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The cation-exchange behavior of heteroionic and homoionic clays of silicate minerals
Published in -
Volume: 55
Issue: 9
Pages: 1429 - 1446
Exchange measurements made with heteroionic bentonites revealed the existence of two categories of cations taking part in the process. In a binary system the cation which is introduced first becomes more firmly bound than the one introduced next. The differentiation between the loosely and the firmly bound portions of any monovalent cation is not noticeable if it does not constitute at least 30 per cent of the binary system; for a bivalent cation this proportion should be about 50 per cent. As a characterizing quantity of homoionic systems the symmetry value has both advantages and limitations. Complications due to the presence of even small amounts of hydrogen ions can be avoided by making measurements at constant pH. The apparent breakdown of the lyotropic effect at low pH, as measured by the symmetry values, has been shown by direct measurements to be caused by a competition between hydrogen ions and other interacting cations. The hysteresis effect often found in exchange measurements was explained on the basis of the loosely and firmly bound categories of cations postulated in the light of studies on heteroionic systems. Peculiarities of the exchange substance are also responsible for such effects. Exchange isotherms over a wide range of electrolyte concentration varying from 1/16 to 4 times the symmetry concentration were obtained with a number of homoionic clays prepared from a bentonite, a mica, and a kaolinite. The isotherms were characterized, unlike the usual ones, by well-defined inflections. Kaolinites showed two inflections; micas and bentonites, three. The significance of these inflections becomes clear in the light of the fact that the exchange spots on these minerals possess different bonding energies. From the point of view of crystal structure kaolinites have two categories of exchangeable positions, whereas micas and bentonites have three such positions. A consequence of the inequality of the exchange spots is that the symmetry value assumes a limited importance and the base-exchange formulations become inadequate. This was shown by applying the experimental results to two such formulations.
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JournalJournal of Physical and Colloid Chemistry