Water Adsorption on Clay Minerals As a Function of Relative Humidity: Application of BET and Freundlich Adsorption Models.

Water adsorption on kaolinite, illite, and montmorillonite clays was studied as a function of relative humidity (RH) at room temp. (298 K) using horizontal attenuated total reflectance (HATR) FTIR spectroscopy equipped with a flow cell. The water content as a function of RH was modeled using the Brunauer, Emmett, and Teller (BET) and Freundlich adsorption isotherm models to provide complementary multilayer adsorption anal. of water uptake on the clays. A detailed anal. of model fit integrity is reported. From the BET fit to the exptl. data, the water content on each of the three clays at monolayer (ML) water coverage was detd. and found to agree with previously reported gravimetric data. However, BET anal. failed to adequately describe adsorption phenomena at RH values >80%, 50%, and 70% RH for kaolinite, illite, and montmorillonite clays, resp. The Freundlich adsorption model was found to fit the data well over the entire range of RH values studied and revealed two distinct water adsorption regimes. Data obtained from the Freundlich model showed that montmorillonite has the highest water adsorption strength and highest adsorption capacity at RH values >19% (i.e., above ML water adsorption) relative to the kaolinite and illite clays. The difference in the obsd. water adsorption behavior between the three clays was attributed to different water uptake mechanisms based on a distribution of available adsorption sites. Probably different properties drive water adsorption under different adsorption regimes resulting in the broad variability of water uptake mechanisms. [on SciFinder(R)]