Interactions of some large organic cations with bentonite in dilute aqueous systems

Abstract--Adsorption studies indicate that paraquat, diquat, and thionine are bound on bentonite by amounts greater than the measured cation-exchange capacity (CEC) of the clay. Methylene blue, new methylene blue, and malachite green are bound by amounts equal to the CEC. The unipositive organo-cations form aggregates on the clay surface. Aggregation i creases with ionic strength and increases the apparent adsorption capacity by 25%. The aggregates are removed by washing with distilled water. De-sorption studies how that the dyes are irreversibly bound, whereas the dipositive organocations are re-versibly bound. Ionic strength variation reduces adsorption by 15 and 36 % in the monovalent and divalent organocation-clay s stems, respectively. In the clay-divalent organocation systems adsorption is greater on Na-saturated clay than on K-saturated clay. Adsorption is unchanged over the pH range 4.5-8.5 and decreases steadily below pH 4.0. Changes in adsorption due to changes in temperature are small. The study indicates that ionic strength is the most important variable in clay-organocation interactions