What is Adsorption Isotherm?
Adsorption isotherms have played a crucial role in environmental protection and adsorption techniques research. The Freundlich and Langmuir isotherms are the two most common approaches used to predict the adsorption capacity of a given substance.
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An adsorption isotherm is a graph that depicts the relationship between the amount of adsorbate(x) adsorbed on the surface of the adsorbent and the pressure change at a constant temperature.
According to Le Chatelier’s principle, the equilibrium direction of a reaction shifts towards the direction of stress relief. So, it can be shown that when extra pressure is applied to the system, the equilibrium moves in the direction of a decrease in the number of molecules, resulting in a decrease in the system’s pressure.
The graph also reveals that once the saturation pressure, Ps, is reached, the variance in the amount of adsorbent adhering to the adsorbate becomes zero. This occurs because the available surface area for adsorption is restricted, and because all sites are occupied, an increase in pressure has no effect.
Different scientists have proposed many adsorption isotherms, including,
- Langmuir isotherm
- Freundlich isotherm
- BET theory
The graph shown above shows the isotherm proposed by Freundlich.
Freundlich Adsorption Isotherm
Freundlich adsorption describes the change in the amount of gas adsorbed by a unit mass of solid adsorbent in response to a change in the system’s pressure at a certain temperature. The following equation represents the expression for the Freundlich isotherm:
where n>1
Where x represents the mass of the gas adsorbed, m represents the mass of the adsorbent, P represents the pressure, and n is a constant that depends on the nature of the adsorbent and the gas at a certain temperature. By applying the logarithm to both sides of the equation, we obtain:
The plot of this equation is a straight line as represented by the following curve.
Langmuir Adsorption Isotherms
Langmuir adsorption isotherms and the BET theory follow the Freundlich adsorption isotherm. The Langmuir adsorption isotherms predict linear adsorption for low adsorption densities and maximum surface coverage at increasing metal solute concentrations.
The Langmuir adsorption isotherm has the following equation:
Where
- θ is the fraction of the surface covered by the adsorbed molecule.
- K is an equilibrium constant known as the adsorption coefficient.
- { K= ka/kd = rate constant for adsorption/ rate constant for desorption}
- p is the pressure.
When there is no interaction between adsorbed species, Langmuir adsorption is applicable for monolayer adsorption onto a homogenous surface.
BET Adsorption Isotherm
Brunauer, Emmett, and Teller’s 1938 theory of multilayer adsorption (BET Theory) assumes that physisorption results in the production of multilayer adsorption. Additionally, the theory posits that the solid surface contains uniform adsorption sites and that adsorption at one site has no effect on adsorption at neighboring sites.
After the development of a monolayer, the adsorption process can continue with the formation of subsequent layers, such as the second, third, etc.
The equation for BET is
Applications of Adsorption
The applications of adsorption include:
- Gas masks: When coal workers use gas masks, toxic gases are adsorbed on the surface of the mask, preventing their interaction.
- Production of vacuum: Traces of air are adsorbed on charcoal and removed from equipment undergoing evacuation to create a vacuum.
- Removal of moisture: In order to manage humidity, silica gel pellets are utilized for the absorption of moisture in pharmaceuticals and new plastic bottles.
- Removal of color: In order to obtain a clear liquid solution, the juice collected from sugarcane is treated with animal charcoal to remove the coloring ingredient.
- As Catalysts: Appropriate materials are utilized as catalysts so that reactants cling to their surface, hence accelerating the reaction and increasing the rate of reaction.
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Frequently Asked Questions – FAQs
Q.1 What is the purpose of adsorption?
Adsorption is the attachment of a gas, liquid, or dissolved substance to an atomic, ionic, or molecular layer. This technique produces an adsorbent coating on the surface of the adsorbent.
Q.2 What is the purpose of isotherms?
An isotherm is a line drawn at the same temperature linking points on a map or graph. Isotherms are frequently used in meteorology to visualize temperature distribution on the Earth’s surface or on a chart displaying constant or steady pressure.
Q.3 What variables influence absorption?
Substratum temperature, pore volume, degree of saturation, type of molecular sieve, type of adsorbent, and unique substratum surface area are adsorption factors. The width of a football field can correspond to the surface area of 1 gram of molecular sieve.
Q.4 Does adsorption increase with temperature?
Yes, as the temperature rises, the adsorption capacity increases. The adsorption capacity of an adsorbent increases with temperature because dye molecules may receive sufficient energy to interact with the active site of the substrate as the temperature increases.
Q.5 What is the adsorption principle?
Adsorption is also the deposition of molecules of a gas or liquid onto a solid surface. Adsorption is a well-established and efficient method for the treatment of home and industrial wastewater. The most prevalent water treatment method is “adsorption” to the activated carbon layer.
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