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Influences of OH/Al Ratios and Loading Rates on Aluminum-Kaolinite Interactions¹

ABSTRACT

The < 2-m fraction of poorly crystalline Georgia kaolinite was reacted with hydrolyzed Al solutions having basicities (OH/Al ratios) of 0.75, 1.50, 2.00, and 2.40 at Al loadings (meq basis) of 40, 60, 80, 100, and 200% of the cation exchange capacity (CEC) of the added kaolinite. The equilihrated solutions and 0.1M BaCl₂ extracts were analyzed for amounts and basicity of Al, and for relative amounts of mononuclear and polynuclear Al. The basicity of adsorbed and fixed Al was used to characterize the Al removed from solution and its reactions with the kaolinite surface. Precipitation of Al was significant in solutions aged 10 d in the absence of kaolinite, but precipitation was not detected in solutions containing kaolinite until the added Al exceeded the CEC of the added clay. The amounts and basicities of Al adsorbed, exchanged, and fixed consistently increased as loading rates and initial solution basicities increased. Relative to the hasicity of the initially added Al, however, decreases were observed at the 40 and 60% loading rates for adsorhed Al at initial hasicities > 2.0 and at all initial basicities for fixed Al. This indicates dehydroxylation of the added Al at these low loading rates. The basicities of adsorbed and fixed Al increased relative to the initial basicities at higher loading rates. The amounts of exchangeable Al were < 2% of the added Al until loading rates were 100% or more. The OH/Al ratios varied from 0 ("trivalent" Al) to 2.13. The proportions of polynuclear to mononuclear Al increased with increasing initial hasicity to values as high as 70% of the Al exchanged. A mechanism based on heterogeneous affinity of exchange sites for Al was proposed to explain the experimental observations. Unlike other specific adsorption reactions, which account for much less total adsorption, approximately 60% of the exchange sites of this kaolinite sample seem to have a high affinity for hydrolyzed Al.

¹ Contribution from the Dep. of Agronomy, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061

² Former Graduate Research Assistant and Professor of Agronomy. The Senior Author is now Assistant Professor of Agronomy and Soils, Clemson University, Clemson, SC 29631.

Received for publication October 28, 1981. Accepted for publication December 3, 1982.