Adsorption performance and mechanisms of ciprofloxacin onto microplastics: effects of different textures and aging degrees

Abstract

Microplastics (MPs) have emerged as pervasive contaminants within aquatic systems. The ability of MPs to adsorb antibiotics may impact pollutant dynamics and thereby ecological risk, yet the impact of various MP types and their aging on interaction mechanisms is not well comprehended. This study systematically examined ciprofloxacin (CIP) adsorption on pristine, 30-day, and 60-day ultraviolet (UV) aged polypropylene (PP, PP-30, PP-60), polyvinyl chloride (PVC, PVC-30, PVC-60), and polyamide 6 (PA6, PA6-30, PA6-60) using batch experiments, material characterization, and density functional theory (DFT) calculations UV aging led to surface cracks, increased specific surface area, and the addition of oxygen-containing functional groups to the MPs, with these changes intensifying over time. The pseudo-second-order model, accounting for both film and intra-particle diffusion, effectively described the kinetics of CIP adsorption. The adsorption process was accurately characterized by both Langmuir and Freundlich isotherms. The maximum adsorption capacity increased from 0.993 (PP) to 1.705 (PP-30) and 2.138 mg g⁻¹ (PP-60), from 1.210 (PVC) to 1.861 (PVC-30) and 2.215 mg g⁻¹ (PVC-60), and from 3.115 (PA6) to (PA6-30) and 4.327 mg g⁻¹ (PA6-60).The improved adsorption of aged MPs is primarily due to oxygen-containing functional groups acting as active sites.Adsorption capacity followed the order of PA6 series > PVC series > PP series, emphasizing the key role of surface functional groups, consistent with DFT analysis.The primary interaction mechanisms included charge-assisted hydrogen bonding, electrostatic attraction, halogen bonding, and CH/π interactions. This work provides systematic insights into CIP adsorption onto pristine and aged MPs, offering a foundation for understanding MPs–antibiotic interactions and assessing environmental risks.