃ loading of 9 wt%, exhibited a specific surface area of 264.82 m² g^-1 and achieved the highest TC removal (≈97%) among the investigated materials. Progressive La₂O₃ incorporation altered pore architecture and surface area, whereas adsorption performance did not increase proportionally, indicating that optimum adsorption depends on balancing structural accessibility with the development of lanthanum-containing adsorption domains. Equilibrium data were best fitted by the Sips and Toth models, with the Langmuir model estimating a maximum adsorption capacity of 91.4 mg g^-1. XPS confirmed successful incorporation of lanthanum as surface La-O species, while lanthanum incorporation shifted the pH_pzc from 5.5 to 7.6, consistent with modification of the interfacial charge environment. FTIR analysis, together with adsorption behavior, suggested contributions from hydrogen bonding, π-π electron donor-acceptor interactions, surface complexation, and electrostatic interactions during TC uptake. Overall, the combined structural and adsorption results suggest that adsorption performance reflects the interplay between structural accessibility and interfacial adsorption affinity. These observations support the interpretation that balancing pore architecture and surface chemistry is important for optimizing lanthanum-functionalized biomass-derived activated carbons for antibiotic removal.