Importance of Al/Fe oxyhydroxide coating and ionic strength in perfluorooctanoic acid (PFOA) transport in saturated porous media.

Understanding subsurface transport of per- and polyfluoroalkyl substances (PFASs) is of critical importance for the benign use and risk management of PFASs. As one of the most commonly found PFASs, perfluorooctanoic acid (PFOA) is used as a representative PFAS and water-saturated column experiments were conducted to investigate the effect of Al/Fe oxyhydroxide coating and ionic strength on its transport at an environmentally relevant PFOA concentration (6.8 μg L). Our results showed a clear increase in PFOA retardation in Al/Fe oxyhydroxide coated sand (retardation factor: Al: 1.87-5.58, Fe: 1.28-4.05) than those in uncoated sand (1.00-1.05), due to the stronger electrostatic attraction between anionic PFOA and Al/Fe oxyhydroxide coated sand surface. Notably, Al oxyhydroxide have a more profound effect on PFOA retention and retardation than Fe oxyhydroxide. Besides, higher ionic strength significantly inhabited PFOA retention and retardation in positively charged sand, and the considerable retention of PFOA (∼90%) in deionized water than those in 1.5 mM and 30.0 mM NaCl (<10%) clearly proves the role of competitive adsorption of Cl on PFOA transport in positively charged sand. In contrast, higher ionic strength (0 mM-30 mM NaCl) slightly increased PFOA retardation in negatively charged sand, illustrating the dominance of electrostatic interaction. Our findings advance current knowledge to understand PFOA transport in natural media with different surface charge property under environmental PFOA concentrations.