| The molecular mechanisms and major geochemical factors controlling the sorption of nonionic organic chemicals (NOCs) to hydrated mineral surfaces in low-organic-carbon environments are not yet fully understood. Although various models have been advanced to describe NOC sorption to mineral surfaces, none of them have been verified by spectroscopic data. The major hypothesis of this research was that the cation-π interactions form the predominant mechanism controlling polycyclic aromatic hydrocarbons (PAHs) sorption to hydrated mineral surfaces. Macroscopic sorption studies were conducted in conjunction with deuterium nuclear magnetic resonance (2H NMR) spectroscopy to examine the influence of mineral surface chemistry and the nature of exchangeable cations on the formation of cation-π bonds. The sorption of naphthalene and pyrene, as representative PAHs, was quantified to specific cation-saturated minerals, including porous silica gels, kaolinite, and vermiculite. Spin-lattice relaxation time (T1) and spin-spin relaxation time (T2) of deuterated benzene in aqueous solutions and clay suspensions were measured, providing spectroscopic evidence for the first time that cation-π interactions exist between PAHs and cations in aqueous solutions. The binding energy of the aqueous metal-benzene complex varied with the hardness/softness of the metal cation. An overall binding energy sequence of Ag+ >> Cs+ > K+ > Na+, Li+ in the aqueous phase was derived based on the T1 measurements of deuterated benzene. Saturating mineral surfaces with softer transition metals or base cations generally increased PAH sorption relative to harder cations. These results suggest that softer metal cations present on hydrated mineral surfaces can directly enhance the sorption of PAHs through cation-π interactions. Quadrupolar splitting of deuterated benzene in Na-montmorillonite suspensions indicated the ordering of benzene molecules with respect to the clay surface, which likely resulted from the cation-π interaction between benzene and exchangeable cations. Further T2 measurements of deuterated benzene in clay suspensions indicated that pH and ionic strength also affects PAH sorption through varying structures of adsorbed water and hence cation-π interactions at mineral surfaces. |
