Key Concepts of Adsorption Isotherms to Know for Separation Processes

Adsorption isotherms describe how substances stick to surfaces, crucial for separation processes. They help predict how different materials interact, guiding the design of efficient systems for purifying air, water, and other resources by understanding adsorption behavior.

1. Langmuir Isotherm

   • Assumes monolayer adsorption on a surface with a finite number of identical sites.
   • The adsorption process is reversible, and there is no interaction between adsorbed molecules.
   • The isotherm is characterized by a maximum adsorption capacity (Qmax) and a constant (KL) related to the affinity of the binding sites.
   • Useful for systems where the surface is homogeneous and the adsorption sites are energetically equivalent.

2. Freundlich Isotherm

   • Empirical model that describes heterogeneous adsorption on a surface with varying affinities.
   • The isotherm is expressed in a logarithmic form, indicating a non-linear relationship between adsorption and concentration.
   • Characterized by two constants: Kf (indicating the adsorption capacity) and n (indicating the adsorption intensity).
   • Applicable to low concentration ranges and often used for multi-layer adsorption.

3. Brunauer-Emmett-Teller (BET) Isotherm

   • Extends the Langmuir model to multilayer adsorption, applicable to porous materials.
   • Describes the relationship between the amount of gas adsorbed and the pressure at constant temperature.
   • Characterized by a BET constant (C) that reflects the energy of adsorption.
   • Widely used for determining surface area and porosity of materials.

4. Henry's Law Isotherm

   • Describes the linear relationship between the concentration of a solute in the liquid phase and its partial pressure in the gas phase.
   • Applicable at low concentrations where the solute behaves ideally.
   • Characterized by the Henry's law constant (KH), which indicates the solubility of the gas in the liquid.
   • Important for understanding gas-liquid interactions in separation processes.

5. Temkin Isotherm

   • Assumes that the heat of adsorption decreases linearly with coverage due to adsorbate-adsorbate interactions.
   • The isotherm is characterized by a constant (B) that reflects the heat of adsorption and a constant (KT) related to the adsorption capacity.
   • Useful for systems where interactions between adsorbed molecules are significant.
   • Often applied in the study of chemisorption processes.

6. Dubinin-Radushkevich Isotherm

   • Focuses on the microporous structure of adsorbents and the energy of adsorption.
   • Characterized by a constant (β) that relates to the adsorption potential and a maximum adsorption capacity (Qm).
   • Suitable for describing adsorption in microporous materials and can indicate the type of adsorption (physical or chemical).
   • Often used in the context of activated carbon and zeolites.

7. Sips Isotherm

   • Combines features of both Langmuir and Freundlich isotherms, suitable for heterogeneous surfaces.
   • Characterized by three parameters: Qmax, KL, and n, allowing for flexibility in modeling.
   • Useful for systems where both monolayer and multilayer adsorption occur.
   • Often applied in complex adsorption scenarios in environmental and industrial processes.

8. Toth Isotherm

   • A modification of the Langmuir isotherm that accounts for heterogeneity in adsorption sites.
   • Characterized by parameters that reflect the maximum adsorption capacity and the degree of heterogeneity.
   • Suitable for describing adsorption in systems with a wide range of pore sizes.
   • Often used in the study of porous materials and catalysts.

9. Redlich-Peterson Isotherm

   • A hybrid model that combines features of both Langmuir and Freundlich isotherms.
   • Characterized by three parameters: A (related to adsorption capacity), B (related to affinity), and g (indicating the degree of non-linearity).
   • Applicable to a wide range of concentrations and useful for complex adsorption systems.
   • Often used in environmental engineering and wastewater treatment studies.

10. Linear Isotherm

    • Represents a direct proportionality between the amount of adsorbate and its concentration in the solution.
    • Characterized by a single constant (K) that reflects the adsorption capacity.
    • Simplest model, often used as a first approximation in adsorption studies.
    • Useful for systems where adsorption is weak or at very low concentrations.