Experimental and theoretical insights into oxytetracycline and sulfamethoxazole adsorption onto four biomass-derived nanosized activated carbons in aquatic environments

Nanosized activated carbons (NACs) have emerged as promising adsorbents for antibiotic removal due to their excellent colloidal stability and low ecotoxicity. This study investigated four biomass-derived NACs for adsorption of oxytetracycline (OTC) and sulfamethoxazole (SMX). Both kinetic and isotherm analysis revealed higher affinity for OTC ( q max = 113.10–196.00 mg/g) than SMX ( q max = 60.66–124.90 mg/g), with adsorption capacity ranking NAC2 > NAC1 > NAC3 > NAC4. Adsorption was favored under mildly acidic conditions (pH 3–4) and at lower temperatures (20 ℃). Increased concentrations of NaCl and CaCl 2 enhanced antibiotic uptake through salting-out and complexation effects, respectively. Humic and fulvic acids significantly inhibited OTC adsorption due to steric hindrance but had minimal impact on SMX. Pore filling, hydrogen bonding, and π–π electron donor-acceptor interactions all contributed to adsorption. Density functional theory (DFT) calculations revealed that OTC had a lower HOMO-LUMO (highest occupied and lowest unoccupied molecular orbitals) energy gap (1.273 eV) and more negative binding energies (-63.5 to −182.2 kJ/mol) than SMX (2.013 eV; −47.1 to −158.2 kJ/mol), confirming its stronger chemical reactivity and affinity for NAC surfaces. The findings provide a comprehensive understanding of NAC-antibiotic interactions and demonstrate the potential of NACs for effective antibiotic remediation in aquatic environments.