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Low Temperature Dehydration Kinetics of Aluminum- and Calcium-saturated Soil Clays¹

ABSTRACT

The kinetic parameters of the isothermal dehydration of standard bentonitic and soil clays with montmorillonitic, mixed, illitic, and kaolinitic mineralogy were studied under Al and Ca saturation. The isothermal temperatures employed under a constant water vapor pressure of P/P_o = 0.995 were 32, 38, 46, and 54°C. Dehydration rates under Al saturation were lower than those observed under Ca saturation in all clays. The differences were greater in clays with abundant interlayer space. The reaction mechanisms best describing the dehydration process were either a first-order, random nucleation (F₁) or a phase-boundary controlled mechanism with spherical (R₃) or cylindrical (R₂) symmetry. The cylindrical symmetry, phase boundary was only applicable to the kaolinitic clays. The mechanisms were mostly independent of the type of the saturation (Al or Ca). Dehydration rate constants (k) were consistently lower and activation energies (E_a) of dehydration were generally higher in Al-saturated clays. These kinetic relationships indicate that increased Al saturation of the exchange phase may slow down considerably the dehydration process by reducing dehydration rate constants and by requiring greater energy for the initiation of the desorption reaction.

¹ Contribution from the Dep. of Agronomy, Univ. of Kentucky, Agric. Exp. Stn., Lexington, KY 40546. The investigation reported in this paper (no. 86-3-8) is in connection with a project of the Kentucky Agric. Exp. Stn. and is published with approval of the Director.

² Assistant Professor and Associate Professor, respectively, Dep. of Agronomy, Univ. of Kentucky, Lexington, KY 40546.

Received for publication August 22, 1986.