In this work subtleties of application of BET isotherm for liquid phase adsorption is presented. It has been shown that direct use of the classical BET equation (which was developed for gas phase adsorption) to liquid phase adsorption leads to ambiguous and erroneous results. Some cases of misuse of BET equation for liquid phase adsorption have been revisited. By close examination of the development of the classical equation, the causes of misunderstandings were elucidated and the suitable form of the BET equation for liquid phase adsorption was developed. As case studies, the classical form of the BET equation along with the correct form of the equation for liquid phase have been applied for modeling liquid phase adsorption of methyl tertbutyl ether (MTBE) on perfluorooctyl alumina, phenol on activated carbon and pentachlorophenol on carbonized bark. It has been shown that direct application of the classical BET isotherm to liquid phase adsorption results in poor and erroneous estimation of the equation parameters. For example, in aqueous phase adsorption of MTBE on perfluorooctyl alumina, the monolayer adsorption capacity of the adsorbent was calculated as 9.7 mg/g instead of 3.3 mg/g or the saturation concentration of MTBE in water was calculated as 1212 mg/L instead of 42000 mg/L.

The performance of two types of perfluorooctyl alumina (PFOAL) adsorbents prepared using c-alumina (PFOAL A ) and alumina-active-basic® (PFOAL B ) is reported. The equilibrium adsorption behavior of the adsorbents was studied for adsorption of methyl tert-butyl ether (MTBE) in a wide range (5-1000 mg/L) of aqueous phase MTBE concentrations. The monolayer adsorption capacities were 3.3 and 3.5 mg MTBE/g adsorbent, and the maximum adsorption capacities were 7.1 and 6.3 mg MTBE/g adsorbent for PFOAL A and PFOAL B, respectively. The application of Freundlich, Langmuir, BET and BDDT (the general form of BET) isotherms were compared for the modeling of aqueous adsorption of MTBE on the surface of PFOAL adsorbents. It was shown that adsorption of MTBE on the surface of PFOAL is a type IV van der Waals adsorption, which is best modeled by the general form of the BET (BDDT) multilayer adsorption isotherm.

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