Adsorption Definition and Principles

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Questions and Answers

Define adsorption.

Adsorption is the process through which a substance, originally present in one phase, is removed from that phase by accumulation at the interface between that phase and a separate (solid) phase.

What is the driving force for adsorption?

Increase in pressure

Increase in solubility

Increase in interfacial tension

Reduction in interfacial tension (correct)

Adsorption involves a chemical alteration of the adsorbed material.

False

_____ is the most common adsorbent used in wastewater treatment.

Activated carbon Signup and view all the answers

Match the adsorbents with their descriptions:

Activated carbon = Char-like material with high surface area Silica gel = Hard, granular, porous material made by precipitation from sodium silicate solutions Activated alumina = Aluminum oxide activated at high temperature and primarily used for moisture adsorption Aluminosilicates (molecular sieves) = Porous synthetic zeolites used mainly in separation processes Signup and view all the answers

What assumption is made for the Langmuir isotherm?

Reversible adsorption and desorption of the adsorbate molecules Signup and view all the answers

What type of isotherm does the Freundlich isotherm represent?

Empirical model Signup and view all the answers

The Langmuir isotherm typically represents data well for single components.

True Signup and view all the answers

In batch adsorption, the spent carbon is separated from the wastewater by ______.

filtration Signup and view all the answers

Match the adsorption process with its description:

Batch adsorption = Consists of contacting activated carbon with the wastewater for a given period of time Multistage crosscurrent adsorption = Fresh carbon is added to each stage of the adsorption process Multistage countercurrent adsorption = Streams leaving each stage are in equilibrium with each other Signup and view all the answers

What is the relationship between the concentration of an adsorbate in solution (C) and the amount of adsorbate adsorbed per unit mass of adsorbent (q)?

V (C_o - C) = M(q - q_o) Signup and view all the answers

What is the Langmuir isotherm used for?

To model the relationship between adsorption and concentration of solute in a solution. Signup and view all the answers

What are the assumptions made in the derivation of the Langmuir model?

Adsorption is a reversible process; the adsorbed layer consists of a single layer of molecules; the adsorbed molecules do not move on the surface. Signup and view all the answers

What is the final expression of the Langmuir isotherm after substitution?

q = q_m K_C / (1 + K_A C) Signup and view all the answers

Which of the following is NOT one of the isotherms typically used to model adsorption?

Dalton isotherm Signup and view all the answers

The Langmuir model assumes that adsorption is an irreversible process.

False Signup and view all the answers

Study Notes

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))

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Description

This quiz covers the definition and principles of adsorption, including its occurrence at solid-fluid interfaces, such as gas-solid and liquid-solid interfaces. Examples of adsorption, including VOCs on activated carbon, are also discussed.