Polypropylene nanoplastics as PFAS carriers: A computational study of the adsorption mechanism

Polypropylene (PP) is a major constituent of nanoplastics (NPs) found worldwide in aquatic environments, where it promotes the co-transport of contaminants. Of particular concern is the co-transport of perfluoroalkyl substances (PFAS), potentially increasing the uptake and bioaccumulation of PFAS in organisms during simultaneous exposure. Since the adsorption mechanism of PFAS molecules on NPs is still only partially understood, we have carried out a thorough systematic investigation of how a range of PFAS interact with PP nanoplastics. To this end, we developed a computational procedure which combines molecular mechanics, semiempirical methods and density functional theory calculations. We were able to describe quantitatively the adsorption process, revealing similarities and differences in the adsorption behavior as a function of the PFAS length, branching and of the nature of the PFAS polar head. Our findings suggest that the nanoplastic possess a certain degree of local flexibility which allows it to effectively adsorb all the investigated compounds, by modifying its form to maximize the interactions with PFAS. The adsorption mechanism is mainly driven by dispersion forces between the PFAS perfluorinated chain and the nanoplastic polymeric chain, with minor electrostatic contributions. These findings represent a significant step forward in the rationalization of PFAS adsorption behavior, which is essential not only to clarify their environmental fate but also to help develop strategies for PFAS removal from contaminated water sources.