Nonideal Transport and Extended Elution Tailing of PFOS in Soil and Aquifer Sediment

The objective of this research was to examine the influence of nonideal sorption/desorption on the transport of PFAS in porous media, with a specific focus on characterizing and quantifying potential extended, mass-transfer-limited elution behavior. PFOS was used as a representative PFAS, and miscible-displacement experiments were conducted with two soils and three aquifer sediments comprising contrasting geochemical properties. The influence of nonlinear, rate-limited, hysteretic, and irreversible sorption/desorption on transport was investigated through experiments and model simulations. The breakthrough curves measured for PFOS transport were asymmetrical and exhibited extensive elution tailing, indicating that sorption/desorption was significantly nonideal. The widely used two-domain sorption-kinetics (TDSK) model could not fully simulate the observed transport behavior, whereas a multi-rate model employing a continuous distribution of sorption domains (CDMR) was successful. The overall results indicated that sorption/desorption was significantly rate-limited, and that nonlinear, hysteretic, and irreversible sorption/desorption had minimal impact on PFOS transport. Comparison of PFOS transport data to data reported for two hydrophobic organic contaminants (HOCs) showed that the HOCs exhibited much more extensive elution tailing, likely reflecting differences in sorption/desorption mechanisms. The projected influence of rate-limited sorption/desorption on PFOS transport at the field scale was investigated through simulation.