EVALUATION OF CATION EXCHANGE CAPACITY OF MONTMORILLONITE USING CAESIUM CHLORIDE

抄録

The use of caesium chloride was investigated for the purpose of measuring cation exchange capacity of montmorillonite. In the preliminary experiment, the montmorillonite was saturated with various monovalent cations by washing with Li Cl, Na Cl, KCl, Rb Cl, Cs CI, and NH₄Cl in order to examine the effect of exchangeable cation species on the dispersibility of montmorillonite. As the electrolyte concentration of the equilibrium solution was stepwise decreased, the Li-, Na-, K- and NH₄-saturated clays began to disperse. Critical concentration for dispersion was 0.1 to 0.25 mol L^-1 for Li and Na saturation and 2.5 to 5 mmol L^-1 for K and NH₄ saturation. Dispersion of clays was thought to cause errors in quantification of the negative charge because of loss of the dispersed clay during the experimental procedure. The use of K⁺, Rb⁺ or NH₄⁺ was considered to have the risk of interlayer fixation by the clay from the viewpoint of the ionic radius. Therefore, Cs⁺ was judged to be appropriate as an index cation for CEC determination. The use of CsCl for evaluation of CEC of montmorillonite was compared with the use of Na Cl and showed the following advantages:(1) since the montmorillonite saturated with Cs exhibited little dispersion or swelling, the manipulation of the experiment was improved, leading to reduction in the experimental efforts;(2) saturation with Cs made the negative adsorption of Cl^- negligible, so that Cs adsorption on the clay could directly estimate the negative charge of the clay without correction for the negative adsorption;(3) there was little risk of contamination in determining Cs.
As the result of the combined effect of these merits, the higher reproducibility was obtained to estimate the amount of the negative charge using Cs⁺ as an index cation compared with the conventional method. In addition, flocculation effect of Cs saturation enabled the measurement of the higher pH system, which had been previously difficult.