ION-EXCHANGE PHENOMENA IN SOME SOILS CONTAINING AMORPHOUS MINERAL CONSTITUENTS

Summary Soils containing allophane, “palagonite”, and amorphous oxides have been found to give cation-exchange capacity values as determined by conventional methods, which depend markedly upon concentration of leaching solution, the cation in the solution, the volume of the washing alcohol, and its water content. Experiments in which the soil is brought to equilibrium with solutions of barium acetate adjusted to pH 7, and containing an excess of barium with respect to the cation-exchange capacity value given by the leaching method, indicate that the uptake of cation conforms to the Brunauer, Emmett, and Teller theory of multilayer physical adsorption. Similar results are also given by barium-chloride and lithium-acetate solutions at pH 7. Adsorption does not take place in the absence of water, and it is thought that it is the hydrated cation which is effective in covering the surface. Adsorption is strongly influenced by equilibrium pH values of the solutions. The adsorption of cations by these soils closely resembles the behaviour of hydrated alumina prepared by precipitation from sodium aluminate with CO2. The acetate and chloride anions are taken up to a much smaller extent than the cations. Titration experiments indicate that soils containing allophane exhibit little if any tendency to hold exchangeable hydrogen ions in the same manner as montmorillonite.