Role of Layer Charge in Organic Contaminant Sorption by Organo-Clays
Stephen A. Boyd, William F. Jaynes, 1994. "Role of Layer Charge in Organic Contaminant Sorption by Organo-Clays", Layer Charge Characteristics of 2:1 Silicate Clay Minerals, A. R. Mermut, S. A. Boyd, W. J. Farmer, W. F. Jaynes, G. Lagaly, D. A. Laird, A. R. Mermut
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The aluminosilicate structure of 2:1 phyllosilicate clays typically contains a deficit of positive charge due to the isomorphous substitution of lower valence cations for higher valance cations. For example, Al−*+ may substitute for Si4+ in the tetrahedral sheets, and Mg2+ may replace Ap+ in the octahedral sheets. This process of isomorphous substitution results in 2:1 layer silicates with structural charge that may vary from zero to >200 cmol/kg (McBride, 1989). The negative structural charge is balanced by exchangeable cations at or near the aluminosilicate surfaces. In nature these exchangeable cations are usually alkali-metal and alkaline-earth-metal cations such as Na+ and Ca∧+. These cations have large heats of hydration and are strongly hydrated in the presence of water. The hydration of inorganic exchange cations present on clays imparts a hydrophilic nature to the clay surfaces. As a result, clays are ineffective as sorbents for poorly water soluble (hydrophobic) organic contaminants dissolved in water.
The hydration of metal exchange cations complicates efforts to evaluate the inherent hydrophobicity or hydrophilicity of the siloxane surfaces of 2:1 layer silicates. Neutral layer phyllosilicates (i.e.; those with no structural charge) such as talc and pyrophyllite appear to be relatively hydrophobic on a macrosophic level. The ability of water to interact directly with surface silicate oxygens has been the subject of considerable debate. Early research on clays emphasized the importance of the surface silicate oxygens on water adsorption via hydrogen bodnding (Handricks and Jefferson, 1938?; Bradley, 1945: Low, 1961) suggesting the hydrophilic or polar nature of the
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Layer charge is recognized as the single most important characteristic of 2:1 phyllosilicates. It indicates a mineral's capacity to retain cations and to adsorb water and various polar organic molecules. The affinity of organic-clays for sorption of organic contaminants in aqueous systems is now attracting considerable attention. The layer charge is also considered an important criterion for the classification of 2:1 silicate clay minerals. It is generally agreed that the magnitude of the layer charge can be taken into account to resolve the classification problem. Studies on structural chemistry also confirm the importance of layer charge for characterization of silicate clays.”