Effects of Monovalent Cations on the Competitive Adsorption of Perfluoroalkyl Acids by Kaolinite: Experimental Studies and Modeling

Abstract

Our hypothesis that longer-chained perfluoroalkyl acids (PFAAs) outcompete shorter-chained PFAAs during adsorption was tested in this study, wherein the adsorption interactions of six frequently detected PFAAs with kaolinite clay were modeled and examined experimentally using various suspension compositions. Competitive adsorption of PFAAs on the kaolinite surface was observed for the first time, and longer-chained PFAAs outcompeted those with a shorter chain. The electrostatic repulsion between adsorbed PFAA molecules is a primary inhibitory factor in PFAA adsorption. An increase in aqueous sodium or hydrogen ion concentration weakened electrostatic repulsions and changed the adsorption free energy. Therefore, the adsorption of a shorter-chained PFAA with weaker hydrophobicity could occur at high sodium or hydrogen ion concentrations. The experimental and modeling data suggest that the adsorption of shorter-chained PFAAs (≤4 perfluorinated carbons) in freshwater with a typical ionic strength of 10(-2.5) is not thermodynamically favorable. Furthermore, by measuring the electrokinetic potential of kaolinite suspension in the presence of PFAAs, we found that the kaolinite surface became more negatively charged because of the adsorption of PFAAs. This observation indicates that the adsorbed PFAA molecules were within the electrical double layer of the kaolinite surface and that they contributed to the potential at the slipping plane. The possible alignments of adsorbed PFAA molecules on the kaolinite surface were then proposed.

Mechanistic study of PFOS adsorption on kaolinite and montmorillonite

“…Upon PFOS adsorption, a slight increase in d(0 0 1) spacing from 1.19 to 1.24 nm was observed, which is consistent with a previous report on expansion from 1.34 to 1.46 nm [46]. The magnitude of the d-spacing increase (0.05 nm) was significant less than the molecular sectional size of PFOS (OD: ≈0.40 nm [9]), which indicated that PFOS may partially insert into interlayers of montmorillonite during the adsorption. In conclusion, our XRD results indicated that interlayer adsorption existed on montmorillonite, whereas no interlayer adsorption occurred on kaolinite.…”

“…This high similarity in FTIR spectra suggested that the adsorption of PFOS as an outer-sphere complex should play a dominant role, in line with a previous report [28]. Here, the driving force for the outer-sphere adsorption of PFOS might be hydrophobic interaction [9,13,15].…”

“…These interactions may be ligand exchange [29], and acid-base reaction [21,30]. Because the sulfonate group is a hard base and clay minerals are hard acids [9,21], the hard base is readily adsorbed on clay surfaces based on the hard-soft-acid-base (HSAB) reaction theory. The results indicate that both the hydrophobic C F chain and the functional sulfonate group could affect PFOS adsorption on clay minerals.…”

“…PFOS adsorption on clay minerals is mainly ascribed to electrostatic interactions and hydrophobic effect [9,[14][15][16]. For example, electrostatic interactions as well as hydrophobic effect contribute PFOS adsorption on Ottawa sand, iron-coated sand, kaolinite, and goethite [14].…”

“…For example, electrostatic interactions as well as hydrophobic effect contribute PFOS adsorption on Ottawa sand, iron-coated sand, kaolinite, and goethite [14]. As a further evidence of electrostatic interactions, the increase in pH which consequently enhanced the electrostatic repulsion, decreased the PFOS adsorption on silica [15] and kaolinite [9]. In addition, PFOS adsorption would rise as the enhancement of the hydrophobic interaction by increasing the organic carbon content of adsorbents and perfluorocarbon chains [16].…”