Interactions of Monovalent Organic Cations with Montmorillonite: Adsorption Studies and Model Calculations

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Abstract
Complexes between organic cations and clays provide a research tool and have ecological applications. The purpose of this study was to elucidate details of interactions between monovalent organic cationic dyes and montmorillonite. Interactions were studied by x‐ray diffraction and ultraviolet and infrared (IR) spectroscopies, IR linear dichroism, and adsorption isotherm measurements with model calculations. The adsorption model combined electrostatic equations with specific binding and considered neutral and positively charged complexes between surface sites and organic cations in a closed system. The model was extended to account for dye aggregation in solution. The adsorption of the dyes to montmorillonite was described by binding coefficients that were at least six orders of magnitude larger than those of inorganic cations such as Na+ and Cd++. The maximal amounts of crystal violet (CV), methylene blue (MB), and acriflavin adsorbed were 200, 150, and 175%, respectively, of the cation‐exchange capacity (CEC) of the clay mineral. The model also simulated the competition between dyes for adsorption sites. The c‐spacing of montmorillonite increased by the adsorption of CV. With MB at loadings of up to 40% of the CEC, the spacing was reduced, indicating desorption of water from the interlayer space. We conclude that MB lies preferentially parallel to the clay mineral plates, whereas CV lies at a slight inclination relative to the plates.

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