Adsorption

What is an Adsorption? An Adsorption is a process in which molecules or particles adhere to the surface of a solid or liquid substance without penetrating its structure. This adherence occurs due to attractive forces between the adsorbate (substance being adsorbed) and the adsorbent (surface on which adsorption occurs). So in this topic we will discuss what are the characteristics of adsorption and types of Absorption.

1.0Introduction to Adsorption

Adsorption definition involves  a surface-based phenomenon where molecules or particles accumulate on the surface of the adsorbent, leading to a higher concentration of the adsorbate at the interface. Adsorption plays a significant role in various fields, including chemical engineering, environmental science, and industrial processes like purification and filtration. In this topic we will discuss some important adsorption characteristics.

Adsorbate refers to the substance that adheres to or accumulates on the surface of another material. Examples include gases like H₂, N₂, and O₂.

Adsorbent, on the other hand, refers to the surface of a substance onto which the adsorbate attaches or accumulates. Materials like charcoal, silica gel, and alumina are examples of surfaces capable of adsorbing substances.

2.0Difference between Absorption and Adsorption

The difference between absorption and adsorption lies in the way substances interact with another substance or surface:

In summary, the process of absorption involves penetration and assimilation of one substance into another, while adsorption involves molecules adhering or accumulating on the surface of another substance without penetrating it.

3.0Types of Adsorption 

Here we will discuss some of the important adsorption types. Which involves mainly Physisorption and Chemisorption. For Further study of adsorption concept we will discuss some other types and characteristics of adsorption-

Physical Adsorption (Physisorption): 

• This is an important and most useful phenomena from adsorption types, Thus we will discuss physical adsorption characteristics in brief.
• This adsorption process involves weak van der Waals forces or London dispersion forces between the adsorbate and the adsorbent. It typically occurs at lower temperatures and is reversible. Physisorption does not involve significant chemical bonding between the molecules and the surface.

Chemical Adsorption (Chemisorption): 

• Chemical adsorption characteristics involve stronger chemical bonds formed between the adsorbate and the adsorbent surface.
• Main important characteristics of chemical adsorption is that It typically occurs at higher temperatures and involves the sharing or exchange of electrons between the adsorbate and the surface atoms. Chemisorption is often more specific and leads to stronger adsorption than physisorption.

Specific Adsorption: 

• This refers to the adsorption of specific molecules onto specific sites on the surface of the adsorbent. It's highly selective and occurs due to particular interactions between the adsorbate and the adsorbent surface.

Non-specific Adsorption: 

• Unlike specific adsorption, this type of adsorption is non-selective and occurs without specific interactions or preference for certain molecules or sites on the surface.

Biological Adsorption: 

• This Adsorption process involves the adsorption of biological molecules, such as proteins or enzymes, onto surfaces. It plays an important role in various biological processes, including cell signaling, drug interactions, and enzyme reactions.

Ion Exchange Adsorption: 

• Ion exchange adsorption is a process where ions in a solution are replaced by ions of similar charge from a solid material, typically a resin. This selective exchange helps in separating or purifying substances in the solution based on their ionic properties.

4.0Adsorption Isotherm

An adsorption isotherm is a graphical or mathematical representation that illustrates the relationship between the amount of adsorbate (substance being adsorbed) and its concentration in the surrounding medium at a constant temperature. This relationship helps describe how adsorbate molecules interact with an adsorbent surface under specific conditions, typically represented as a plot of the amount of adsorbate adsorbed versus the pressure (for gases) or concentration (for liquids) at equilibrium.

Adsorption Process Diagram  

An adsorption process diagram typically illustrates the relationship between the amount of adsorbate (substance being adsorbed) and either pressure (for gases) or concentration (for liquids) at a constant temperature. It shows how the adsorbate quantity changes concerning the external conditions while the adsorption occurs.

Several models and equations describe adsorption isotherms, with the most common being:

Langmuir Isotherm: Assumes a monolayer adsorption onto a homogeneous surface, suggesting a finite number of identical sites on the adsorbent surface and no interaction between adsorbed molecules.

•  It expresses a relationship between the adsorbed quantity and the pressure or concentration of the adsorbate.

Freundlich Isotherm: Describes heterogeneous surface adsorption, indicating that the adsorption intensity decreases as the surface coverage increases.

• It represents a non-ideal adsorption scenario and describes multilayer adsorption.
• Freundlich adsorption isotherm is obeyed by the adsorption where the adsorbate forms single layer on the surface of the adsorbent.
• Freundlich adsorption isotherm equation; n ≥ 1

And, in logarithmic form 

$\frac{\mathrm{x}}{\mathrm{m}}={\mathrm{kP}}^{\frac{1}{\mathrm{n}}}\log \frac{x}{m}=\log \mathrm{k}+\frac{1}{n}\log \mathrm{P};0\le \frac{1}{n}\le 1$

In general, $\frac{1}{n}\text{ is }0.1\text{ to }0.5\text{. }$

where x is the weight of the gas adsorbed by m g of the adsorbent at a pressure p, thus x/m represents the amount of gas adsorbed on the unit mass of adsorbent, k and n are constant at a particular temperature and for a particular adsorbent and adsorbate (gas), n is always greater than one, indicating that the amount of the gas adsorbed does not increase as rapidly as the pressure.

BET (Brunauer-Emmett-Teller) Isotherm: Often used for physical adsorption on a solid surface, especially for monolayer adsorption onto porous materials. 

• It assumes a homogeneous surface and provides insights into the specific surface area and the formation of multilayers.

5.0Applications of Adsorption 

In this section we will discuss some important and widely used applications of Adsorption in brief.

1. Water and Wastewater Treatment: Adsorption is widely used to remove pollutants, heavy metals, organic compounds, and dyes from water and wastewater. Materials like activated carbon, zeolites, and various adsorbent polymers are employed for purification.

2. Air Purification: Adsorption is utilized in air purifiers and filters to remove volatile organic compounds (VOCs), odors, and harmful gases from indoor air, improving air quality.

3. Gas Storage and Separation: Adsorption is crucial in storing gases, like hydrogen or methane, in porous materials for applications in fuel storage, natural gas storage, and gas separation processes.

4. Catalysis and Chemical Processes: Adsorption plays a vital role in catalysis by providing active sites for chemical reactions. Catalysts often involve surfaces where reactants adsorb, facilitating reactions and enhancing their efficiency.

5. Pharmaceuticals and Drug Delivery: Adsorption is employed in drug delivery systems, where pharmaceuticals are adsorbed onto carrier materials to control drug release rates and improve drug stability.

6. Food and Beverage Industry: Adsorption is used for various purposes in food technology, including decolorization, removal of impurities, and preserving food quality by adsorbing moisture or undesirable flavors.

7. Chromatography : Adsorption process used in chromatography method to separate and analyze mixtures based on differential adsorption affinities of components to a stationary phase, aiding in scientific research and analytical chemistry.

8. Energy Storage : Scientists are looking into using adsorption for storing energy, specifically by creating effective adsorbents. These materials would be designed to capture and store carbon dioxide (CO₂) emissions from industrial activities, helping reduce their impact on the environment.

9. Surface Modification : Adsorption processes are used to modify surfaces, making them hydrophobic, improving adhesion, or enhancing properties like hardness and wear resistance in materials science.

10. Biomedical Applications : Adsorption is applied in various biomedical fields for biomolecule purification, DNA separation, and diagnostic assays.