Saturday, 15 July 2006

Investigating the Role of Organo-Clay Complexes in Phenanthrene Sorption.

Xiaojuan Feng, André J. Simpson, and Myrna J. Simpson. Dept of Physical and Environmental Sciences, Univ of Toronto, Scarborough College, 1265 Military Trail, Toronto, ON M1C 1A4, Canada

Sorption of Hydrophobic Organic Compounds (HOCs) is an important process that regulates the fate and transport of many problematic chemicals in the environment. Recent studies of contaminant-soil interactions have indicated that selective sorption and physical conformation of organic matter at the solid-aqueous interface are important in governing HOC sorption. A new Nuclear Magnetic Resonance (NMR) technique, High Resolution-Magic Angle Spinning (HR-MAS) NMR, which enables one to obtain liquid-state quality spectra on semi-solid samples, provides a powerful tool to examine the role of organo-clay compounds in sorption interactions with HOCs. In our study, organo-clay complexes were constructed by coating montmorillonite and kaolinite with Peat Humic Acid (PHA) in Na+ or Ca2+ dominated solutions with varying pH and ionic strength values, and used in the batch sorption studies with phenanthrene. The solution conditions encourage the dissolved PHA to adopt “coiled” or “stretched” conformation prior to interacting with the clay mineral surface and facilitates the investigation of the role of organic matter physical conformation in HOC sorption. Evidence from 1H HR-MAS NMR indicates that polymethylene carbon (-CH2-) is prevalent at the surface of the organo-clay complexes, which may enhance interactions, such as hydrogen bonding, in the sorption of HOCs. Selective sorption of polymethylene groups on kaolinite and aromatic compounds on montmorillonite may contribute to the difference in phenanthrene sorption by PHA associated with these two types of clay. Moreover, less peptides are sorbed to clay surfaces at lower pH values, and hence, a relatively higher ratio of polymethylene carbon is exposed at the solid-aqueous interface. This is likely to contribute to the enhanced phenanthrene sorption at lower pH values because sorbents rich in polymethylene carbon have been reported to exhibit very high sorption capacity (Koc) for phenanthrene. Alternatively, organic matter conformation in various solution conditions is accounted for the difference in phenanthrene sorption capacity of different organo-clay complexes. This study demonstrates that solution conditions and clay mineral type are important for the selective sorption of humic materials at the soil-water interface, and indirectly control the HOC sorption behavior.

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