Gas Sorption in Clay Mineral Systems

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Abstract
Sorption isotherms for four gases (N2, A, Kr and CO2) , commonly used in specific surface area and pore structure measurements, have been accurately determined on a number of clay mineral and oxide systems. Specific surface areas obtained by application of the BET theory to these isotherms illustrate the extent to which the apparent cross-sectional areas for these sorbed gases vary with surface structure, ex- changeable cation and microporosity. V-n plots for nitrogen adsorption on these materials using nitrogen adsorption on crystalline materials of large crystal size as a standard isotherm provide appreciable ranges of linearity in each case. The specific surface areas obtained from these straight line plots agree well with the corresponding BET values. The linearity of these plots for illite clays indicates the absence of capillary condensation and that adsorption in slit-shaped pores takes place largely by the formation of physically adsorbed layers on the surfaces. Much larger BET specific surface areas were obtained from carbon dioxide sorption at 196K on goethite, hematite and gibbsite than from nitrogen, argon and krypton sorption at 78"K. It is suggested that enhanced sorption of CO2 into microporous regions of the oxides, inaccessible to the other gases, occurs in a similar fashion to that frequently observed for coal and charcoal materials. V-n plots for CO2 sorption in these materials using that for an illite clay as a standard isotherm, support this conclusion. Considerably lower BET specific surface areas were obtained for CO 2 sorption on kaolinite than were obtained for nitrogen, argon and krypton sorption. The shape of the V-n plots for CO., sorption on kao- linite compared with illite suggest that an initial specific adsorption of CO2 on the kaolinite is followed by a change in state with the completion of this layer, allowing normal multilayer formation to proceed.

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