19F magnetic resonance imaging-informed fate models of PFAS in porous media

Per- and polyfluoroalkyl substances (PFAS) are persistent contaminants. Predicting their fate in natural or engineered porous media, using accurate models, is essential for effective remediation and contamination management strategies. The mechanisms of transport and retention included in such models, and the associated parameters, are mostly inferred from PFAS concentration vs. time breakthrough curves (BTCs) measured during transport experiments. Still, the interpretation of BTCs may not be unique as they result from a succession of mechanisms taking place inside the porous media. We addressed this issue using ¹⁹F magnetic resonance imaging (MRI) to monitor the transport of perfluorobutanoic acid (PFBA) inside a sand-packed column. The experimental BTC was slightly asymmetric, suggesting that some PFBA may have been adsorbed onto the sand. Hence, a transport model based on the hypothesis that PFBA behaved as a non-sorbing tracer slightly overestimated the concentrations in two regions of the BTC. Surprisingly, the same model matched well the MRI profiles, pointing out that the BTC asymmetry stemmed from an imperfect column exit. Although ¹⁹F MRI requires PFAS concentration above those found in environmental samples, this study showed that the combination of this technique and modeling constitutes a powerful tool to determine the mechanisms involved in PFAS transport in natural or engineered porous media andselect appropriate fate models.