Effects of ionic strength and cation type on the transport of perfluorooctanoic acid (PFOA) in unsaturated sand porous media

Current understanding of perfluorooctanoic acid (PFOA) transport in unsaturated porous media is still limited with significant variability in solution chemistry. Column experiments were conducted to systematically evaluate the impacts of ionic strength (1.5-30 mM) and cation type (Na+ and Ca2+) on PFOA transport in unsaturated quartz sand. The results showed that an increase in ionic strength (1.5-30 mM) led to greater PFOA retardation in unsaturated columns. Meanwhile, Ca2+ caused more PFOA retardation than Na+ at the same unsaturated conditions. These findings were supported by bubble column experiments, which indicated greater PFOA adsorption at the air-water interface with increasing ionic strength or in the presence of Ca2+ in comparison to Na+. Furthermore, the air-water interfacial (AWI) adsorption coefficients calculated from surface tension isotherms also increased with increasing ionic strength or in the presence of Ca2+ in comparison to Na+. These results clearly confirm that higher ionic strength or cation valence significantly promoted PFOA adsorption at the air-water interface, and thus caused greater PFOA retardation during transport in unsaturated porous media. This work points out the importance of considering solution ionic strength and cation type in assessing the transport behavior of PFOA in unsaturated porous media.