Unveiling the adsorption mechanism of perfluorooctane sulfonate onto polypropylene nanoplastics: A combined theoretical and experimental investigation

Polypropylene (PP) is a key component of nanoplastics detected globally in water, which can carry pollutants through co-transport. In this regard, the co-transport of perfluoroalkyl substances (PFAS) by nanoplastics (NPs) raises significant concern, as NPs can act as vectors that enhance PFAS uptake and bioaccumulation in organisms during co-exposure. In this context, research has shown interactions between NPs and PFAS, but the adsorption mechanism remains still unclear. In this work, a powerful synergic approach combining several computational and experimental techniques has been used to unveil the adsorption mechanism of perfluorooctanesulfonate (PFOS), which is one of the most widespread contaminants of emerging concerns (CECs) on PP nanoparticles. According to our DFT results, PFOS adsorbs onto the outer and inner surface of the nanoparticle, with a maximum adsorption energy of ≈ 18 kcal/mol and an adsorption mechanism mainly governed by dispersion forces between the two fragments. Batch experiments have confirmed that PFOS rapidly adsorbs on PP nanoparticle, showing that pH can reduce the adsorption capacity thus affecting the co-transport. Moreover, the dipole moment of the PFOS-nanoparticle complex has been found to be significantly larger as compared to the bare nanoparticle, resulting in a more pronounced transport in aqueous environment and making the PFOS-PP nanoparticle complex much more dangerous than the bare PP nanoparticle. Altogether, our results allowed us to disentangle the adsorption mechanism of PFAS on PP nanoparticles, which is a fundamental step to understand the co-occurrence of such dangerous pollutants in environmental matrices, as well as to obtain new information for toxicity and risk-models development.