Molecular insight into per- or polyfluoroalkyl substances (PFAS) sorption on mineral oxides in soil under varying geochemical conditions.

Abstract

The environmental persistence and widespread distribution of per- and polyfluoroalkyl substances (PFAS) are driven by their unique physicochemical properties, including the exceptional strength of carbon‑fluorine (CF) bonds, amphiphilicity, and surface activity, which govern their resistance to degradation, environmental partitioning, and interactions with soil constituents. However, a detailed molecular-level understanding of PFAS sorption on representative mineral surfaces under varying geochemical conditions remains limited. In this study, quartz crystal microbalance with dissipation monitoring (QCM-D) was employed to characterize the sorption behaviors of trifluoroacetic acid (TFA), heptafluorobutyric acid (HFBA), and perfluorooctanoic acid (PFOA) on silicon dioxide (SiO2) and aluminum oxide (Al2O3), two major soil minerals. Sorption on SiO2 was generally weak and primarily governed by hydrophobic interactions, while sorption on Al2O3 was relatively stronger and nonlinear, involving electrostatic attraction between carboxylate groups and positively charged surface sites, and hydrophobic interactions of fluorocarbon chains. PFOA showed the highest sorption affinity, followed by HFBA and TFA. Sorption on SiO2 showed minimal sensitivity to pH and temperature, whereas sorption on Al2O3 decreased markedly with increasing pH and temperature. Divalent cations exerted contrasting effects. On SiO2, Ca2+ and Mg2+ enhanced sorption of long-chain PFAS (PFOA) through cation bridging and reduced electrostatic repulsion, while they suppressed short-chain PFAS (TFA, HFBA). Cu2+ showed a dual effect, promoting PFOA sorption at moderate concentrations but inhibiting it at higher concentrations due to competition for surface sites. On Al2O3, in contrast, all three cations suppressed sorption by compressing the electrical double layer and weakening electrostatic attraction, with the extent varying by chain length. These findings highlight the central role of interfacial processes in controlling PFAS behavior in subsurface environments and provide mechanistic insights that improve the prediction of PFAS fate and mobility, support environmental risk assessment, and guide the development of more effective site-specific remediation strategies for PFAS-contaminated soils and groundwater.