Aging and aggregation enhance antibiotics persistence and sequestration on polyethylene microplastics.

Microplastics (MPs) are increasingly recognised as chemically active interfaces that influence the environmental fate, transport, and persistence of coexisting contaminants. However, the molecular mechanisms governing antibiotic adsorption on environmentally transformed microplastics remain poorly understood. In this study, molecular dynamics (MD) simulations were employed to investigate the adsorption behaviour of the third-generation cephalosporin antibiotic cefixime on polyethylene (PE) microplastics, explicitly considering the effect of polymer aggregation and environmental aging. Pristine single-chain PE exhibited weak and reversible cefixime adsorption dominated by van der Waals interactions, resulting in high molecular mobility and frequent desorption events. In contrast, polymer aggregation substantially enhanced antibiotic retention by creating confined inter-chain domains that amplify cooperative dispersion interactions and reduced solvent competition at the polymer-water surface. Environmental aging further strengthened adsorption through the introduction of oxygen-containing functional groups capable of electrostatic interactions and hydrogen bonding with polar moieties of cefixime. These structural and chemical modifications collectively reduced molecular fluctuations, promoted the formation of stable binding sites, suppressed molecular diffusion, and significantly increased interaction energies. The findings demonstrate that environmental transformation of polyethylene microplastics enhance molecular adsorption of cefixime on polyethylene, with potential implications for contaminant transport, persistence, and ecological exposure in aquatic systems. The results provide mechanistic insight into the role of polymer ageing and aggregation in antibiotic adsorption and contribute to the understanding of antibiotic microplastic interactions.