Kinetic investigation of water physisorption on natural clinoptilolite at room temperature

Charge balancing extra-framework cations play a fundamental role in the adsorption/desorption of polar molecules on zeolites. The way cations interact with polar molecules (e.g. water) in combination with the negatively charged framework and the kinetics of such interaction is extremely important for designing novel devices based on these functional materials. Kinetic investigation and mechanistic modeling of these processes can be conveniently performed by electrical tests, based on biasing the clinoptilolite sample with a constant a.c. voltage (20V, 5kHz) and monitoring the temporal evolution of the current intensity during the adsorption/desorption processes. According to this kinetic analysis, adsorption is described by the Lagergren pseudo-first-order model with a rate constant of (5.86 ± 0.09)·10 min, while water desorption in dry air follows a first-order kinetic model with a specific rate of (2.02 ± 0.003)·10 min at 25°C. According to these models, the water molecule to be adsorbed is first caught by the cation electric field and then transferred at the cation-framework interface to maximize non-bonding interactions.