Kinetics and mechanism of dehydration of natrolite and its potassium exchanged form

A kinetic analysis was done of the dehydration in natrolite and its corresponding potassium exchanged form. Kinetic evaluation of the thermogravimetric data based on Ozawa's methods yielded the following results: the activation energy is 126.83 kJ mol^-1 for natrolite and 74.01 kJ mol^-1 for the potassium form, the frequency factor is 1.92 × 10⁹ s^-1 for natrolite and 3.31 ×10⁷ s^-1 for the potassium form, and the dehydration follows zero-order kinetics in natrolite and first-order kinetics in the potassium form. Considering the crystal structures of the hydrated and dehydrated phases of natrolite and its potassium form, together with the results of the kinetic analysis, it is suggested that the dehydration in natrolite proceeds at a constant rate in one direction parallel to the c axis, while in the potassium exchange form the dehydration is instantly completed in each particle and then proceeds particle by particle.