PURIFICATION IN THE LIQUID PHASE

Adsorption with a LIQUID PHASE carbon is useful to remove organic compounds that originate unwanted color, smell or taste. This technique is, in most cases, the simplest and less expensive option as compared to other techniques, for example: distillation, crystallization, etc.

Although most of the compounds that carbon adsorbs are organic in nature, there are some important inorganic exceptions. Adsorption in the
LIQUID PHASE is the result of two phenomena:

a) Physical Adsorption: caused by Van Der Waals forces.

b) Chemical Adsorption: caused by chemical links

It should be mentioned that adsorption in the LIQUID PHASE is the result of equilibrium between adsorption and desorption, consequently, it is a complex phenomenon that can be influence by many variables.

FREUNDLINCH EQUATION
There is an empirical equation that has proven useful to predict the behavior of activated carbon in most of the liquid phase applications.

Freundlich Equation

(A constant temperature)
X = amount of adsorbed impurities
M = carbon dosage
C = residual impurity concentration
Kn = constants

If we draw this equation in logarithm paper, we will get a straight line.

This graph is known as Isotherm of Freundlich and is very useful when evaluating the behavior of activated carbon for a certain application, and to find the appropriate dosage.

It is interesting to emphasize that if the data of carbon dosage vs.The average of removed impurities for a certain application are drawn in a graph, a graph like this one is obtained:

Where it can be seen that there is a range in which activated carbon is efficient, but there comes a time when although more carbon is added, the rate of removal is lower.

An activated carbon generally adsorbs from 10% to 60% of its weight in impurities. Unfortunately, in the LIQUID PHASE the impurity that has to be removed is usually a mixture of compounds whose precise composition is rarely known.

Therefore, producing isotherms is highly important. It should be mentioned that the isotherm will only apply to the same conditions under which it was produced, and by changing any one of them, the isotherm may change significantly.

It is obvious that for a compound to be adsorbed by the activated carbon, its molecules must penetrate the carbon pores, thus, their diameter must be bigger than the impurity molecules themselves.

It has been noticed that in the LIQUID PHASE most of the molecules are medium size or big, and that they require carbon with a great amount of mesopores.

Considering that adsorption is an equilibrium process, any impurity that has a similarity with the product in which it is present, will make adsorption difficult. For example, a highly soluble pollutant in the medium it is present will be more difficult to adsorb than one with a medium or low solubility.

During the adsorption process one of the generally more critical steps is the diffusion of the impurities to be removed towards the external surface of the carbon, consequently any variable that affects diffusivity can also affect adsorption.

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