Adsorption of Indole on Kaolinite in Nonaqueous Media: Organoclay Preparation and Characterization, and 3D-RISM-KH Molecular Theory of Solvation Investigation

Abstract

Current oil sand mining operations in the Athabasca basin are predominantly aqueous-based. Extracts containing large amounts of fines lead to the formation of stable organoclay suspensions in froths giving lower yields and greater tailing wastes and making the development of more efficient extraction methods desirable from both economical and environmental perspectives. We examine an indole-kaolinite system as a model for these oil fines and their resistance to washing in nonaqueous solvents. The prepared organoclays show indole loading exclusively on the external surface of the clay. Micron-scaled vermicular structures, similar to natural kaolinite, are observed. Their formation is believed to be driven by strong adsorbate–adsorbate interactions. Indole is the primary adsorbate, as solvent adsorption is shown to be minimal based on both experimental and computational results. Isotherms are constructed and parameters calculated from linear regression analysis fitted to the Brunauer–Emmett–Teller equation. Monolayer quantities calculated match well to the theoretical amount calculated from surface areas measurements. Washing the organoclays with both toluene and isopropanol results in a 50% decrease of loaded organic material, leaving a monolayer equivalent of organic matter. The statistical-mechanical 3D-RISM-KH molecular theory of solvation is employed to perform full sampling of solvent orientations around a kaolinite platelet and gain insights into the preferred orientation and adsorption thermodynamics of indole on kaolinite in toluene and heptane solvents. In its preferred orientation, indole is hydrogen-bonded to one or two O atoms at the aluminum hydroxide surface of kaolinite. The calculated solvation free energy and potential of mean force for adsorption of indole and solvents on kaolinite in solution yield the increasing adsorption strength order of heptane < toluene < indole on the aluminum hydroxide surface. Multilayer adsorption profiles are predicted based on the integrated three-dimensional distribution functions of indole, toluene, and heptane