Tuesday, November 14, 2006

Influence of Smectite Hydration and Interlayer Cation on Substituted Benzenes Sorption.

Vaneet Aggarwal1, Hui Li2, Brian Teppen2, Stephen Boyd2, Cliff T. Johnston3, and David Laird4. (1) Plant & Soil Sci Bldg., Michigan State University, Michigan State University, Dept. of Crop & Soil Sciences, East Lansing, MI 48824, (2) MSU, Dept of Crop & Soil Sciences, Plant & Soil Science Building, East Lansing, MI 48824, (3) Purdue Univ, Crop Soil and Environmental Sciences, Dept of Agronomy, West Lafayette, IN 47907, (4) USDA-ARS, National Soil Tilth Lab, 2150 Pammel Dr, Ames, IA 50011

Smectite clays demonstrate a high affinity with organic contaminants and pesticides from water. Recent research has shown that the extent of crystalline swelling (or interlayer hydration status) along with surface charge density and exchangeable cations plays a determinant role in controlling the sorption of organic pollutants by smectites.  This study was undertaken to determine the effect of clay hydration on the sorption of 1,3-dinitrobenzene (DNB) and dimethyl 1,3-benzenedicarboxylate (DMB) by a reference Wyoming smectite saturated with K+ and Ca2+.  The results indicate that Ca2+-saturated never-dried clay had higher DNB sorption than Ca2+-saturated freeze-dried clay, while the opposite was observed for DMB.  In case of K+-saturated clay, there was no apparent difference between freeze-dried and never-dried clays in sorption of DNB and DMB. For both compounds, K-saturated clays sorbed more than Ca-saturated clays.  This can be attributed to the lower hydration energy and hence smaller hydrated radius of K+, which maximizes the siloxane surface available for sorption.  The relatively weak hydration of K causes K-saturated clay to swell less in water, thus providing a more favorable sorption domain that may allow the organic substances to compete better with water for coordination sites around the cations.