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Soil Environmental Remediation LabChonbuk National UniversitySoil Environmental Remediation LabChonbuk National University

Soil Environmental Remediation LabChonbuk National University

Adsorption isotherm

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Adsorption isotherm

In order to optimize the design of an adsorption system to remove the adsorbate, it isimportant to establish the most appropriate correlations for the equilibrium data foreach system. The following isotherm models were used to describe the equilibriumrelationship between the dye adsorbed onto the adsorbent and that left in solution.

Soil Environmental Remediation LabChonbuk National University

Adsorption isotherm

Fig. 14. Examples of various adsorption isotherm.

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Soil Environmental Remediation LabChonbuk National UniversitySoil Environmental Remediation LabChonbuk National University

Soil Environmental Remediation LabChonbuk National University

Langmuir isotherm

The Langmuir equation relates the coverage or adsorption of molecules on a solid surface togas pressure or concentration of a medium above the solid surface at a fixed temperature.The equation was developed by Irving Langmuir in 1916. The equation is stated as:

or theta is the fractional coverage of the surface, P is the gas pressure or concentration, alpha is a constant.The constant is the Langmuir adsorption constant and increases with an increase in thebinding energy of adsorption and with a decrease in temperature.

Fig. 15. Langmuir isotherm.

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The Freundlich adsorption isotherm is an adsorption isotherm, which is a curve relatingthe concentration of a solute on the surface of an adsorbent, to the concentration ofthe solute in the liquid with which it is in contact.

In 1909, Freundlich gave an empirical expression representing the isothermal variation ofAdsorption of a quantity of gas adsorbed by unit mass of solid adsorbent with pressure.

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Freundlich isotherm

Fig. 16. Freundlich isotherm.

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Soil Environmental Remediation LabChonbuk National University

Freundlich isotherm

The Freundlich Adsorption Isotherm is mathematically expressed asIt is also written as orIt is also written as

wherex = mass of adsorbate m = mass of adsorbent.p = Equilibrium pressure of adsorbate.

c = Equilibrium concentration of adsorbate in solution.K and n are constants for a given adsorbate and adsorbent at a particular temperature.

At high pressure 1/n = 0 Hence extent of adsorption is independent of pressureBut at high pressure it is dependent on pressureLimitation of Freundlich adsorption isotherm

Experimentally it was determined that extent of adsorption varies directly with pressure tillsaturation pressure Ps is reached. Beyond that point rate of adsorption saturates even afterapplying higher pressure. Thus Freundlich Adsorption Isotherm failed at higher pressure.

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BET theory aims to explain the physical adsorption of gas molecules on a solid surface andserves as the basis for an important analysis technique for the measurement of the specific

surface area of a material.

In 1938, Stephen Brunauer, Paul Hugh Emmett, and Edward Teller published an article aboutthe BET theory in a journal for the first time; BET consists of the first initials of their familynames.

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BET isotherm

Fig. 17. BET isotherm.

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X is the pressure divided by the vapor pressure for the adsorbate at that temperature(usually denoted P / P0), V is the STP volume of adsorbed adsorbate, Vmon is the STP

volume of the amount of adsorbate required to form a monolayer and C is theequilibrium constant Kwe used in Langmuir isotherm multiplied by the vapor pressure ofthe adsorbate.

The key assumption used in deriving the BET equation that the successive heats ofadsorption for all layers except the first are equal to the heat of condensation of the

adsorbate.The Langmuir isotherm is usually better for chemisorption and the BET isotherm worksbetter for physisorption for non-microporous surfaces.

Soil Environmental Remediation LabChonbuk National University

BET isotherm