The adsorption data of Gorbach et al. (1) and Morris (2) for the adsorption of water on 4A zeolite pellets is re-analyzed. Model isotherms are derived considering a two site hypothesis, one for the alpha cage and one for the beta cage. Four simple model isotherms are fitted to the data. Both a dual site Toth or Langmuir isotherm model fit the data adequately.

The optimized standard enthalpy and entropy of adsorption parameters derived from the data are surprising for the beta cage,. The optimized standard enthalpy of the beta cage is 1/3rd of that observed calorimetrically, and the standard entropy of adsorption is positive, a physical impossibility. Substituting the calorimetric enthalpy of adsorption corrected the standard differential entropy of sorption values resulting in the standard entropy of sorption values varying significantly with temperature. This variation is postulated to be due to the formation of clusters of water, such as dimers, trimers etc.

KEYWORDS:

Adsorption, water, 4A (NaA) zeolite, Langmuir isotherm, Toth isotherm, Ruthven isotherm, dual site isotherm, standard enthalpy of adsorption, standard entropy of adsorption,