ADSORPTION

By MD SAJID ALI

ADSORPTION

What is Adsorption?Adsorption is the phenomenon of accumulation of large number of molecular species at the surface of liquid or solid phase in comparison to the bulk.

How Adsorption occurs?

The process of adsorption arises due to presence of unbalanced or residual forces at the surface of liquid or solid phase.

These unbalanced residual forces have tendency to attract and retain the molecular species with which it comes in contact with the surface. Adsorption is essentially a surface phenomenon.

Adsorption Vs Absorption

Adsorption is a term which is completely different from Absorption .While absorption means uniform distribution of the substance throughout the bulk, adsorption essentially happens at the surface of the substance. When both Adsorption and Absorption processes take place simultaneously, the process is called sorption.

Adsorption process involves two components Adsorbent and Adsorbate. Adsorbent is the substance on the surface of which adsorption takes place.Adsorbate is the substance which is being adsorbed on the surface of adsorbent. Adsorbate gets adsorbed.

Adsorbate + Adsorbent gives rise to Adsorption

AdsorbentsNatural or syntheticAmorphous or microcrystalline structureVery high specific surface areaExamples:•Synthetic Polymers (vinyl based, absorb polar and or non-polar org.)•Bentonite (clay adbsorbent, adsorbs proteins readily for winemaking)•Activated carbon (organics, microcrystalline, 300 to 1200 m2/g)•Silica gel (dry gas and liq., fractionate hydrocarbons, 600 to 800 m2/g)•Activated alumina (dry gas and liq., 200 to 500 m2/g)•Zeolite (molecular sieves with specific pore sizes, separation, drying)

Adsorption in liquids

Adsorption can be understood by considering a simple example. In case of liquid state, water molecule present on the surface is attracted inwards by the molecules of water present in the bulk. This gives rise to surface tension. While the molecule of water present within the bulk is equally attracted from all the sides and the net force experienced by the water molecule in bulk is zero. This clearly shows that particles at surface and particles at the bulk are in different environment.

Water molecule on surface experiencing unbalanced forces as compared to molecule inside which experiences forces from all direction.

Adsorption in solids

In case of solid state these residual forces arises because a of unbalanced valence forces of atoms at the surface. The generation of these forces on solid surface can be explained diagrammatically as follows:

Due to cleavage of a big crystal into smaller unit, residual forces or vacancies gets generated on the surface of the solid. Occupancy of these vacancies by some other molecular species results into Adsorption.

Facts about Adsorption Process

•Adsorption is a spontaneous process

For reaction or process to be spontaneous, there must be decreases in free energy of the system i.e. ΔG of the system must have negative value.

•Adsorption is an exothermic process

Adsorption process takes place by adsorbate getting adsorbed on adsorbent .Forces of attraction exist between adsorbate and adsorbent and due to these forces of attraction, heat energy is released. So adsorption is an exothermic process.

Types of Adsorption

Forces of attraction exist between adsorbate and adsorbent. These forces of attraction can be due to Van der waal forces of attraction which are weak forces or due to chemical bond which are strong forces of attraction. On the basis of type of forces of attraction existing between adsorbate and adsorbent, adsorption can be classified into two types: 1.Physical Adsorption2.Chemical Adsorption.

Physical Adsorption or Physisorption

• When the force of attraction existing between adsorbate and adsorbent are weak Vanderwaal forces of attraction, the process is called Physical Adsorption or Physisorption.

•Physical Adsorption takes place with formation of multilayer of adsorbate on adsorbent. It has low enthalpy of adsorption i.e. ΔHadsorption is 20-40KJ/mol.• It takes place at low temperature below boiling point of adsorbate.As the temperature increases in, process of Physisorption decreases. Physical Adsorption vs.

Temperature graph

Chemical Adsorption or Chemisorption

• When the force of attraction existing between adsorbate and adsorbent are chemical forces of attraction or chemical bond, the process is called Chemical Adsorption or Chemisorption.

• Chemisorption takes place with formation of unilayer of adsorbate on adsorbent. It has high enthalpy of adsorption. i.e.

It can take place at all temperature. With the increases in temperature, Chemisorption first increases and then decreases. Chemical Adsorption vs. Temperature Graph

Applications of Adsorption

1. Charcoal is used as a decoloriser as it adsorbs the coloring matter from the coloured solution of sugar.

2. Silica gel adsorbs moisture from the desiccators.

3. Silica and alumina gels are used as adsorbents for removing moisture and for controlling humidity of rooms.

4. Activated charcoal is used in gas masks as it adsorbs all the toxic gases and vapours and purifies the air for breathing.

5. Adsorption processes are useful in carrying out heterogeneous catalysis.

6. Adsorption of drugs and toxins on finely divided substance is considered as a useful interaction

when these substances are used as:1. Antidote for toxins (Charcoal).2. Antidiarrheal mixtures (Kaolin, Pectin,

Attapulgite).3. Antacids (Magnesium trisilicate, Al (OH)3 ,

Mg(OH)2 ).

However when antidiarrheal mixtures, Charcoal, antacids possessing adsorptive properties are concurrently administered with other drugs, possible adsorption leads to decreased bioavailability of these drugs.

Factors on which Adsorption Depends

1. TemperatureAdsorption increases at low temperature conditions.Adsorption process is exothermic in nature. According to Le Chatleir principle, low temperature conditions would favour the forward direction.

2. PressureAs depicted by Adsorption Isotherm, with the increases in pressure, adsorption increases up to a certain extent till saturation level is achieved. After saturation level is achieved no more adsorption takes place no matter how high the pressure is applied.

3. Surface AreaAdsorption is a surface phenomenon therefore it increases with increase in surface area.

4. Activation of AdsorbentActivation of adsorbent surface is done so as to provide more number of vacant sites on surface of adsorbent. This can be done by breaking solid crystal in small pieces, heating charcoal at high temperature, breaking lump of solid into powder or other methods suitable for particular adsorbent.

5. Surface Area of Adsorbent

Adsorption Isotherm

The process of Adsorption is usually studied through graphs called as adsorption isotherm. It is the graph between the amounts of adsorbate (x) adsorbed on the surface of adsorbent (m) and pressure at constant temperature.

From the graph, we can predict that after saturation pressure Ps, adsorption does not occur anymore. This can be explained by the fact that there are limited numbers of vacancies on the surface of the adsorbent. At high pressure a stage is reached when all the sites are occupied and further increase in pressure does not cause any difference in adsorption process. At high pressure, Adsorption is independent of pressure.

ADSORPTION EQUILIBRIA

If the adsorbent and adsorbate are contacted

long enough an equilibrium will be established

between the amount of adsorbate adsorbed

and the amount of adsorbate in solution. The

equilibrium relationship is described by

isotherms.

Define the following:qe = mass of material adsorbed (at equilibrium)

per mass of adsorbent.

Ce = equilibrium concentration in solution

when amount adsorbed equals qe.

qe/Ce relationships depend on the type of

adsorption that occurs, multi-layer, chemical, physical adsorption, etc.

Langmuir Isotherm:

This model assumes monolayer coverage and

constant binding energy between surface and

adsorbate. The model is:

0a e

ee

K Q Cq

1 K C

represents the maximum adsorption

capacity (monolayer coverage) (g solute/g

adsorbent).

Ce has units of mg/L.

K has units of L/mg

0aQ

BET (Brunauer, Emmett and Teller) isotherm:

This is a more general, multi-layer model. It

assumes that a Langmuir isotherm applies to each

layer and that no transmigration occurs between

layers. It also assumes that there is equal energy

of adsorption for each layer except for the first

layer.

)}C/C)(1K(1){CC(

QCKq

SeBeS

0aeB

e

CS =saturation (solubility limit) concentration of

the solute. (mg/liter)

KB = a parameter related to the binding intensity

for all layers.

Note: when Ce << CS and KB >> 1 and K =

KB/Cs BET isotherm approaches Langmuir

isotherm.

Freundlich Isotherm:For the special case of heterogeneous surface energies (particularly good for mixed wastes) in

which the energy term, “KF”, varies as a function

of surface coverage we use the Freundlich model.

n and KF are system specific constants.

n1

eFe CKq

Type of Adsorption Isotherm

Type I Adsorption Isotherm

The above graph depicts Monolayer adsorption.This graph can be easily explained using Langmuir Adsorption Isotherm.If BET equation, when P/P0<<1 and c>>1, then it leads to monolayer formation and Type I Adsorption Isotherm is obtained.Examples of Type-I adsorption are Adsorption of Nitrogen (N2) or Hydrogen (H) on charcoal at temperature near to -1800C.

Type II Adsorption Isotherm

Type II Adsorption Isotherm shows large deviation from Langmuir model of adsorption.The intermediate flat region in the isotherm corresponds to monolayer formation.In BET equation, value of C has to be very large in comparison to 1.Examples of Type-II adsorption are Nitrogen (N2 (g)) adsorbed at -1950C on Iron (Fe) catalyst and Nitrogen (N2 (g)) adsorbed at -1950C on silica gel.

Type III Adsorption Isotherm

Type III Adsorption Isotherm also shows large deviation from Langmuir model.In BET equation value if C <<< 1 Type III Adsorption Isotherm obtained.This isotherm explains the formation of multilayer.There is no flattish portion in the curve which indicates that monolayer formation is missing.Examples of Type III Adsorption Isotherm are Bromine (Br2) at 790C on silica gel or Iodine (I2) at 790C on silica gel.

Type IV Adsorption Isotherm

At lower pressure region of graph is quite similar to Type II. This explains formation of monolayer followed by multilayer.The saturation level reaches at a pressure below the saturation vapor pressure .This can be explained on the basis of a possibility of gases getting condensed in the tiny capillary pores of adsorbent at pressure below the saturation pressure (PS) of the gas.Examples of Type IV Adsorption Isotherm are of adsorption of Benzene on Iron Oxide (Fe2O3) at 500C and adsorption of Benzene on silica gel at 500C.

Type V Adsorption Isotherm

Explanation of Type V graph is similar to Type IV.Example of Type V Adsorption Isotherm is adsorption of Water (vapors) at 1000C on charcoal.Type IV and V shows phenomenon of capillary condensation of gas.