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Aluminum Fixation in Montmorillonite¹

ABSTRACT

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 AlCl₃-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.

¹ Contribution from the Agronomy Department, Virginia Polytechnic Institute, Blacksburg. Presented before Div. II Soil Science Society of America, Chicago, Ill. Dec. 5, 1960.

² Former Graduate Assistant and Professor of Agronomy, respectively.

Received for publication March 8, 1961. Accepted for publication May 31, 1961.