Interactions between Nanoplastics and Antibiotics: Implications for Nanoplastics Aggregation in Aquatic Environments

Nanoplastics and antibiotics frequently co-occur in aquatic environments, and their interactions could alter nanoplastics' surface properties, affecting nanoplastics aggregation, fate, and ecotoxicity. However, the mechanisms driving antibiotics-induced nanoplastics aggregation under environmentally relevant conditions remain unclear. This study investigated the effects of ciprofloxacin (CIP) and sulfamethoxazole (SMX) on the aggregation of four environmentally relevant nanoplastics (pristine and aged polystyrene, polyethylene, and polypropylene). At pH 5.0, both CIP and SMX significantly promoted nanoplastics aggregation, with CIP being more potent. CIP enhanced nanoplastics aggregation through charge shielding driven by electrostatic attraction, hydrogen bonding (HB), and charge-assisted HB (CAHB), whereas SMX promoted aggregation solely through molecular bridging involving HB and CAHB. At pH 7.0, only CIP facilitated aggregation, while neither antibiotic induced aggregation at pH 9.0. Aged polystyrene aggregated more readily than pristine polystyrene due to increased surface functional groups. Polyethylene and polypropylene showed weaker aggregation due to fewer surface functional groups. High organic matter (OM) levels (1.65 mg/L TOC) inhibited antibiotics-induced aggregation, whereas low OM levels (16.5 μg/L TOC) were more conducive. These findings highlight that antibiotic characteristics, pH, OM levels, plastic types, and environmental aging collectively influence nanoplastics aggregation, and improve the understanding of the fate and risk of nanoplastics in natural waters.