Model-Based Assessment of Adsorption and Diffusion of PFASs through Concrete as Continuous Source of PFASs to the Environment

Per- and polyfluoroalkyl substances (PFASs) are persistent and pervasive contaminants of concern that are frequently introduced into the environment through aqueous film-forming foams (AFFF) on concrete pads. Previously published laboratory results of PFASs diffusion into Portland cement concrete are modeled using an analytical solution of the 1-D diffusion equation under water-saturated conditions. The diffusion equation was fitted to empirical data to determine the solid phase partitioning coefficient and the observed coefficient of diffusion for weighted and unweighted fits. The solid phase partitioning coefficient is used to calculate the tortuosity of the material from the observed coefficient of diffusion as a benchmark to validate this approach. Results show that while the majority of PFAS mass is well-fitted to a standard diffusion profile, a smaller fraction of mass may have accelerated diffusion into concrete from non-Fickian transport processes. Solid phase adsorption to concrete was small to negligible for anionic mixtures of PFASs but significant for AFFF releases, suggesting that PFASs in AFFF releases may be more strongly retained on concrete surfaces due to additional chemical interactions. Synthetic weathering of the concrete surface also produced higher rates of diffusion than in finished concrete. Back diffusion through concrete was identified as a key source of resupply to surface runoff.