Mechanistic Insights into Competitive Adsorption of Antibiotics on PET, PP, and HDPE Microplastics

The co-occurrence of microplastics and antibiotics in aquatic environments has raised increasing concern because their interactions remain poorly understood. This study investigated the interactions between two co-existing aquatic pollutants, antibiotics and microplastics, by evaluating the adsorption of nine commonly detected antibiotics onto PET, PP, and HDPE microplastics. The microplastics were considered environmentally contaminants rather than adsorbents intended for water treatment. Adsorption kinetics, equilibrium isotherms, and multi-component competitive models were employed, while the effects of the microplastic size, dosage, and water matrix composition were also studied. The adsorption kinetics for most antibiotics were best described by the pseudo-second order model (R2 > 0.90), with the modelling suggesting contributions from film diffusion. The Freundlich and multi-component Sheindorf–Rebuhn–Sheintuch (SRS) models represented the equilibrium adsorption, indicating heterogeneous and competitive adsorption behaviour. PET exhibited the highest adsorption capacity, reaching 0.80 mg/g for doxycycline and 0.75 mg/g for oxytetracycline at 2000 mg/L microplastic. The competitive adsorption suggested that aromatic and moderately hydrophobic antibiotics showed greater adsorption, while highly polar antibiotics exhibited weaker adsorption and greater displacement. Potential interactions included electrostatic interactions, hydrogen bonding, and π–π interactions. These findings provide insights into antibiotic–microplastic interactions and emphasise the roles of the polymer type, molecular structure, and environmental conditions in influencing antibiotic fate and transport in aquatic environments.