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Basicity of Exchangeable Aluminum, Formation of Gibbsite, and Composition of the Exchange Acidity in the Presence of Exchangers¹

ABSTRACT

Montmorillonite, vermiculite, and a sulfonic resin were treated with different concentrations of Al(OH)_BCl_3-B with the basicity B ranging from 0 to 2.2. The Al exchanged by vermiculite and the resin was always the nonbasic Al³⁺, independent of B and the amounts of Al(OH)_BCl_3-B added. The Al exchanged by montmorillonite was Al³⁺ when exchanged from Al(OH)_BCa_0.5B-montmorillonite containing 32 mmoles Al/100 meq CEC and became basic when large amounts of Al(OH)_BCl_3-B had been added before the exchange reaction. In contrast to vermiculite and the resin, the montmorillonite adsorbed preferentially relatively large OH-Al polymers and, therefore, accumulated much more Al in the interlayers than vermiculite and resin which excluded large OH-Al polymers because of their smaller d spacings and/or high charge densities. When more OH-Al accumulated in the interlayers of montmorillonite, the Al was adsorbed less strongly, and relatively large amounts of basic Al became exchangeable. It was shown also that an Al³⁺-saturated vermiculite is much more unstable than an Al³⁺-montmorillonite. These results are employed to discuss the widely-observed formation of gibbsite when smectite reacts with basic Al. Finally, the exchange acidity from an Al-saturated exchanger was shown to consist of both Al³⁺ and H⁺. The protons come from (i) the hydrolysis of exchanged Al³⁺ which, in turn, can be readsorbed as OH-Al; (ii) from the hydrolysis of adsorbed Al; and (iii) by the exchange of H⁺ from permanent charges. The latter portion mechanism provides an amount of exchangeable acidity that, if it exists, is relatively small.

¹ Contribution from the Institut für Ökologie-Bodenkunde-der Technischen Universität Berlin, Engler-Allee 19–21, 1 Berlin 33, W. Germany.

² Soil Scientist. Requests for reprints should be sent to Dr. J. A. Veith, Institut fuer Anorg. Chemie der Freien Universitaet Berlin, Fabeckstrasse 34–36, 1 Berlin 33 (W. Germany).

Received for publication October 29, 1976. Accepted for publication May 31, 1977.