Colloid-facilitated release of PFAS from polluted-soil monoliths

Firefighting Aqueous Film Forming Foams contain per- and polyfluoroalkyl substances (PFAS). Because PFAS are persistent, firefighting training sites constitute a source of soil and groundwater contamination. Colloidal soil particles are known to facilitate the mobility of contaminants that have a strong affinity for soils. Indeed, colloid-facilitated transport of PFAS has been brought up repeatedly to discuss experimental observations regarding PFAS mobility. Still, it has only been observed experimentally for two PFAS molecules, using model experimental situations: repacked soil columns spiked with PFAS. We investigated for the first time the determinants and extent of colloid-facilitated transport of PFAS from AFFF-impacted undisturbed-soil monoliths during simulated rainfalls. These experiments showed that: (i) colloids contributed to the mobility-up to 70% of their concentration in leachates-of 12 PFAS having a perfluorinated carbon chain length n_c ≥ 6, out of the 20 PFAS present in leachates; (ii) PFAS content in colloids was up to 25 times higher than in bulk soil, this enrichment stemmed from the adsorption on colloids of PFAS present in the soil solution; (iii) colloid-facilitated transport was exacerbated during transient flow regimes, which was probably related to the adsorption of PFAS and colloids to the moving air-water-interfaces. In addition, diffusion was the mechanism limiting the mobility of eight other PFAS with n_c ≤ 7 and hydrodynamic conditions, linked to the soil structure, strongly affected the mobility of PFAS. The importance of colloidal particles as a carrier phase of PFAS calls for a renewed conceptual model of PFAS fate in soils.