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 temperature (298 K)using horizontal attenuated total reflectance (HATR)Fourier transform infrared (FTIR)spectroscopy equipped with a flow cell percent 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 analysis of water uptake on the clays percent A detailed analysis of model fit integrity is reported percent From the BET fit to the experimental data, the water content on each of the three clays at monolayer (ML)water coverage was determined and found to agree with previously reported gravimetric data percent However, BET analysis failed to adequately describe adsorption phenomena at RH values greater than 80 percent, 50 percent, and 70 percent RH for kaolinite, illite, and montmorillonite clays, respectively percent 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 percent Data obtained from the Freundlich model showed that montmorillonite has the highest water adsorption strength and highest adsorption capacity at RH values greater than 19 percent (i percente percent, above ML water adsorption)relative to the kaolinite and illite clays percent The difference in the observed water adsorption behavior between the three clays was attributed to different water uptake mechanisms based on a distribution of available adsorption sites percent It is suggested that different properties drive water adsorption under different adsorption regimes resulting in the broad variability of water uptake mechanisms percent