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(P888) Cation-pi Bonding: A New Perspective on the Sorption of Polycyclic Aromatic Hydrocarbons to Hydrated Geologic Materials.
Herbert, Bruce*,1, Zhu, Dongqiang2, Schlautman, Mark3,4, Carraway, Elizabeth4, 1 Geology and Geophysics, Texas A&M University, College Station, Texas, USA2 The Connecticut Agricultural Experiment Station, New Haven, CT, USA3 Agricultural & Biological Engineering, Clemson University, Clemson, SC4 Environmental Toxicology and the Clemson Institute of Environmental Toxicology, Pendleton, SC, USA
ABSTRACT- 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 contaminant fate and transport, bioavailability and uptake, and deposition of allochthonous organics at continental margins. While the importance of dipole-dipole interactions in controlling the sorption of organic molecules containing polar functional groups such as phenols, amines or nitro groups, is well accepted, the sorption of aromatic organic molecules is generally assumed to be controlled by hydrophobic effects. 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 of other PAHs, 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.
Key words: aromatics, NMR, noncovalent interactions, distributed sorption models
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