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Liquid phase and Freundlich isotherms

Liquid phase adsorption isotherm testing

Liquid phase isotherms: Shaking water bath

Liquid Phase Isotherms are based on an empirical Freundlich isotherm that shows the relationship between the residual concentration of a compound in a liquid against the loading of a compound on the carbon. A liquid phase isotherm is a plot of the amount of impurities adsorbed per unit weight of carbon versus the residual concentration in the solution.

For single compound solutions, generally, straight line plots can be obtained by making use of the empirical Freundlich equation, which relates the amount of impurity in the liquid phase to the amount adsorbed onto the activated carbon by using the expression:

X/M = kC1/n

where X/M = loading, is the amount of impurity adsorbed (X) per unit weight of carbon (M);

C = equilibrium concentration after adsorption;

k, n = constants.

Taking logarithms, we obtain:

log X/M = log k + 1/n log C.

This is the equation of a straight line whose slope is 1/n and whose intercept is k at C = 1. Therefore, if X/M is plotted against C on log-log paper, a straight line should be obtained. There are, however, occasions when deviations will occur. For example, isotherms of liquids containing multi-components may have a line with multiple slopes.

Liquid phase adsorption isotherms and activated carbon

Liquid phase isotherm of ORGANOSORB® activated carbon

A liquid phase adsorption isotherm is obtained experimentally by adding increasing amounts of pulverised activated carbon to different flasks containing fixed volumes of the liquid to be treated. The activated carbon is pulverised to reduce kinetic effects allowing the carbon to reach equilibrium as quickly as possible. One of the flasks does not contain carbon - this is the blank.

The flasks are then stirred, normally overnight, at a constant temperature to reach equilibrium. The pulverized activated carbon is then filtered from the sample and the remaining liquid analysed for the concentration of the compound being adsorbed. The concentration in each sample and in the blank is then determined. From these measurements, all the values necessary to plot an isotherm may be calculated and the theoretical amount of the granular activated carbon (GAC) necessary to meet the treatment objective can be calculated.

The detailed experimental procedure can be downloaded here. This experimental procedure is intended as a guideline only and not as a substitute for personal contact with our experienced technical staff. The DESOTEC engineers can always help to select the right activated carbon and correct operation conditions. You can always reach us!

Compare different activated carbon types thanks to isotherm tests!

From an isotherm we can obtain information such as:

  1. Can the desired treatment objective be obtained?
  2. What is the theoretical activated carbon consumption?

An isotherm test is an easy and quick method for comparing different activated carbons and for investigating the effects of pH and temperature to select the best isotherm test conditions or to run an isotherm in our technical support laboratory.

On the other hand, an isotherm test is a static test that gives no information on:

  • The impact of plant fluctuations;
  • The adsorption kinetics and thus the length of the Mass Transfer Zone (MTZ);
  • Presence of suspended solids;
  • The impact of microbiology causing biological degradation;
  • Heat of adsorption for organics during the wetting.

This information can be obtained by running a pilot test using a MOBICON® mobile adsorption system. Interested? Don’t hesitate to contact us!