Adsorption properties and influencing factors of antibiotics on microplastics under simulated gastric fluid environment.

Microplastics (MPs) and antibiotics (ATs) are emerging pollutants. Recent studies have confirmed that MPs can act as carriers for toxic pollutants, releasing and accumulating these substances within organisms upon ingestion. Additionally, MPs undergo aging processes within organisms, affecting their ability to adsorb pollutants. This study investigated the adsorption behavior of three aged MPs including polypropylene (PP), polylactic acid (PLA) and polystyrene (PS) for typical ATs in a simulated human gastric fluid environment. The results indicated that the adsorption capacity of MPs for ATs decreases slightly with increasing pH and significantly with higher ionic strength. Smaller particle sizes exhibited higher adsorption capacities. MPs aged in simulated gastric fluid exhibited significantly greater adsorption capacities than their original counterparts. Among the four antibiotics tested, amoxicillin (AMX), ciprofloxacin (CIP), levofloxacin (LEV), and sulfadiazine (SD), the three microplastics exhibited the highest adsorption capacity for AMX. The adsorption capacities were determined as follows: PP-AMX (1.65 mg g), PLA-AMX (1.47 mg g), and PS-AMX (1.48 mg g). Furthermore, the adsorption of ATs on aged MPs followed a pseudo-second-order model, and isotherm analysis aligned with the Freundlich model, suggesting a non-uniform, multilayer surface adsorption process. These findings deepen the understanding of interactions between aged MPs and ATs in the human gastric fluid environment and provide crucial information for ecological risk assessments of MPs.