Stephen Boyd1, Brian Teppen1, Hui Li1, Cliff T. Johnston2, and David Laird3. (1) Dept of Crop & Soil Sciences, Michigan State University, East Lansing, MI 48824-1325, (2) Purdue Univ, Crop Soil and Environmental Sciences, Dept of Agronomy, West Lafayette, IN 47907, (3) USDA-ARS, National Soil Tilth Lab, 2150 Pammel Dr., Ames, IA 50011
Clay minerals and soil organic
matter (SOM) are generally considered to be the most important soil components
in the sorption of aqueous phase organic contaminants. During the past 25 years, much emphasis
has been placed on the dominant role of SOM in sorption. However, there is increasing evidence
that many organic contaminants are substantially sorbed from bulk water by
clays, often in excess of that by SOM when compared on a unit mass basis. Nitroaromatic compounds (NACs) are an
interesting case in point. We have
examined in detail the molecular scale mechanisms and forces leading to the
very high affinities of smectite clays for NACs. These studies have employed a complimentary set of
investigatory approaches including detailed bulk phase sorption measurements,
novel x-ray diffraction techniques, infrared spectroscopy, molecular simulation
and thermodynamic measurements to reveal the molecular scale determinants of
NAC sorption by smectite clays.
Our studies have revealed that NAC affinity for smectites is strongly
dependent on the nature of the inorganic exchangeable cations. Clays saturated with cations having low
hydration energies (e.g. K+) have high affinities for NACs. For smectites, these cation effects are
well understood at the molecular level.
Sorption occurs primarily in the clay interlayers, and cation hydration
determines the interlayer distance, size of adsorption domains and ability of
-NO2 groups to complex directly with exchangeable cations. Lower clay charge densities result in larger
adsorption domains that promote adsorption. Partial solute dehydration afforded by the subaqueous clay
interlayer environment is also energetically favorable.