Submitted to: Journal of Environmental Science and Technology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 7/9/2008
Publication Date: 12/15/2008
Citation: Chatterjee, R., Laird, D.A., Thompson, M. 2008. Interactions among K+-Ca2+ Exchange, Sorption of m-Dinitrobenzene, and Smectite Quasicrystal Dynamics. Journal of Environmental Science and Technology. 42(24):9099-9103. Interpretive Summary: When organic contaminants such as pesticides enter soils they may be degraded, adsorbed to soil particles, or leached to ground water. Organic matter and clay minerals are the most important components of soils for adsorption of organic contaminants. Recent research has shown that the ability of smectites, one of the most common types of soil clay minerals, to adsorb organic contaminants varies dramatically. Sometime smectites adsorb almost all of the contaminant and sometimes they adsorb little or none of the contaminant. Because of this variation, it is difficult to predict whether there is a risk of an organic contaminant leaching to ground water. We discovered that adsorption of organic contaminants by smecties is controlled by a complex process that is influenced by how much the clay has swollen by imbibing water and the type of cations that are present in the system. Also the pathway or history of the clay is very important in influencing the ability of the smectite to adsorb organic contaminants. This study will help scientists to understand the mechanisms that control sorption of organic contaminants by soil materials and may help engineers to design more effective containment systems. The results also indicate that pesticide application rates based on the amount or even type of clay in soils will not be effective for preventing ground water contamination in all circumstances.
Technical Abstract: The fate of organic compounds in soils and sediments is influenced by sorption of the compounds on surfaces of soil materials. We investigated the interaction among sorption of an organic compound, cation exchange reactions, and both the size and swelling of smectite quasicrystals. Two reference smectites that vary in location and amount of layer charge, SPV (a Wyoming montmorillonite) and SAz-1, were initially Ca- and K-saturated and then equilibrated with mixed 0.01 M KCl and 0.005 M CaCl2 salt solutions both with and without the presence of 200 mg/L m-dinitrobenzene (m-DNB). In general, sorption of m-DNB increased with the amount of K in the system for both clays, and the SPV sorbed more m-DNB than the SAz-1. Sorption of m-DNB increased the preference of Ca-SPV for K relative to Ca but had little effect on K-Ca selectivity for K-SPV. Selectivity for K relative to Ca was slightly higher for both K-SAz-1 and Ca-SAz-1 in the presence of m-DNB than in its absence. Distinct hysteresis loops were observed for the K-Ca cation exchange reactions for both clays, and the legacy of having been initially Ca- or K-saturated influenced sorption of m-DNB by SPV but had little effect for SAz-1. Suspension x-ray diffraction was used to measure changes in d-spacing and the relative thickness of smectite quasicrystals during the cation exchange and m-DNB sorption reactions. The results suggest that interactions among cation exchange and organic sorption reactions are controlled by inherently hysteretic complex feedback process that is regulated by changes in the size and extent of swelling of smectite quasicrystals.