Cation-Exchange Hysteresis and Dynamics of Formation and Breakdown of Montmorillonite Quasi-Crystals
- Kirsten Verburg,
- Philippe Baveye and
- Murray B. McBride
The phenomenon of cation-exchange hysteresis seems to suggest that a number of cation-exchange reactions are characterized by an extremely slow kinetic. This, however, is in direct conflict with the fast equilibration times that have been reported in the literature in recent years. These seemingly contradictory observations have been reconciled recently in a conceptual model in which the exchanger is assumed to first undergo a fast reaction, followed by a much slower, second kinetic stage. This conceptual model links cation-exchange hysteresis to hysteresis in the formation and breakdown of quasicrystals during the forward and backward exchange reactions. Because the results in the literature on the kinetics of these processes are contradictory, the objective of this study was to assess their dynamics and long-term kinetics. Initial rates of quasi-crystal formation and breakdown were found to be fast, but the processes proceeded thereafter for at least 5 mo at a slower rate. The results also suggest that the slow kinetics of quasi-crystal formation and breakdown is related to the presence of activation energy barriers for intercrystalline swelling and can cause quasi-crystals to be inherited from the preparation procedure.
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