Adsorption and Heat of Adsorption of Organic Compounds on Montmorillonite from Aqueous Solutions

Abstract The adsorption of 1,4‐dioxane, urea, ethanol, phenol, and the acetate anion by <2µm Na‐ and Ca‐saturated Upton montmorillonite from dilute aqueous solutions was determined at different concentrations by using their 14 C‐labeled isotopes. The heat of adsorption of each compound at each concentration was also determined. All the neutral organic molecules were positively adsorbed and the acetate anion was negatively adsorbed. Except for the adsorption of 1,4‐dioxane and phenol on Ca montmorillonite, the adsorption isotherms of the neutral molecules were linear. The linear adsorption isotherms mean that the adsorbed molecules were not held to the surface by specific bonds but were distributed proportionally between interfacial and bulk phases. Preference of the neutral molecules for the interfacial phase was due to their increased interaction with the surrounding water molecules. Preference of the acetate ion for the bulk phase was attributed to its electrostatic repulsion by the clay. The adsorption reactions of the 1,4‐dioxane and urea with the Na montmorillonite were exothermic; the other adsorption reactions were endothermic. Had specific bonds formed between the organic molecules and the surface of the montmorillonite, the adsorption reactions would have been exothermic. Hence, the endothermic adsorption reactions confirm the absence of such bonds for the ethanol, phenol, and acetate anions. The adsorption isotherm of 1,4‐dioxane on Ca montmorillonite was of the Langmuir type, explained in the conventional way. The adsorption isotherm of phenol on the same clay exhibited upward curvature. We postulate the existence of interlayer regions containing phenol as a separate phase that grew disproportionally as the phenol concentration in the bulk phase increased.