Adsorption of Aluminum by a Smectite: II. An Al3+‐Ca2+ Exchange Model

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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.

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