Interaction of alkali, alkaline-earth and sulphate ions with clay minerals and sedimentary rocks

Ion-exchange isotherms of kaolinite and montmorillonite for calcium, barium and sulphate ions in a solution at constant pH near neutral have been examined. The ionic strength of the solutions ranged from seawater to formation waters from oil reservoirs in the North Sea. 45Ca-, 133Ba- or 35SO4-tagged solutions with added NaCl as supported electrolyte were used for adsorption measurements. From distribution coefficients found for CaCl2NaCl or BaCl2NaCl in the clay suspensions, the adsorbed quantities of divalent cations can be predicted for sedimentary rocks with a given mineralogical composition. These results show that the equivalent ionic fractions of alkaline-earth ions adsorbed on clay pretreated with seawater, or formation water, fit the normalized isotherms obtained for the calcium-sodium exchange on the clay within uncertainties of ± 10%. The same correlation was not found for sulphate adsorption from seawater because the clay minerals pretreated with seawater were less reactive for sulphate adsorption. These results suggest that in addition to adsorption due to electrostatic forces, some ions were chemisorbed to the clay minerals during the pretreatment with seawater. The lower sulphate adsorption may be due to ion pair formation.