Thermodynamic analysis of sorption isotherms of bentonite

Sorption isotherms of water vapour on commercial bentonite clay are determined at T = (303, 323, and 343) K. The sorption isotherms have a sigmoid shape (Type II). At a given water activity, moisture content decreases with increasing temperature. Hysteresis between adsorption and desorption isotherms is shown over most the range of water activity varying from 0.1 to 0.9. The fitting of the experimental data by using two theoretical models (Guggenheim–Anderson–DeBoer (GAB) and Henderson) shows that the two models reproduce experimental data with acceptable accuracy. The GAB model, however, is largely superior. The isosteric sorption enthalpy is determined and its dependency on the amount of water retention is investigated. It is found that the enthalpy reaches a maximum value when a monolayer of water covers the adsorbant surface. The enthalpy decreases asymptotically to a finite value when the amount of adsorbed water increases.