CHARACTERIZING THE SORPTION OF AROMATIC HYDROCARBONS TO GEOLOGIC MATERIALS: NOVEL APPROACHES USING 2H-ISOTOPIC LABELING, MASS SPECTROMETRY AND 2H-NUCLEAR MAGNETIC RESONANCE SPECTROSCOPY

Sorption of aromatic organic molecules to geologic materials, such as mineral surfaces and natural organic matter, affects a large number of important geologic processes including petroleum migration, contaminant fate and transport, and deposition of allochthonous organics at continental margins. While significant strides have been made in characterizing the sorption of ionic organics to geosorbents through macroscopic and spectroscopic techniques, the molecular mechanisms controlling the sorption of nonionic organic chemicals (NOCs) to geosorbents are not yet fully understood due to the nature of the weaker, dipole interactions that characterize their sorption.

Progress in understanding the sorption of polycyclic aromatic hydrocarbons (PAH) and other aromatics has recently been accomplished using deuterium-labeled aromatics coupled with mass spectrometry and 2H-nuclear magnetic resonance spectroscopy (2H NMR) techniques. Deuterium nuclear magnetic resonance spectroscopy is sensitive enough to collect the deuterium spectra of natural and 2H-labeled organics. Because deuterium is a quadrupolar nucleus, with a nuclear spin of one, relaxation of deuterium is sensitive to its molecular environment and provides direct characterization of sorption mechanisms. Deuterium NMR was used to characterized the sorption of PAH to hydrated mineral surfaces. Saturating mineral surfaces with a soft transition metal (e.g., Ag+) or softer base cations (e.g., Cs+ > K+ > Na+; Ba2+ > Mg2+) generally increased PAH sorption relative to harder cations. Relaxation of d6-benzene decrease with increasing aqueous salt concentration with an overall binding energy sequence of Ag+ >> Cs+ > K+ > Na+, Li+. Quadrupolar splitting of d6-benzene distributed in suspensions of Na-montmorillonite supported the ordering of benzene molecules with respect to the mineral surface. The NMR results for benzene, coupled with the results of the batch sorption experiments, provides the first spectroscopic evidence for the formation of cation-pi interactions between PAHs and exchangeable cations at mineral surfaces and control PAH sorption to hydrated mineral surfaces.