CATION-EXCHANGE HYSTERESIS AND DYNAMICS OF FORMATION AND BREAKDOWN OFMONTMORILLONITE QUASI-CRYSTALS

The phenomenon of cation-exchange hysteresis seems to suggest that a n umber of cation-exchange reactions are characterized bg an extremely s low kinetic. This, however, is in direct conflict with the fast equili bration times that have been reported in the literature in recent year s. These seemingly contradictory observations have been reconciled rec ently in a conceptual model in which the exchanger is assumed to first undergo a fast reaction, followed by a much slower, second kinetic st age. This conceptual model links cation-exchange hysteresis to hystere sis in the formation and breakdown of quasicrystals during the forward and backward exchange reactions. Because the results in the literatur e on the kinetics of these processes are contradictory, the objective of this study was to assess their dynamics and long-term kinetics. Ini tial rates of quasi-crystal formation and breakdown were found to be f ast, but the processes proceeded thereafter for at least 5 mo at a slo wer rate. The results also suggest that the slow kinetics of quasi-cry stal formation and breakdown is related to the presence of activation energy barriers for intercrystalline swelling and can cause quasi-crys tals to be inherited from the preparation procedure.