Cation-pi bonding: a new perspective on the sorption of polycyclic aromatic hydrocarbons to mineral surfaces

Recent molecular modeling and spectroscopic studies have suggested that relatively strong interactions can occur between aromatic donors and metal cations in aqueous solutions. The objective of this study was to characterize potential cation-pi interactions between donors and exchangeable cations accumulated at mineral surfaces via both spectroscopic and batch sorption methods. Quadrupolar splitting in deuterium nuclear magnetic resonance (2H NMR) spectroscopy for d2-dichloromethane, d6-benzene, and d8-toluene (C6D5- 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 (deltav1/2) of 2H 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+ > Mg2+, Ba2+). 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+; Ba2+ > Mg2+). Sorption of phenanthrene to Ag+-saturated montmorillonite was much stronger compared with 1,2,4,5-tetrachlorobenzene, a coplanar non-pi 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 2H NMR experiments with montmorillonite, strongly suggest that cation-pi bonding forms between PAHs and exchangeable cations at mineral surfaces and affects PAH sorption to hydrated mineral surfaces.