Hydrolysis and Exchange Reactions of the Aluminum Ion in Hectorite and Montmorillonite Suspensions

The hydrolysis and exchange reactions of the aluminum ion in hectorite and montmorillonite clay suspensions were studied in an attempt to characterize the nature of exchangeable and nonexchangeable aluminum. In agreement with theoretical considerations of the effect of charge on the electrostatic adsorption of ions, the degree of hydrolysis of the aluminum ions adsorbed on the clay was found to be lower than that in the equilibrium dialyzate. The degree of hydrolysis of the aluminum ions in the dialyzate was, in turn, shown to be lower than that of an aqueous aluminum chloride solution having the same total concentration of aluminum as the dialyzate. The hydrolysis of aluminum ions in clay suspensions is thus suppressed rather than increased as has been suggested. Consistent with this observation, virtually all of the exchangeable aluminum was found to be present in the form of trivalent aluminum ions without any evidence for the formation of basic polymeric aluminum ions even upon addition of moderate amounts of base. It was also shown why the region of maximum buffer action of an Alsaturated clay may differ markedly from that given by the pK value corresponding to the first‐stage hydrolysis of the aluminum ion. The fixation of aluminum by clays in nonexchangeable form was not found to produce any substantial reduction in exchange capacity, and was, therefore, attributed to precipitation of aluminum hydroxide.