Maximum Adsorption Capacity of Perfluorooctanoic Acid (PFOA) on Clays

Per- and polyfluoroalkyl substances (PFAS) are emerging contaminants that persist in soil environments, necessitating reliable models to predict their fate and transport. This study evaluates the performance of three theoretical models in estimating the maximum adsorption capacity (Qmax) of perfluorooctanoic acid (PFOA) on kaolinite and montmorillonite clay minerals. The models assessed include a van der Waals interaction-based approach, a monolayer adsorption capacity model, and a surface site density model emphasizing reactive hydroxyl groups at mineral edges. Benzene, nitrogen, and glyphosate molecules were used as reference compounds for model validation. Results indicated that the van der Waals model significantly underestimated Qmax (0.0007 mg·g−1 for kaolinite), while the monolayer capacity model produced substantial overestimations (17.51 mg·g−1) compared to the experimental range (0.10–10.0 mg·g−1). The surface site density model provided the most accurate predictions (3.39 mg·g−1 for kaolinite), although it slightly underestimated values for montmorillonite (0.20 mg·g−1) by excluding interlayer adsorption. These discrepancies demonstrate that simplified models cannot adequately capture the complex adsorption behavior of PFAS. Accurate prediction requires site-specific approaches incorporating electrostatic forces, hydrogen bonding, and steric effects. As PFAS accumulation in soil directly contributes to groundwater contamination, improving adsorption models is essential for accurate risk assessment and the development of effective remediation strategies.