Powdered zeolites are used as a desiccant in the preservation of many types of vegetable foods (e.g., cereal grain, corn, etc.). Natural clinoptilolite is a very abundant, inexpensive, nontoxic, regenerable, and environmentally friendly zeolite with good desiccant properties. Here, water adsorption/desorption properties of natural clinoptilolite have been investigated by a novel technique based on a.c. electrical measurements. In particular, owing to the presence of extra-framework cations, zeolites are ionic conductors. The presence of water in cationic sites significantly modifies cation mobility, because strong electrostatic interactions act between cations and nucleophilic areas in 3D-frameworks, and non-hydrated cations have a near zero mobility, while hydrated cations have enough mobility at room temperature. The type of law controlling the adsorption/desorption process has been established by monitoring the real-time behavior of relative current intensity moving in the sample surface biased by a sinusoidal voltage signal of 20Vpp (5 kHz) and exposed to a constant moisture atmosphere (75%) at 25 °C. An intergranular diffusion control was active at the beginning of hydration because of the lamellar texture, then Lagergren irreversible pseudo-first-order kinetics took place. To confirm the adsorption mechanism and possibility of regenerating the clinoptilolite desiccant, dehydration by silica gel was electrically monitored and an exponential kinetic law found.
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