The exchange capacities of kaolinite and the preparation of homoionic clays

Abstract

The concepts of cation and anion exchange capacity, as applied to kaolinite, are critically examined. By attention to experimental detail it was possible to obtain reproducible measurements of ion uptake. Contrary to classical ideas the new data show no evidence for any definite cation exchange capacity. The cation uptake depends upon the cation chosen, the electrolyte concentration and pH, and on whether the experiment is done from aqueous or 95%-ethanol solvent. Apparent “capacities” as high as 25 μequiv/g of kaolinite were obtained with Ca²⁺ and Cs⁺ from the nonaqueous solvent and as low as 12 μequiv/g of kaolinite with Li⁺ from aqueous solutions. The data suggest complex ion exchange reactions on heterogeneous -SiOH and -A1OH sites. They do not support the simple model of isomorphous substitution charge plus basic edge sites so often assumed in the past.