Mechanistic Insights into PFAS Adsorption on Microplastics: Effects of Contaminant Properties and Water Chemistry

Microplastics (MPs) and perfluoroalkyl substances (PFAS) frequently coexist in aquatic environments, where their co-occurrence has raised concern due to combined exposure risks. MPs provide a large surface area and reactive interface for contaminant adsorption, making them potential carriers for PFAS transport. This study investigates the adsorption behavior of two widely detected PFAS, perfluorooctane sulfonic acid (PFOS) and perfluorooctanoic acid (PFOA), onto five common MPs. Batch experiments were conducted to evaluate the influence of MPs, PFAS molecular properties, and key environmental factors including natural organic matter (NOM), ionic strength, and pH. The adsorption capacity followed the order PFOS > PFOA for all polymers, with maximum adsorption ranging from 35.54 to 43.34 μg⋅g^-1 for PFOA and 38.16 to 49.92 μg⋅g^-1 for PFOS and the overall affinity increased in the order PVC < PA < PS < PP < PE. Sorption kinetics were best described by the pseudo-second order model, while equilibrium data were more consistent with the Langmuir model, suggesting predominantly monolayer-type adsorption behavior dominated by hydrophobic and electrostatic interactions. Environmental conditions strongly influenced PFAS adsorption, with increasing pH and NOM reducing adsorption by more than 65 percent and up to 76 percent respectively, while higher ionic strength increased adsorption. Fourier transform infrared spectroscopy and principal component analysis revealed changes on MP surfaces after adsorption, while molecular orbital analysis confirmed differences in reactivity between PFOS and PFOA. Overall, these findings demonstrate that MPs can act as carriers for PFAS, especially in saline waters, highlighting their importance in PFAS fate and exposure pathways.