The extent and mechanisms by which hydroxy‐Al interlayers may reduce the CEC of Chambers montmorillonite were studied using 22‐Na labeled clay. Clay‐hydroxy‐Al complexes were formed by added AlCl 3 and NaOH at pH 7.5–9.5 (OH/Al ratio of about 2.80). AlCl 3 was added at 4, 8, or 16 meq/g clay and either before, after, or simultaneously with NaOH. Addition of 16 meq AlCl 3 /g clay followed by NaOH resulted in CEC reductions of 60, 40, and 10% at pH 7.5, 8.5, and 9.5 as measured by Na adsorption. However, there was no trapped or nonexchangeable 22‐Na observed in any of the systems. Since the charge on the hydroxy‐Al is negligible at these pH's, the CEC reduction is apparently due to trapped Al. CEC reduction was much less when NaOH was added prior to rather than after AlCl 3 even though X‐ray results indicated the presence of Al‐interlayers after drying in all except physical mixtures of hydroxy‐Al and clay. Since CEC reduction is apparently due to formation of hydroxy‐A1 interlayers and entrapment of A1 on the surface of the clay, it will vary with the amount and sequence in which AlCl 3 and NaOH are added and the pH of the system due to varying solubility of the hydroxy‐Al and charge on the precipitate. As a result, studies where AlCl 3 and NaOH are added simultaneously may give varying results depending on which is leading during the titration. Studies using 137‐Cs and 85‐Sr labeled Cs‐and Sr‐clay were conducted to further confirm the mechanism of hydroxy‐Al interlayer formation and subsequent CEC reduction.
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Keren et al. (1977) studied this question.