Adsorption is a phenomenon in which molecules or atoms from a fluid (gas or liquid) adhere to the surface of a solid material. This process occurs due to attractive forces between the adsorbate (the substance being adsorbed) and the adsorbent (the material on which adsorption takes place). It is a crucial process in various industries, including environmental remediation, gas separation, and purification. Understanding the principles of adsorption is essential for designing effective adsorption systems and optimizing their performance.

Fundamentals of Adsorption : 

The fundamentals of adsorption involve several key aspects:

1. Surface Area: Adsorption occurs on the surface of the adsorbent, so a higher surface area provides more active sites for molecules to adhere to.

2. Adsorbate-Adsorbent Interaction: The strength and nature of the interaction between the adsorbate and adsorbent determine the adsorption capacity and selectivity.

3. Equilibrium: Adsorption reaches equilibrium when the rate of adsorption equals the rate of desorption, and no net change in adsorbate concentration occurs on the surface.

4. Temperature: Adsorption is influenced by temperature; some adsorption processes are exothermic, while others are endothermic.

5. Pressure: In gas-phase adsorption, pressure plays a significant role in determining the adsorption capacity and is often described by adsorption isotherms.

6. Adsorption Isotherms: These mathematical models describe the relationship between the amount of adsorbate adsorbed and the equilibrium concentration in the fluid phase.

7. Types of Adsorbents: Different materials can act as adsorbents, such as activated carbon, zeolites, silica gel, and metal-organic frameworks (MOFs).

8. Adsorption Kinetics: The rate at which adsorption occurs and the time required to reach equilibrium depend on the specific adsorption system.

9. Effect of Surface Chemistry: The chemical properties of the adsorbent surface, such as functional groups and polarity, influence the adsorption process.

Understanding these fundamentals is essential for designing and optimizing adsorption processes in various applications, ranging from water and air purification to gas separation and pharmaceutical purification.


Type of adsorbents Development of Adsorption isotherms: Freundlich

Type of adsorbents:

There are various types of adsorbents used in adsorption processes, including:

1. Activated Carbon: A highly porous form of carbon with a large surface area, widely used for gas and water purification.

2. Zeolites: Crystalline materials with a unique porous structure, used for molecular sieving and catalysis.

3. Silica Gel: A non-toxic and chemically stable adsorbent commonly used in desiccants and chromatography.

4. Metal-Organic Frameworks (MOFs): Porous materials composed of metal ions coordinated with organic ligands, suitable for gas storage and separation.

5. Charcoal: Produced from organic materials like wood or coconut shells, used in various adsorption applications.

6. Silica Alumina: A mixture of silica and alumina used as a catalyst and adsorbent.

Development of adsorption isotherms: 
The Freundlich isotherm is one of the earliest and most commonly used models to describe the adsorption process. It was developed by German chemist Kuno Freundlich in 1906. The Freundlich isotherm is an empirical equation and is applicable for adsorption on heterogeneous surfaces, where the adsorption energy varies across the surface.

The Freundlich isotherm equation is given as:

q = K * C^(1/n)

Where:
  • q is the amount of adsorbate adsorbed per unit mass of adsorbent (adsorption capacity),
  • C is the equilibrium concentration of the adsorbate in the fluid phase,
  • K and n are Freundlich constants representing adsorption capacity and intensity, respectively.

The Freundlich isotherm is particularly useful when there is no specific monolayer formation on the adsorbent surface and when the adsorption does not follow Langmuir-type behavior. While it's a valuable tool for understanding adsorption, it has limitations and may not accurately describe certain adsorption processes, especially at high concentrations or in cases of strong interactions between the adsorbate and adsorbent.


Langmuir : 

The Langmuir isotherm is another widely used model to describe the adsorption process, especially when adsorption occurs on a homogeneous surface with a finite number of identical adsorption sites. It was introduced by Irving Langmuir in 1918 and is based on the assumption that only one layer of adsorbate molecules can form on the adsorbent surface.

The Langmuir isotherm equation is given as:

q = (K * C) / (1 + K * C)

where:
  • q is the amount of adsorbate adsorbed per unit mass of adsorbent (adsorption capacity),
  • C is the equilibrium concentration of the adsorbate in the fluid phase,
  • K is the Langmuir constant, which represents the equilibrium constant for adsorption.
The Langmuir isotherm assumes that adsorption and desorption are reversible processes, and there is no interaction between the adsorbate molecules on the surface. It predicts that adsorption will reach a saturation point where all available adsorption sites are occupied.

The Langmuir isotherm is particularly useful for understanding monolayer adsorption on surfaces and is commonly used in gas adsorption and surface chemistry studies. While it provides valuable insights into adsorption behavior, it may not accurately represent multilayer adsorption or adsorption on heterogeneous surfaces, where the Freundlich isotherm or other models might be more suitable. Researchers often use both the Langmuir and Freundlich isotherms to compare their predictions and determine which model better fits experimental data.

BET Activated carbon adsorption : 

BET (Brunauer, Emmett, and Teller) theory is a method commonly used to analyze the surface area of porous materials, including activated carbon. It was developed in 1938 by Stephen Brunauer, Paul Hugh Emmett, and Edward Teller.

Activated carbon is a highly porous form of carbon with a vast internal surface area, created through a process called activation. This surface area provides numerous adsorption sites, making activated carbon an excellent adsorbent for a wide range of substances.

The BET theory is based on the assumption that adsorption of gas molecules on the surface of a solid occurs in a multilayer manner. It describes the relationship between the amount of adsorbate gas adsorbed on the surface of the solid (activated carbon) and the relative pressure of the gas.

The BET isotherm equation is given as:

C = C_m * P / (1 - P)

where:
  • C is the amount of adsorbate gas adsorbed on the surface of the solid,
  • C_m is the monolayer adsorption capacity,
  • P is the relative pressure of the gas.

By plotting the experimental data of adsorption at various relative pressures and using the BET equation, researchers can determine the monolayer adsorption capacity (C_m) and, from there, calculate the surface area of the activated carbon.

This analysis is crucial in characterizing the effectiveness of activated carbon as an adsorbent, especially in applications such as water and air purification, where the surface area and adsorption capacity play a critical role in removing contaminants. BET analysis helps optimize the selection and usage of activated carbon for specific adsorption processes.

Granular carbon adsorption Regeneration/reactivation of adsorbents

Granular carbon adsorption:

Granular activated carbon (GAC) is a common form of activated carbon used in adsorption processes. It consists of small granules of carbon with a large surface area, providing an efficient medium for adsorption.

GAC is widely used in water treatment, air purification, and various industrial processes to remove organic contaminants, chlorine, volatile organic compounds (VOCs), and other pollutants from liquids and gases.

Regeneration/reactivation of adsorbents:

Over time, adsorbents like activated carbon can become saturated with adsorbed substances and lose their effectiveness. To maintain their adsorption capacity and extend their lifespan, adsorbents need to undergo regeneration or reactivation processes.

The regeneration process involves desorbing the adsorbed substances from the adsorbent surface, making it available for reuse. Depending on the adsorbent and the adsorbate, regeneration methods can vary, including thermal regeneration, steam regeneration, solvent regeneration, or a combination of these methods.

For granular activated carbon, the regeneration process typically involves thermal reactivation. The spent GAC is subjected to elevated temperatures in a controlled environment to remove the adsorbed substances. This high-temperature treatment breaks the adsorbate-adsorbent bonds, allowing the adsorbate to be driven off as gas or vapor, leaving the carbon surface clean and regenerated.

Once the activated carbon is regenerated, it can be reused for further adsorption cycles. Proper regeneration is essential to maintain the efficiency and cost-effectiveness of using granular activated carbon in adsorption applications. It helps reduce waste generation and ensures that the adsorbent remains effective for an extended period, making it a sustainable solution for environmental remediation and purification processes.