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CATION EXCHANGE CAPACITY MEASUREMENTS ON ILLITE USING THE SODIUM AND CESIUM ISOTOPE DILUTION TECHNIQUE: EFFECTS OF THE INDEX CATION, ELECTROLYTE CONCENTRATION AND COMPETITION: MODELING

Laboratory for Waste Management, Paul Scherrer Institut, CH-5232 Villigen PSI, Switzerland

^* E-mail address of corresponding author: bart.baeyens{at}psi.ch

The isotope dilution technique using Na and Cs as index cations was used to determine the cation exchange capacity (CEC) of illite du Puy as a function of background electrolyte composition. The work showed, in accord with previous studies, that the CEC values were in the order Cs-CEC > Na-CEC. Sodium is commonly chosen as the index cation in CEC determinations using the isotope dilution method. The experimentally measured Na-CEC values for Na-illite increased from ~75 to ~200 meq kg^–1 for NaClO₄ concentrations in the range 5.6x10^–4 to 1.25x10^–2 M. Cesium CEC determinations showed a much less pronounced trend over a CsNO₃ concentration range from 10^–3 to 10^–2 M. A reference Cs-CEC value of 225 meq kg^–1 was chosen. Careful chemical analyses of the supernatant solutions revealed that Ca and Mg at the (sub)µmolar level were present in all the determinations, despite the extensive conditioning procedures used. Competition between (Ca + Mg) and Na for the exchange sites was put forward as an explanation for the variation of Na-CEC values. This hypothesis was confirmed in a series of single (⁴⁵Ca) and double (⁴⁵Ca plus ²²Na) labeling experiments. Calcium-sodium selectivity coefficients (_Na^CaK_c) were calculated from the experimental data for NaClO₄ concentrations from 5.6x10^–4 to 0.1 M and exhibited a variation from 1.6 to 14.3. A two-site cation exchange model was developed with site capacities and (_Na^CaK_c) values for each site: planar site capacity =180 meq kg^–1, _Na^CaK_c^Ps = 2; type II site capacity = 45 meq kg^–1, _Na^CaK_c^II = 80. The model was able to predict the Na and Ca occupancies in the Na-CEC experiments over the whole range of NaClO₄ concentrations. It is recommended that Cs should be used instead of Na as the index cation for determining the CEC of illite.

Key Words: Cation Exchange Capacity (CEC) • Cesium • Illite du Puy • Isotopic Dilution Technique • Two-site Cation-exchange Model