Cation–π Bonding: A New Perspective on the Sorption of Polycyclic Aromatic Hydrocarbons to Mineral Surfaces

Abstract

Recent molecular modeling and spectroscopic studies have suggested that relatively strong interactions can occur between aromatic π donors and metal cations in aqueous solutions. The objective of this study was to characterize potential cation–π interactions between π donors and exchangeable cations accumulated at mineral surfaces via both spectroscopic and batch sorption methods. Quadrupolar splitting in deuterium nuclear magnetic resonance (2 H NMR) spectroscopy for d2–dichloromethane, d6–benzene, and d8–toluene (C 6 D 5– moiety) in aqueous suspensions of a Na‐saturated reference montmorillonite unambiguously indicated the ordering of solute molecules with respect to the clay surface. The half line broadening (Δν 1/2) of 2 H NMR of d6–benzene in montmorillonite suspensions showed that soft exchangeable cations generally resulted in more benzene sorption compared with harder cations (e.g., Ag+ > Cs+ > Na+ > Mg 2+, Ba 2+). In batch sorption experiments, saturating minerals (e.g., porous silica gels, kaolinite, vermiculite, montmorillonite) with a soft transition metal or softer base cations generally increased the polycyclic aromatic hydrocarbon (PAH) sorption relative to harder cations (e.g., Ag+ >> Cs+ > K+ > Na+; Ba 2+ > Mg 2+). Sorption of phenanthrene to Ag+–saturated montmorillonite was much stronger compared with 1,2,4,5‐tetrachlorobenzene, a coplanar non‐π donor having slightly higher hydrophobicity. In addition, a strong positive correlation was found between the cation‐dependent sorption and surface charge density of the minerals (e.g., vermiculite, montmorillonite >> silica gels, kaolinite). These results, coupled with the observations in 2 H NMR experiments with montmorillonite, strongly suggest that cation–π bonding forms between PAHs and exchangeable cations at mineral surfaces and affects PAH sorption to hydrated mineral surfaces.