Adsorption: A surface phenomenon

1
Adsorption A surface phenomenon
Adsorbate
Adsorbent
Selective solute binding
Removal of bound material is called desorption
i.e. opposite of adsorption
2
Selectivity of adsorption Criteria

• Molecular weight or size
• Solute shape
• Polarity
• Electrostatic charge

Steric
Chemical properties
Interactions involved in adsorption

• van der Waals forces
• Electrostatic interactions
• Hydrophobic interactions
• Hydrogen bonding
• Chemical bond formation (chemisorption)

Non-covalent interactions
3
van der Waals forces
4
Hydrogen bonding
5
Long range electrostatic interactions
6
Adsorbents

• Natural or synthetic
• Amorphous or microcrystalline structure
• Very high specific surface area
• Examples
• Charcoal
• Kaolin
• Bentonite
• Activated carbon
• Silica gel
• Activated alumina
• Zeolite (molecular sieves),

7
Structure of adsorbents
Silica gel
Activated carbon
Zeolites
8
Adsoprtion

• Advantages
• High selectivity (e.g. affinity adsorption)
• Ability to handle very dilute feed concentrations
• Disadvantages
• It is a surface phenomenon i.e. the interior of
  adsorbent is not available
• Batch or semi-batch operations only
• Adsorbent regenerated
• Loss of product quality

9
Adsoprtion Applications

• Gas separation using molecular sieves by pressure
  swing adsorption
• Removal of toxic gases from air (e.g. gas masks)
• Fractionation of industrial chemicals using gas
  or liquid chromatography
• Removal of phenolics and other toxic chemical
  from waste water
• Removal of chlorinated hydrocarbons from waste
  gas streams
• De-hydration or de-humidification of gases
• Water purification by deionization and
  ion-exchange
• Ion exchange separation of proteins and nucleic
  acids
• Affinity separations

10
Adsoprtion Isotherms

• Relationship between bound and free solute
  concentrations at equilibrium
• Analogous to equilibrium line in extraction
• Isotherm refers to data being obtained at
  constant temperature

Irreversible
Very favourable
Linear
Solute Concentration on adsorbents surface (q)
Favourable
Unfavourable
Solute concentration in solution (y)
11
Adsorption isotherms
Linear
Freundlich
Langmuir
Freundlich
q
Linear
Langmuir
y
12
Determination of isotherm constants
Linear Determine K from plot of q versus
y Freundlich Determine K and n from plot of
(log q) versus (log y) Langmuir Determine q0
and K from plot of (1/q) versus (1/y)