PFAS immobilization with soil amendments - How immobilized is immobilized?

In this study, the efficacy of RemBind®300 (applied at 5% w/w) to immobilize perfluorooctane sulfonic acid (PFOS) in AFFF-impacted soil (750-7455 μg PFOS kg^-1; n = 8) was assessed using in vitro bioaccessibility (SBRC-I) and in vivo relative bioavailability (Sprague-Dawley rat bioassay) approaches. In unamended soil, PFOS bioaccessibility and relative bioavailability ranged from 29.5 to 86.5% (mean = 63.6%; median = 63.5%) and 44.8-107% (mean = 83.3%; median = 88.3%) respectively, reflective of the influence of soil physicochemical properties (total organic carbon and reactive Al and Fe content) on PFOS desorption under gastrointestinal conditions. However, following the addition of RemBind®300, PFOS bioaccessibility (range = 2.4-11.6%; mean = 5.6%; median = 5.4%) and PFOS relative bioavailability (range = 5.1-24.5%; mean = 13.0%; median = 14.8%) were significantly reduced (p < 0.05) due to RemBind®300 enhanced hydrophobic and electrostatic interactions which decreased PFOS desorption. When PFOS immobilization efficacy was calculated and compared to previously conducted leaching (99.1-100%) and earthworm (92.2-99.0%) assays, immobilization efficacy was significantly lower (p < 0.05) for bioaccessibility (80.0-95.4%) and relative bioavailability (69.0-95.0%) calculations attributable to factors such as assay complexity, presence/absence of enzymes/microbiota, pH, gut transit time and solid-to-solution ratio which influence PFOS desorption kinetics and thermodynamics.