Adsorption Isotherms of Unknown Mixtures

For the sake of simplicity, the adsorption of an aqueous solution of COD which consists of two organic components on activated carbon is considered here. One of the components, whose concentration is Ci, is not adsorbed at all while the other component of concentration Ci has an adsorption isotherm of the Freundlich type, q = when it exists as a single component. COD of the mixture, COA, is the weighted sum of Cl and C2. 

Illustration of shape of the isotherms of mixtures measured hy different methods 

If adsorption isotherm is measured by the batch adsorption technique, i.e. measurement of COD concentration change in vessels with different carbon load, then adsorption equilibrium is attained for Component 2 but coexistence of Component 1 results in an unfamiliar shape of adsorption isotherm with regard to adsorption of COA, as shown in Fig. 3.21. 

Another method of measurement of isotherm is the dilution method, which is performed by preparing several bottles containing raw water diluted to different levels and adding the same amount of adsorbent in the bottles. Then the amount adsorbed in each flask is determined by comparing the initial concentrations and the final concentrations. By this method the 

The difference in isotherms by the two methods which never occurs in single-component measurement is due to the dilution of the unadsorbed component in the latter case. In other words, the existence of an unadsorbable component may be checked by measunng isotherms by these two different methods. 

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