Previous studies have indicated that Al is fixed by a synthetic cation exchanger as a positively‐charged hydroxy‐Al polymer with an OH/Al molar ratio of 2. The present study with montmorillonite showed that this mineral also can fix Al in nearly the same OH/Al ratio but at a lower pH. Graded amounts of NaOH were added to an AlCl 3 ‐montmorillonite suspension and after 6 months the clay was separated. In the solution with the highest (1.35) OH/Al ratio, the apparent CEC of the clay had decreased from 98 to 20.5 me. per 100 g. and a 15.5 Å. X‐ray diffraction spacing was retained at 500°C. This sample, however, expanded to 18 Å. with ethylene glycol at 25°C. The increase in fixed Al with decrease in CEC was 24.6 mg. per me. Additional evidence is presented which indicates that the CEC was reduced by positively‐charged hydroxy‐Al groups occupying exchange sites rather than by Al(OH) 3 blocking pathways to these sites.
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Shen et al. (1962) studied this question.