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Adsorption of Aluminum by a Smectite: II. An Al³⁺-Ca²⁺ Exchange Model¹

ABSTRACT

The adsorption of Al by Ca²⁺-montmorillonite has been found to be described by a Langmuir-type equation in which the activity of Al³⁺ in solution is a function of the coverage of the mineral surfaces by Al. The classical mass-action equations were inadequate in describing the exchange reaction where the Ca²⁺ activity was allowed to vary, weighting the effect of the Ca²⁺ ion activity on the equilibrium too heavily. The model suggests the relative solutionlike nature of adsorbed Ca²⁺ compared to the much more strongly adsorbed Al³⁺. Hydrolysis and precipitation reactions of Al³⁺ at higher pH complicated the exchange model by introducing significant quantities of adsorbed hydroxyl-Al into the system.

¹ Contribution of the Dep. of Agron., Cornell Univ., Ithaca, NY 14853. Agron. Paper no. 1204.

² Assistant Professor of Soil Chemistry and Graduate Research Assistant, respectively.

Received for publication December 3, 1976. Accepted for publication July 15, 1977.