UV-induced aging creates adsorption hotspots: Oxygen-containing functional groups on nanoplastics dictate the adsorption behavior of ciprofloxacin.

Nanoplastics (NPs), as emerging contaminants in aquatic environments, critically influence the environmental fate and ecological risks of antibiotics via adsorption. Continuous environmental aging markedly alters NP surface properties, including functional groups and surface charge, leading to increasingly complex interactions with antibiotics. In this study, batch adsorption experiments combined with density functional theory (DFT) calculations were employed to systematically elucidate how aging-induced oxygen-containing functional groups-including carbonyl (C=O), hydroxyl (-OH), phenolic hydroxyl (-OHm), and carboxyl (-COOH)-govern the adsorption of ciprofloxacin (CIP) onto polystyrene nanoplastics (PSNPs). The characterization results revealed that UV-induced aging enhanced adsorption performance by introducing additional adsorption sites. Adsorption kinetics followed a pseudo-second-order model (R2 > 0.973), while isotherms were well described by the Freundlich model (R2 > 0.986), confirming heterogeneous multilayer adsorption. Compared with pristine PSNPs (49.42 mg/g), the maximum adsorption capacity of aged PSNPs increased to 86.38 mg/g, accompanied by an increase in Freundlich constant kF from 25.94 to 35.06 L/mg. DFT calculations further showed that aging-induced functional groups stabilized PS-CIP complexes, with binding energies of -13.46 to -18.08 kJ/mol, higher than pristine PSNPs (-13.31 kJ/mol). Electrostatic potential mapping and IGMH analysis revealed that van der Waals interactions dominant in pristine PSNPs were weakened after aging, whereas hydrogen bonding and electrostatic contributions were strengthened. The synergistic interplay of these interactions explains the enhanced adsorption capacity of aged PSNPs. This study provides mechanistic insights into how aging-induced surface functionalization controls antibiotic adsorption on NPs and highlights its implications for environmental risk assessment of aged NPs.