Mechanism and Implication of the Sorption of Perfluorooctanoic Acid by Varying Soil Size Fractions

Sorption of perfluorooctanoic acid (PFOA), a toxic and persistent organic pollutant, by various size fractions of an agricultural soil at environmentally relevant concentrations was evaluated. PFOA sorption to all fractions involved both film diffusion and intraparticle diffusion with the rate-limiting step by the latter. PFOA isotherm data fitted a linear model. Organic matter (OM), cation exchange capacity, pore volume, and the Brunauer–Emmett–Teller area played key roles in PFOA sorption. The sorption capacity followed the order of humic acid > clay (0.15–4.4 mm) > fine silt (1.9–39.8 mm) > coarse silt (17.3–79.4 mm) > fine sand (45.7–316.2 mm) > coarse sand (120–724.4 mm), opposite to their contributions to overall PFOA sorption due to the influence of their percentage weight in the original soil. Percentage OM content was the dominant factor controlling the fraction contributions to overall PFOA sorption, demonstrating influence of the hydrophobic force on sorption. PFOA should be highly mobile and bioavailable in soil-crop systems due to the low log Koc values.