Adsorption of Macrolide Antibiotics by Aged Microplastics of Different Sizes: Mechanisms and Effects

Microplastics (MPs) and antibiotics are widely detected in water bodies. However, the adsorption behavior and mechanism of different particle size polystyrene (PS) MPs on macrolide antibiotics under natural aging remain to be elucidated. In this study, potassium persulfate (K₂S₂O₈) was used to simulate the natural aging process of PS MPs. The adsorption behavior and mechanism of different size PS (80 and 400 μm) toward azithromycin (AZI), clarithromycin (CLA), and erythromycin (ERY) were investigated. Results of SEM showed that the surface roughness of aged PS MPs increased with the appearance of cracks, pits, and pores. XPS and FTIR analyses showed enhanced C=O functional groups in the aging process. The adsorption isotherm models revealed that the aging processes enhanced the AZI, CLA, and ERY adsorption tendency, as evidenced by the highest adsorption capacity for aged-80 μm (645, 665, 184 mg/kg) > original-80 μm (412, 420, 120 mg/kg), and aged-400 μm (280, 330, 110 mg/kg) > original-400 μm (197, 308, 100 mg/kg). Kinetic model fitting revealed that the adsorption process occurred in three stages: rapid, slow, and saturation. Adsorption kinetic curves for original and aged PS MPs conformed to the pseudo-second-order kinetic model. In contrast, the adsorption isotherm data fit the Langmuir model, indicating that the process primarily involved uniform monolayer chemical adsorption. Our findings provide insights into the substantial changes in the interactions between PS and macrolide antibiotics with aging processes.