Electrical Investigation of the Mechanism of Water Adsorption/Desorption by Natural Clinoptilolite Desiccant Used in Food Preservation

Zeolites are used as desiccant in the preservation of many type of vegetable foods (e.g., corns, onions, etc.). Natural clinoptilolite is a very abundant, inexpensive, nontoxic, regenerable, and environmentally friendly type of zeolite with good desiccant properties. Here, the 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 the cationic site significantly changes the cation mobility, in fact owing to the strong electrostatic interaction acting between cations and the nucleophilic area in the 3D-framework, non-hydrated cations have a nearly zero mobility, while hydrated cations have a good mobility value at room temperature. Therefore, in a zeolite sample the carrier density (that can be simply determined by measuring the temporal evolution of the current intensity, I) increases during the isothermal process of water physical adsorption and such an increase is exactly corresponding to the adsorbed water increase. The type of law controlling the adsorption/desorption process can be established by monitoring the real time behavior of the relative current intensity (I/I₀) moving in the sample surface biased by a sinusoidal voltage signal of 5kHz and exposed to a constant moisture atmosphere. This approach has been applied to a natural clinoptilolite slab exposed to a 75% humidity atmosphere at 25°C. An intergranular diffusion control was active at beginning of hydration because of the natural clinoptilolite fine-grained microstructure of lamellar texture, then a Lagergren pseudo-first-order kinetics took place. To confirm a reversible adsorption process and the possibility of re-using clinoptilolite as desiccant, the slab dehydration (by silicagel) was electrically monitored and an exponential kinetic law found.