Studies Of Ion Exchange Equilibria
STUDIES OF ION EXCHANGE EQUILIBRIA
AT LOW- AND HIGH-SILICON
CLINOPTILOLITES INVOLVING Na+, K+ AND Ca2+ IONS
Y. I. Tarasevich, D. A. Krysenko, V. E. Polyakov
A.V. Dumansky Institute of Colloid Chemistry and Chemistry of Water, Ukrainian National Academy of Sciences,
42 Vernadsky Avenue, Kyiv (Kiev) 03680, Ukraine
For a number of low- and high-silicon clinoptilolites possessing different SiO2:Al2O3 and (Na++K+):Ca2+ ratio in the exchange complex, taken from the deposits in Bulgaria, Georgia, Russia, Ukraine and United states, the studies of ion exchange equilibria involving Na+, K+ and Ca2+ ions are reported.
The ion exchange isotherms were built and concentration exchange constants К/С were determined. The dependencies of lgК/С() on the degree of occupation of solid phase by adsorbed cation , proposed by Kjelland, were used to calculate the thermodynamical exchange constants KQ.
From the analysis of the obtained results it was shown that the high-silicon clinoptilolites, which exist in their natural state as the sodium/potassium substances, exhibit the increased selectivity with respect to K+ cations, while the low-silicon clinoptilolites are characterised by increased selectivity with respect to Ca2+ ions. The Na-K exchange isotherms for all samples exhibit сonvex shape with various curvatures. The Na-Ca exchange isotherms at high-silicon zeolites possess sigma-like shape, with the variations of selectivity for different values.
The purely sodium clinoptilolites in their natural state are characterised by the variation of selectivity for low occupation of solid phase by Ca2+ cations (≈0.2), while the sodium-calcium kaolinites which possess (Na+K):Ca ratio 50:50 exhibit the selectivity variation at (≈ 0.4-0.6). The clinoptilolites which in the natural state are purely calcium ones are characterised by the сonvex shape of the isotherm (K>1) throughout the whole range of the occupation by Ca2+ cations.
It is concluded that the crystallisation of clinoptilolites in the natural conditions with Na+ and Ca2+ ions has an essential effect on the fine features of
the mineral structure. These features are pronouncedly exhibited by memory effect with respect to the ions which were present during the crystallisation process.
The studies summarised above demonstrate the importance of the combined analysis of ion exchange and structural data in what regards the discussions of the results obtained for the ion exchange equilibria at mineral ion exchangers.
Related articles::