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 d 2 –dichloromethane, d 6 –benzene, and d 8 –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 d 6 –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.