| Criteria | Absorption | Adsorption |
| Definition | Assimilation of the molecular system throughout the bulk of the solid or liquid medium. | Accumulation of molecular species at the bottom instead of the liquid or solid. |
| Adsorption is different from absorption( which involves the incorporation of a substance into the bulk of a material). | In adsorption, the adsorbate molecules are held onto the adsorbent surface by weak intermolecular forces, such as
· van der Waals forces · electrostatic forces · hydrogen bonding |
|
| Phenomenon | A bulk phenomenon. | A surface phenomenon. |
| Rate of reaction | Occurs at a uniform rate. | Increases steadily and reaches equilibrium. |
| Temperature | The temperature has no effect. | Influenced by low temperature |
| Concentration | It is constant throughout the medium. | The Concentration at the bottom of adsorbent is different from that in bulk. |
| Heat exchange | Endothermic process | Exothermic process |
Adsorption
Adsorption is a phenomenon by which a substance in a gas, liquid, or dissolved solid state showing their attraction and stick to the surface of a solid material, usually referred to as an adsorbent.
This attraction among the adsorbent and adsorbate results in a accumulation of the adsorbate at the surface of the adsorbent, which can cause a changes in the characteristics of both the adsorbent and adsorbate.
Factor Affecting Adsorption
factor affecting the absorption are given below:
Surface area of the adsorbent:
Adsorption has direct relation with the surface area of the adsorbent. A smaller surface area means that there are less sites available for the adsorbate to interact with, which decrease the adsorption capacity.
Temperature:
Temperature can affect adsorption by altering the kinetic energy of the adsorbate molecules, which as result affect their capability to react with the adsorbent surface. In general, adsorption decreases with increasing temperature, as the adsorbate molecules have less kinetic energy and shows more attraction towards the adsorbent surface.
Pressure:
For gas-phase adsorption, the pressure of the adsorbate can affect adsorption. As the pressure decreases(there are less gas molecules available to interact with the adsorbent surface)
which decreases the adsorption capacity.
Nature of the adsorbate and adsorbent:
The chemical and physical properties of both the adsorbate and adsorbent would have impact on adsorption. For example
- polar adsorbents tend to adsorb polar molecules
- while nonpolar adsorbents tend to adsorb nonpolar molecules.
pH:
The pH of the solution can affect adsorption
- by altering the surface charge of the adsorbent.
- Depending on the nature of the adsorbent
- changing the pH can either increase or decrease adsorption.
Presence of other substances:
The presence of other substances in the solution can affect adsorption by
- competing for adsorption sites on the adsorbent surface
- or by altering the properties of the adsorbate.
Adsorption Isotherm
Adsorption isotherm is a graphical description of the relationship among the concentration of an adsorbate in a solution and the quantity of adsorbate that is adsorbed onto the surface of an adsorbent at a constant temperature. Commonly it is expressed as a plot of the adsorbed amount (adsorbate per unit mass of adsorbent) against the equilibrium concentration of the adsorbate in the solution.
The shape of the adsorption isotherm can provide insights
- into the nature of the adsorption process
- and the properties of the adsorbent and adsorbate.
Types of adsorption isotherms
There are many types of adsorption isotherms, including:
Langmuir isotherm:
This isotherm suppose that the adsorption occurs at specific sites on the adsorbent surface, and that there is a limited number of sites open for adsorption. The Langmuir isotherm is characterized by a saturation point at high concentrations, where more increases in concentration have no impact on the quantity of adsorbate adsorbed.
The Langmuir isotherm model is one of the most broadely used types to explain the adsorption of a solute onto a solid surface. It is founded on the following assumption that
- there is a fixed number of identical energetically equivalent adsorption sites on the surface
- and the adsorption occurs in a step-wise fashion without interactions between adsorbate molecules.
Mathematical representation:
The Langmuir isotherm can be explained mathematically as:
θ = (K_adsC)/(1+K_adsC)
where
- θ is the fraction of the surface covered by adsorbate
- K_ads is the Langmuir constant
- and C is the concentration of the adsorbate in the bulk solution.
important characteristics of Langmuir isotherms:
This isotherm has some important characteristics which are given below:
No lateral interactions:
The Langmuir isotherm suppose that there is no sideways interactions between adsorbed molecules.
Homogeneous surface:
The Langmuir isotherm suppose that the surface of the adsorbent is homogeneous, and all the adsorption sites have the equal energy.
Monolayer adsorption:
The Langmuir isotherm suppose that the adsorbate molecules form a monolayer on the surface of the adsorbent, signifying that there is a maximum adsorption ability beyond which no more adsorbate can be adsorbed.
Adsorption is reversible:
The Langmuir isotherm suppose that the adsorption is reversible, meaning that the adsorbate can desorb from the surface under appropriate conditions.
Equilibrium adsorption:
The Langmuir isotherm suppose that the adsorption system reaches equilibrium, signifying that the rate of adsorption becomes equals to the rate of desorption.
Uses of Langmuir isotherm
The Langmuir isotherm can be used to
- determine the maximum adsorption capacity of an adsorbent,
- as well as the affinity of the adsorbate for the adsorbent surface.
- The Langmuir constant, K_ads, is related to the adsorption energy, and a higher value of K_ads indicates a stronger interaction between the adsorbate and the adsorbent surface
- . The Langmuir isotherm is commonly used in the design and analysis of adsorption processes in various applications, such as water treatment and gas separation.
Freundlich isotherm:
This isotherm is used to explain heterogeneous adsorption systems where the adsorption occurs on different kinds of sites with different energies. The Freundlich isotherm is characterized by
- a non-linear relationship between the amount of adsorbate adsorbed
- the equilibrium concentration
- and it does not assume a maximum adsorption capacity.
Explanation:
The Freundlich isotherm is another commonly used model to explain the adsorption of a solute onto a solid surface. It was given by Herbert Freundlich in 1906 and is founded on the statement that the adsorption occurs on heterogeneous sites of changing energies and strengths.
Mathematical representation:
The Freundlich isotherm can be expressed mathematically as:
q = K_f*C^(1/n)
where
- q is the amount of solute adsorbed per unit mass of adsorbent
- C is the concentration of the solute in the bulk solution
- K_f is the Freundlich constant
- and n is the Freundlich exponent.
Important characteristics of Freundlich isotherm
The Freundlich isotherm has many important characteristics such as:
Heterogeneous surface:
The Freundlich isotherm suppose that the surface of the adsorbent is heterogeneous, and the adsorption can happen on sites of varying energies and strengths.
Multilayer adsorption:
The Freundlich isotherm suppose that the adsorbate molecules can form many layers on the surface of the adsorbent.
Non-equilibrium adsorption:
The Freundlich isotherm does not suppose;
- that the adsorption system reaches equilibrium
- and the rate of adsorption may not equal the rate of desorption.
No lateral interactions:
The Freundlich isotherm suppose that there are no sideways interactions between adsorbed molecules.
Uses of freundlich isotherm
The Freundlich isotherm can be used to;
- determine the affinity of the adsorbate for the adsorbent surface
- as well as the heterogeneity of the surface.
- The Freundlich exponent, n, is related to the surface heterogeneity, and a higher value of n indicates a more heterogeneous surface.
- The Freundlich isotherm is commonly used in the design and analysis of adsorption processes in various applications such as;
- wastewater treatment
- gas separation
- and catalysis.
Drawback of freundlich isotherm:
However, it is important to mention that the Freundlich isotherm is not appropriate for defining systems where the adsorption happen on a limited number of actively equivalent sites.
BET isotherm:
This isotherm is used to explain the adsorption of gases onto solid surfaces, and it suppose that the adsorbate molecules form a monolayer on the surface of the adsorbent. The BET isotherm is characterized by a plateau at high concentrations, where the adsorption volume is limited by the creation of a monolayer.
The BET (Brunauer, Emmett, and Teller) isotherm is a widely used model to describe the physical adsorption of gases onto solid surfaces.
Explanation:
It was developed by Stephen Brunauer, Paul Emmett, and Edward Teller in 1938 and is founded on the assumption that the adsorption occurs on a homogeneous surface with a finite number of actively equivalent sites.
Mathematical representation;
The BET isotherm can be expressed mathematically as:
p/p_0 = (C_1 – C_1*x)/(1 – x) * (1/(1 + (B – 1)x))
where
- p is the equilibrium pressure of the adsorbed gas
- p_0 is the saturation vapor pressure of the gas at the temperature of adsorption
- C_1 is a constant related to the monolayer capacity of the adsorbent
- B is a constant related to the heat of adsorption
- and x is the fraction of the surface covered by the adsorbate.
Important characteristics of BET isotherm
The BET isotherm has many important characteristics:
Monolayer adsorption:
The BET isotherm suppose that the adsorbate molecules shape a monolayer on the surface of the adsorbent, signifying that there is a maximum adsorption capability beyond which no more adsorbate can be adsorbed.
Homogeneous surface:
The BET isotherm suppose that the surface of the adsorbent is homogeneous, and all the adsorption position have the same energy.
No lateral interactions:
The BET isotherm suppose that there are no sideways interactions between adsorbed molecules.
Multilayer adsorption:
The BET isotherm taking into account the production of multiple layers of adsorbate on the surface of the adsorbent.
Uses of BET isotherms;
The BET isotherm can be used
- to determine the monolayer capacity
- and the heat of adsorption of the adsorbent.
- The BET constant, B, is related to the heat of adsorption, and a higher value of B indicates a stronger interaction between the adsorbate and the adsorbent surface.
- The BET isotherm is commonly used in the characterization of porous materials, such as
- activated carbon
- zeolites
- and metal-organic frameworks.
- It is also used in the design and analysis of gas separation and purification processes.
Note:
Understanding the shape and characteristics of adsorption isotherms is necessary for designing and optimizing adsorption processes for various applications, such as
- water treatment
- gas separation
- and catalysis.
