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Salt-Exchangeable and Titratable Acidity in Bentonite-Sesquioxide Mixtures¹

ABSTRACT

Precipitation of ferric or aluminum "hydroxides" in bentonite suspensions resulted in the formation of claysesquioxide complexes with exchange and titratable acidity reminiscent of many acid soils high in iron and aluminum oxides. Tenth-normal AlCl₃-treated samples containing different amounts of sesquioxide had exchange acidities between 0 and 0.42 me. per g. of bentonite, titratable but nondisplaceable acidities from 0.41 to 0.74 me. per g. The sum of exchange and titratable acidity was close to the cation-exchange capacity of the clay. Substantial amounts of aluminum or ferric hydrous oxide existed as interlayers, aluminum ones being better ordered. Apparently ferric or aluminum hydrous oxides deposited on montmorillonite surfaces can exist there as R(OH)_x, with x 3. At pH 8 to 9, there are 3 hydroxyls per trivalent ion, and the effective CEC is that of the clay. As pH is lowered, OH is removed or H is added to the sesquioxide coating, which becomes positively charged and interacts with exchange sites of the clay. At pH around 5, the effective exchange capacity is near zero. Addition of base to the acid system restores OH to the sesquioxide and recreates exchange capacity. The salt displacement and titration results in the bentonite-sesquioxide systems parallel in many ways similar observations made with acid soils, suggesting that similar reactions occur in natural and synthetic systems.

¹ Paper No. 1479, University of California Citrus Research Center and Agr. Exp. Sta., Riverside.

² Professor of Soils and Plant Nutrition, Riverside; Visiting Assistant Professor on leave from Department of Agronomy, Virginia Polytechnic Institute; and Graduate Research Assistants, respectively. The work was done under National Science Foundation Research Grant NSF-GP 323-Coleman.

Received for publication April 12, 1963. Accepted for publication June 17, 1963.