Adsorption of perfluoroalkyl substances on polyamide microplastics: Effect of sorbent and influence of environmental factors

Microplastics (MPs) and perfluoroalkyl substances (PFASs) are two types of pollutants coexisting in the environment. Their co-exposure is a source of increasing concern. MPs present in the natural environment suppose an ideal surface for the sorption of hazardous contaminants. This study investigates the adsorption behaviour of six PFASs on polyamide (PA) MPs. Adsorption experiments under various internal (PA and PFASs dosage, PA particle size) and environmental (pH, ionic strength, dissolved organic matter) factors were carried out. Isotherm results (from 0.1 to 25 mg/L of PFASs) showed that the maximum adsorption capacity of the selected PFASs on the PA was as follows: perfluorooctanesulfonic acid (PFOS, 0.873 mg/g) > perfluorooctanoic acid (0.235 mg/g) > perfluoroheptanoic acid (0.231 mg/g) > perfluorohexanoic acid (0.201 mg/g) > perfluoropentanoic acid (0.192 mg/g) > perfluorobutanoic acid (0.188 mg/g) (pH 5.88, 0% salinity and 0% of dissolved organic matter). The PFOS has more tendency to be sorbed onto PA than perfluorocarboxilic acids. The MP characterization by scanning electron microscopy, X ray diffraction and Fourier transform infrared spectroscopy showed changes in the PA surface after adsorption assays. Pore filling, hydrophobic interactions and hydrogen bonds governed sorption process. The sorption capacity of PFASs was crucially affected by the PA size (from 19.6% to 99.9% for 3 mm and 50 μm particle size, respectively). The process was not significantly influenced by salinity while the dissolved organic matter exerted a negative effect (decrease from 100% to 26% for PFOS in presence of 25 mg/L of humic acid). Finally, adsorption rates of PFASs were quantified in real water matrices (influent and effluent wastewater, surface and tap water samples). The results revealed interactions between PA and PFASs and evidenced the role of PA as a vector to transport PFASs in the aquatic environment.