Adsorption Definition and Principles

Definition of Adsorption

• Adsorption is the process of removing a substance from one phase and accumulating it at the interface with another phase (typically a solid phase).
• Examples of adsorption include:
  • Gas-solid interface (e.g., adsorption of VOCs on activated carbon)
  • Liquid-solid interface (e.g., adsorption of organic pollutants on activated carbon)

Driving Force for Adsorption

• The driving force for adsorption is the reduction in interfacial (surface) tension between the fluid and the solid adsorbent.
• The surface or interfacial tension (σ) is defined as the change in free energy (G) resulting from an increase in the area between two phases (A).

Adsorption vs. Absorption

• Adsorption is the accumulation of a substance at the interface between two phases, whereas absorption is the dissolution of a substance into another phase.

Motivation for Adsorption in Wastewater Treatment

• Adsorption is used in wastewater treatment to remove:
  • Toxic or hazardous substances that are difficult to remove through conventional treatment
  • Volatile substances that cannot be transferred to the atmosphere
  • Substances that create noxious vapors or odors, or impart color to the wastewater
  • Substances present in very small concentrations that are difficult to remove through other methods

Adsorption in Wastewater Treatment

• Adsorption is used to remove:
  • Toxic or recalcitrant organic pollutants (especially halogenated)
  • Inorganic contaminants from wastewater
  • Adsorption is used as a polishing step in tertiary wastewater treatment
  • Adsorption is commonly used in industrial wastewater treatment

Adsorption as a Physical Process

• Adsorption is a physical separation process in which the adsorbed material is not chemically altered.
• The adsorbed material is removed from one phase and transferred to another phase.
• Appropriate actions must be taken to treat the spent activated carbon, such as regeneration, disposal, or destruction.

Common Adsorbents

• Activated carbon: a char-like material with high surface area
• Silica gel: a hard, granular, porous material made from sodium silicate solutions
• Activated alumina: aluminum oxide activated at high temperature, used primarily for moisture adsorption
• Aluminosilicates (molecular sieves): porous synthetic zeolites used primarily in separation processes

Activated Carbon as an Adsorbent

• Activated carbon is the most commonly used adsorbent in wastewater treatment.
• The adsorbing capacity of activated carbon is associated with very high surface area per unit volume.
• Activated carbon is produced through a two-step process: pyrolysis of raw material, followed by activation of the charred residue.

Factors Affecting Adsorption

• Surface area of adsorbent: larger surface areas imply greater adsorption capacity
• Particle size of adsorbent: smaller particle sizes reduce internal diffusional and mass transfer limitations
• Contact time or residence time: longer times result in more complete adsorption
• Solubility of solute in liquid: substances with low solubility in water are more easily removed
• Affinity of solute for adsorbent: non-polar substances are more easily removed than polar substances
• Number of carbon atoms: larger number of carbon atoms is associated with greater adsorption potential
• Size of molecule with respect to size of pores: larger molecules may be too large to enter small pores
• Degree of ionization of adsorbate molecule: more highly ionized molecules are adsorbed to a smaller degree
• pH: affects the degree of ionization of a species, which in turn affects adsorption

Adsorption Equilibrium

• Adsorption equilibrium relates the amount of adsorbate adsorbed per unit mass of adsorbent (q) to the equilibrium concentration of the adsorbate in solution (C).
• The equilibrium relationship is a function of temperature.

Adsorption Isotherm

• The adsorption isotherm is a curve that relates q to C at a given temperature.
• The Langmuir, BET, and Freundlich isotherms are commonly used to describe adsorption equilibrium.

Experimental Determination of Adsorption Equilibrium

• A bottle is filled with a solution of the adsorbate and a carbon pellet is added.
• The solution is sampled and analyzed for the adsorbate at equilibrium.
• A mass balance is used to relate the initial and equilibrium concentrations of the adsorbate.

Langmuir Isotherm

• The Langmuir isotherm is a model that describes adsorption equilibrium.
• The assumptions of the Langmuir model include:
  • Adsorption is a reversible process
  • The adsorbed layer is made up of a single layer of molecules
  • The adsorbed molecules do not move on the surface of the adsorbent
  • The enthalpy of adsorption is the same for all molecules
• The Langmuir isotherm can be rearranged to give a linear plot, from which parameters can be determined.

BET Isotherm

• The BET isotherm is a model that describes adsorption equilibrium.
• The assumptions of the BET model include:
  • The adsorbed molecules are arranged in multiple layers on the surface of the adsorbent
  • The adsorbed molecules do not move on the surface of the adsorbent
  • A layer does not need to be completed before the next layer starts to fill up
  • The enthalpy of adsorption is the same for all molecules in a given layer
  • All molecules in layers other than the first have the same adsorption energy
• The BET isotherm can be rearranged to give a linear plot, from which parameters can be determined.

Freundlich Isotherm

• The Freundlich isotherm is a model that describes adsorption equilibrium.
• The Freundlich equation is q = KF C^(1/n), where KF and n are constants.### Freundlich Isotherm
• The Freundlich isotherm is an empirical model that represents the adsorption of solutes onto a surface.
• The equation for the Freundlich isotherm is: q = KF * C ^ (1/n)
• Where q is the amount of solute adsorbed per unit weight of adsorbent, KF is the Freundlich constant, C is the concentration of solute in solution, and n is a dimensionless constant.
• The Freundlich isotherm can be rearranged to give: log q = log KF + (1/n) * log C
• A plot of log q vs. log C should produce a straight line with a slope of 1/n and an intercept of log KF.

Determination of Parameters in Freundlich Isotherm

• The Freundlich isotherm parameters can be determined by plotting log q vs. log C and finding the slope and intercept.
• The slope of the line is 1/n, and the intercept is log KF.

Freundlich Parameters at Neutral pH

• Various compounds have different Freundlich parameters (KF and 1/n) at neutral pH.

Comparison of Freundlich and Langmuir Isotherms

• The Langmuir isotherm has a theoretical justification, while the Freundlich isotherm is an empirical model.
• The Langmuir isotherm assumes reversible adsorption and desorption, while the Freundlich isotherm does not make any assumptions.
• The Langmuir isotherm is typically used for single components, while the Freundlich isotherm can be used for mixtures of compounds.

Adsorption with Powdered Activated Carbon (PAC)

• Powdered activated carbon (PAC) is commonly used for wastewater treatment.
• PAC can be added to wastewater in a single stage or multiple stages to allow pollutants to be adsorbed.
• The process can be classified as batch adsorption, multistage crosscurrent adsorption, or multistage countercurrent adsorption.

Batch Adsorption

• Batch adsorption involves contacting finely divided activated carbon with wastewater for a given period of time.
• The activated carbon is typically supplied in a powdered form to maximize surface area for mass transfer.
• After the process is complete, the spent carbon is separated from the wastewater and regenerated or disposed of.

Unsteady State Mass Balance for Batch Adsorption

• The mass balance equation for batch adsorption is: V (Co - C) = B (q - qo)
• The equation can be rearranged to give: C = Co - (B/V) * (q - qo)
• At equilibrium, the equation becomes: Ceq = Co - (B/V) * (qeq - qo)

Multistage Crosscurrent Adsorption

• Fresh carbon is added to each stage of the adsorption process.
• The mass balance equation for each stage is: V (Ci - C(i+1)) = B(i) (qi - qi+1)
• If the adsorption follows a known adsorption model (e.g. Freundlich isotherm), the mass balances can be rewritten to solve for the equilibrium concentration.

Multistage Countercurrent Adsorption

• The streams leaving each stage are assumed to be in equilibrium with each other.
• The rates of wastewater and activated carbon moving from stage to stage are constant.
• The operating line can be obtained by solving the mass balance equations for the entire process.
• The operating line is given by: Cj = Co - (B/V) * (qi - q(j+1))