The calculation of the absolute adsorption isotherm of binary solution on zeolite NaX based on the experimental limiting adsorption values of pure adsorbates
Abstract
Adsorption from solution of nonelectrolytes by microporous adsorbents such as zeolites is not only of practical interest. Because of the well-defined internal structure of zeolite, it can be a good model system which provides the possibility to measure absolute adsorption in a direct experiment. Such data may lead to a better understanding the properties of the liquid solution in the micropores. Whereas the excess adsorption isotherm can easily be measured, the exact determination of the absolute adsorption isotherm requires rather painstaking and time-consuming experiments. So the main problem is to convert excess adsorption data to the absolute adsorption isotherm. The purpose of this work was to suggest an approximate simple method for determining the absolute adsorption isotherm for solutions using as initial data the excess adsorption isotherm and the limiting (absolute) adsorption values for pure components of binary solutions m1 and m2 which are much easier to measure than absolute adsorption isotherms of solutions. The method is based on the Dubinin-Radushkevich equation modified for adsorption from solutions. The equation gives the concentration dependence of the absolute adsorption of solution wsand includes two parameters m1 and m2 and two constants k and n which can be determined if the c1m value (the concentration corresponding to the maximum excess adsorption) is known. Thus, the modified Dubinin-Radushkevich equation is a non-fitting three-parameter m1 and m2 are measured by the pycnometric method. The experimental set-up is described and the procedure and results are reported. Thus, to calculate the absolute (individual) adsorption isotherm w1s by suggested method, one has to determine c1m from the excess adsorption isotherm w1e then calculate ws and finally using the well-known expression relating w1e to w1s find the latter. The method is tested by 4 absolute adsorption isotherms measured earlier experimentally. The calculated values agree very well with the experimental ones (the deviations do not exceed 1.5-2.0 %). The results can be used for calculation of the main physicochemical characteristics of the adsorption phase
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References
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