Adsorption
How Does Adsorption occur?
As mentioned above adsorption is a surface phenomenon. It occurs due to the imbalance of forces at
the surface of a material. This lead to formation of bonds (Covalent, ionic, Van der Waals, Hydrogen
bonds etc.) between the surface molecules (adsorbents) and the molecules in the fluid phase
(adsorbate).
Physisorption
Adsorption in which the forces involved are intermolecular (i.e., van der Waals, hydrogen bonding) of
the same kind as those responsible for the non-ideality of real gases and the condensation of vapours
etc. , and which do not involve a significant change in the electronic orbital patterns of the species
involved is called physisorption.
Chemisorption
A chemical process in which a reacting molecule forms a definite chemical bond with an unsaturated
atom, or a group of atoms (an active centre) on a catalyst surface, and electron transfer is involved is
known as chemisorption.
Distinguish between Physical Adsorption and Chemisorption
Difference between physical adsorption and chemisorption are as following:
PHYSICAL ADSORPTION CHEMISORPTIONS
The forces operating in these are weak vander The forces operating in these cases are similar to
Waal’s forces. those of a chemical bond.
The heat of adsorption are low i.e. about 20 – 40 kJ The heat of adsorption are high i.e. about 40 – 400
mol-1 kJ mol-1
No compound formation takes place in these cases. Surface compounds are formed.
The process is reversible i.e. desorption of the gas The process is irreversible. Efforts to free the
occurs by increasing the temperature or decreasing adsorbed gas give some definite compound.
the pressure.
It does not require any activation energy. It requires any activation energy.
This type of adsorption decreases with increase of This type of adsorption first increases with increase
temperature. of temperature. The effect is called activated
adsorption.
It is not specific in nature i.e. all gases are adsorbed It is specific in nature and occurs only when there is
on all solids to some extent. some possibility of compound formation between
the gas being adsorbed and the solid adsorbent.
The amount of the gas adsorbed is related to the There is no such correlation exists.
ease of liquefaction of the gas.
It forms multimolecular layer. It forms unimolecular layer.
Applications of Adsorption
Adsorption finds extensive applications both in research laboratory and in industry. A few applications
are discussed below:
In clarification of sugar:
Sugar is decolorized by treating sugar solution with charcoal powder. The latter adsorbs the undesirable
colors present.
In paint industry:
The paint should not contain dissolved gases as otherwise the paint does not adhere well to the surface
to be painted and thus will have a poor covering power. The dissolved gases are therefore, removed by
suitable adsorbents during manufacture. Further, all surfaces are covered with layers of gaseous, liquid
or solid films. These have to be removed before the paint is applied. This is done by suitable liquids
which adsorbs these films. Such liquids are called wetting agents. The use of spirit as wetting agent in
furniture painting is well known.
In chromatographic analysis:
The selective adsorbent of certain substances from a solution by a particular solid adsorbent has helped
to develop technique for the separation of the components of the mixture. This technique is called
chromatographic analysis. For example: in column chromatography a long and wide vertical tube is filled
with a suitable adsorbent and the solution of the mixture poured from the top and then collected one
by one from the bottom.
In catalysis:
The action of certain solids as catalysts is best explained in terms of adsorption. The theory is called
adsorption theory. According to this theory, the gaseous reactants are adsorbed on the surface of the
solid catalyst. As a result, the concentration of the reactants increases on the surface and hence the rate
of reaction increases. The theory is also able to explain the greater efficiency of the catalyst in the finely
divided state, the action of catalyst promoters and poisons.
In preserving vacuum:
In Dewar flasks activated charcoal is placed between the walls of the flask so that any gas which enters
into the annular space either due to glass imperfection or diffusion though glass is adsorbed.
In glass masks:
All gas masks are devices containing suitable adsorbent so that the poisonous gases present in the
atmosphere are preferentially adsorbed and the air for breathing is purified.
In adsorption indicators:
Various dyes which owe their use to adsorption have been introduced as indicators particularly in
precipitation titrations. For example: KBr is easily titrated with AgNO3 using eosin as an indicator.
In softening of hard water:
The use of ion exchangers for softening of hard water is based upon the principle of competing
adsorption just as in chromatography.
In removing moisture from air in the storage of delicate instruments:
Such instruments which may be harmed by contact with the moist air are kept out of contact with
moisture using silica gel.
In controlling humidity of rooms
Silica and alumina gels are used as adsorbents for removing moisture and for controlling humidity of
rooms.
In gas mask
Activated charcoal is used in gas masks as it adsorbs all the toxic gases and vapours and purifies the
air for breathing.
Adsorption Isotherms
Now let us study a bit more of the physics of adsorption.
Adsorption is usually described through adsorption isotherms that is the amount of adsorbate on the
adsorbent as a function of its pressure (if gas) or concentration (if liquid) at constant temperature.
The amount of gas adsorbed per unit of adsorbent at equilibrium is measured against the partial
pressure of the adsorbate in the gas phase gives equilibrium adsorption isotherm
The adsorption isotherm is the equilibrium relationship between the concentration in the fluid phase
and the concentration in the adsorbent particles at a given temperature. The quantity adsorbed is nearly
always normalized by the mass of the adsorbent to allow comparison of different materials. Some
typical adsorptions are shown in the figure below
• In general, an adsorption isotherm relates the volume or mass adsorbed to the partial pressure
or concentration of the adsorbate in the main gas stream at a given temperature
• The equilibrium concentration adsorbed is very sensitive to T
• There are many equations proposed to fit analyticaly the various experimental istoherms
Freundlich Adsorption Isotherm
In 1909, Freundlich gave an empirical expression representing the isothermal variation of adsorption of
a quantity of gas adsorbed by unit mass of solid adsorbent with pressure. This equation is known as
Freundlich Adsorption Isotherm or Freundlich Adsorption equation or simply Freundlich Isotherm.
Where x is the mass of the gas adsorbed on mass m of the adsorbent at pressure p and k, n are
constants whose values depend upon adsorbent and gas at particular temperature. Though Freundlich
Isotherm correctly established the relationship of adsorption with pressure at lower values, it failed to
predict value of adsorption at higher pressure.
Langmuir Adsorption Isotherm
In 1916 Langmuir proposed another Adsorption Isotherm known as Langmuir Adsorption isotherm. This
isotherm was based on different assumptions one of which is that dynamic equilibrium exists between
adsorbed gaseous molecules and the free gaseous molecules.
Where A(g) is unadsorbed gaseous molecule, B(s) is unoccupied metal surface and AB is Adsorbed
gaseous molecule.
Based on his theory, he derived Langmuir Equation which depicted a relationship between the number
of active sites of the surface undergoing adsorption and pressure.
Where θ the number of sites of the surface which are covered with gaseous molecule, P represents
pressure and K is the equilibrium constant for distribution of adsorbate between the surface and the gas
phase .The basic limitation of Langmuir adsorption equation is that it is valid at low pressure only.
At lower pressure, KP is so small, that factor (1+KP) in denominator can almost be ignored. So Langmuir
equation reduces to
θ = KP
At high pressure KP is so large, that factor (1+KP) in denominator is nearly equal to KP. So Langmuir
equation reduces to
BET adsorption Isotherm
BET Theory put forward by Brunauer, Emmett and Teller explained that multilayer formation is the true
picture of physical Adsorption.
One of the basic assumptions of Langmuir Adsorption Isotherm was that adsorption is monolayer in
nature. Langmuir adsorption equation is applicable under the conditions of low pressure. Under these
conditions, gaseous molecules would possess high thermal energy and high escape velocity. As a result
of this less number of gaseous molecules would be available near the surface of adsorbent.
Under the condition of high pressure and low temperature, thermal energy of gaseous molecules
decreases and more and more gaseous molecules would be available per unit surface area. Due to this
multilayer adsorption would occur. The multilayer formation was explained by BET Theory. The BET
equation is given as
The another form of BET equation is
Where Vmono be the adsorbed volume of gas at high pressure conditions so as to cover the surface with
a unilayer of gaseous molecules,
the ratio is designated C. K1 is the equilibrium constant when single molecule adsorbed per vacant site
and KL is the equilibrium constant to the saturated vapor liquid equilibrium.
Type of Adsorption Isotherm
Five different types of adsorption isotherm and their characteristics are explained below.
Type I 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
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
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
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
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.
Factors affecting Adsorption
Temperature
Adsorption increases at low temperature conditions.
Adsorption process is exothermic in nature. According to Le Chatleir principle, low temperature
conditions would favour the forward direction.
Pressure
As 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.
Surface Area
Adsorption is a surface phenomenon therefore it increases with increase in surface area.
Activation of Adsorbent
Activation 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.
Surface Area of Adsorbent
As adsorption is a surface phenomenon, surface area offered by Adsorbent becomes important factor
for consideration.
Volume of an ideal gas at STP = 22.4 L= 22.4 dm³
Also the number of gaseous molecules present at STP = 6.023*10²³ molecules
Vmono be the adsorbed volume of gas at high pressure conditions so as to cover the surface with a
unilayer of gaseous molecules. Let the total number of molecules of gas adsorbed corresponding to
volume Vmono be N whose value is given as
Now area of a molecule having density ρ and occupying volume V is calculated as follows
Where Vm is the total volume of the surface
Now Total volume Vm is equal to volume of each molecule (V) multiplied by total number of molecule
(NA) adsorbed on the surface at volume, V.
Molecule being spherical in nature, Volume of the molecule V is also given as
The radius, r occupied by each gas molecule is
Area occupied by each gas molecule is
This is the area occupied by one molecule. If the value multiplied by the total number of molecules
adsorbed on the surface of adsorbent we will get the total surface area of the adsorbent.
Or,
Adsorbents
The material upon whose surface the adsorption takes place is called an adsorbent
Major types of adsorbents in use are: activated alumina, silica gel, activated carbon, molecular sieve
carbon, molecular sieve zeolites and polymeric adsorbents.
Most adsorbents are manufactured (such as activated carbons), but a few, such as some zeolites, occur
naturally. Each material has its own characteristics such as porosity, pore structure and nature of its
adsorbing surfaces.
Adsorbents are used usually in the form of spherical pellets, rods, moldings, or monoliths with
hydrodynamic diameters between 0.5 and 10 mm.
They must have high abrasion resistance, high thermal stability and small pore diameters, which results
in higher exposed surface area and hence high surface capacity for adsorption.
The adsorbents must also have a distinct pore structure which enables fast transport of the gaseous
vapors.
Most industrial adsorbents fall into one of three classes:
Oxygen-containing compounds - Are typically hydrophilic and polar, including materials such as silica gel
and zeolites.
Carbon-based compounds - Are typically hydrophobic and non-polar, including materials such as
activated carbon and graphite.
Polymer-based compounds - Are polar or non-polar functional groups in a porous polymer matrix.
Activated carbon is used for adsorption of organic substances and non-polar adsorbates and it is also
usually used for waste gas (and waste water) treatment. It is the most widely used adsorbent since most
of its chemical (eg. surface groups) and physical properties (eg. pore size distribution and surface area)
can be tuned according to what is needed. Its usefulness also derives from its large micropore (and
sometimes mesopore) volume and the resulting high surface area.
Pore sizes in adsorbents may be distributed throughout the solid. Pore sizes are classified generally into
3 ranges: macroporeshave "diamaters" in excess of 50-nm, mesopores (also known as transitional pores)
have "diameters" in the range 2 - 50-nm, andmicropores have "diameters" which are smaller than 2-nm.
Many adsorbent materials, such as carbons, silica gels and aluminas are amorphous and
contain complex networks of inter-connected micropores, mesopores and macropores. In contrast,
pores in zeolitic adsorbents have precise dimensions.
Types of Adsorbents
Absorbents and Adsorbents can be categorized by specific material types. Here are some types of
absorbents and adsorbents:
Activated alumina is an adsorbent made of aluminum oxide (Al2O3). It is used as a desiccant for drying
gases and air and as a fluoride filter for drinking water. It has specific use as a silica gel replacement in
certain environments due to its thermal shock resistance and physical constancy when immersed in
water.
Activated carbon is roasted organic material (coconut shell, bone, wood) that forms porous granules. It
is a versatile and inexpensive adsorbent that comes in many sizes and has a range of applications from
gas, water, and metal purification to air filtration.
Calcium sulfate is a natural mineral that is chemically stable, and readily retains its captured moisture. It
costs little but also has a low adsorbency capacity and is best suited for small moisture capture
operations or laboratory use.
Calcium oxide is a slow but strong and high capacity desiccant also known as quicklime. It is caustic and
expands as it adsorbs and does so over several days. It is most effective in high humidity environments.
Clay or clay silicates are natural mineral absorbents and adsorbents that are used as spill cleaning
agents, sealants, and packing materials because they are inexpensive, inert, and have a quick capture
rate. However they begin to desorb at temperatures above 120°F.
Molecular sieves or zeolites are naturally occurring adsorbents with uniform pore size that can be tuned
to be highly selective. They are used as dehumidifiers and air purifiers due their high retention and
adsorption capacities even at high temperatures. Zeolites are often combined with activated carbon for
combined effectiveness.
Organic polymers are chains of repeating carbon based molecules used as adsorbents in size-exclusion
chromatography and gas separation processes with high retention power and selectivity. Most do not
require disposal and the regeneration process is environmentally friendly.
Silica gel or silicon dioxide is a common desiccant used in food preservation, humidity control, and
various medical devices. It has a higher water absorption capacity than clay silicates, it is very inert, and
it can be regenerated through heating.
Selection Criteria
When selecting adsorbents and absorbents, the most important properties to consider are the
selectivity, surface area, and regeneration ability.
Selectivity is the amount of specificity a sorbent has in the materials that it can capture. A very
unselective sorbent captures many substances and a selective sorbent only removes specific ones.
Surface area is the amount of material a sorbent has available for contact and determines the capacity
of material a sorbent can capture, the capture rate of the sorbent, and the retention rate of the
substance within the sorbent.
Regeneration is the ability of a sorbent to be reused after capturing to its capacity. Many desiccants can
be heat treated in order to regenerate the material after reaching water capture capacity.
Other factors to consider when selecting sorbents are bulk density, chemical inertness, and ease of
application.
Typical applications of commercial adsorbents:
Adsorbent Applications
Drying of gases, refrigerants, organic solvents, transformer
Silica Gel oils
Desiccant in packings and double glazing
Dew point control of natural gas
Activated Drying of gases, organic solvents, transformer oils
Alumina Removal of HCl from hydrogen
Removal of fluorine in alkylation process
Nitrogen from air
Hydrogen from syngas
Ethene from methane and hydrogen
Vinyl chloride monomer (VCM) from air
Carbons Removal of odours from gases
Recovery of solvent vapours
Removal of SOX and NOX
Purification of helium
Clean-up of nuclear off-gases
Water purification
Oxygen from air
Drying of gasses
Removing water from azeotropes
Sweetening sour gases and liquids
Purification of hydrogen
Separation of ammonia and hydrogen
Recovery of carbon dioxide
Separation of oxygen and argon
Zeolites Removal of acetylene, propane and butane from air
Separation of xylenes and ethyl benzene
Separation of normal from branched paraffins
Separation of olefins and aromatics from paraffins
Recovery of carbon monoxide from methane and
hydrogen
Drying of refrigerants and organic liquids
Pollution control, including removal of Hg, NOX and SOX
Recovery of fructose from corn syrup
Water purification
Recovery and purification of steroids, amino acids
Separation of fatty acids from water and toluene
Polymers &
Separation of aromatics from aliphatics
Resins
Recovery of proteins and enzymes
Removal of colours from syrups
Removal of organics from Hydrogen peroxide
Treatment of edible oils
Clay Removal of organic pigments
Refining of mineral oils
Removal of polychlorinated biphenyls (PCBs)
The Freundlich Equation and its application to single stage
adsorption
The Freundlich Adsorption Isotherm is mathematically expressed as
It is also written as
or
It is also written as
where
x = mass of adsorbate m = mass of adsorbent p = Equilibrium pressure of adsorbate
c = Equilibrium concentration of adsorbate in solution.
K and n are constants for a given adsorbate and adsorbent at a particular temperature.
At high pressure 1/n = 0, hence extent of adsorption becomes independent of pressure.
It is used in cases where the actual identity of the solute is not known, such as adsorption of colored
material from sugar, vegetable oil etc.
Limitation of Freundlich adsorption isotherm
Experimentally it was determined that extent of adsorption varies directly with pressure until saturation
pressure Ps is reached. Beyond that point rate of adsorption saturates even after applying higher
pressure. Thus Freundlich adsorption isotherm failed at higher pressure.
Adsorption hysteresis
Hysteresis can also occur during physical adsorption processes. In this type of hysteresis, the quantity
adsorbed is different when gas is being added than it is when being removed. The specific causes of
adsorption hysteresis are still an active area of research, but it is linked to differences in the nucleation
and evaporation mechanisms inside mesopores. These mechanisms are further complicated by effects
such as cavitation and pore blocking.
In physical adsorption, hysteresis is evidence of mesoporosity-indeed, the definition of the mesopores
(2–50 nm) is associated with the appearance (50 nm) and disappearance (2 nm) of mesoporosity in
nitrogen adsorption isotherms as a function of Kelvin radius. An adsorption isotherm showing hysteresis
is said to be of Type IV (for a wetting adsorbate) or Type V (for a non-wetting adsorbate), and hysteresis
loops themselves are classified according to how symmetric the loop is. Adsorption hysteresis loops also
have the unusual property that it is possible to scan within a hysteresis loop by reversing the direction of
adsorption while on a point on the loop. The resulting scans are called "crossing," "converging," or
"returning," depending on the shape of the isotherm at this point
