ADSORPTION
LESSON FIVE
WRU07432
Adsorption
Mechanism of adsorption
Types of adsorption
Applications of adsorption
Adsorption isotherms
Different types of isotherms
Applications of isotherms
Summary
References
Adsorption
The term adsorption
was
introduced
by
Kayser in 1881.
The phenomenon of
higher concentration of
any species of solid,
liquid or gas at the
surface than in the bulk
of a material, is known
as adsorption.
Some Important Terms
Adsorbent-The solid
that takes up a gas,
vapour or a solute from
a solution, e.g. silica
gel, charcoal, clay, etc.
Desorption-The
process of removal
of adsorbed
substances from
the surface on
which it is
absorbed.
Adsorbate-The
gas, vapour or the
solute, which is
held to surface of
the solid. e.g.
poisonous gases
Some commonly used adsorbents
silica
Activated
charcoal
Alumina
Bentonite clay
Zeolites
Bagasse
Examples of adsorption
Accumulation of dust on our skin when we
travel in regions of heavy traffic
Adsorption of dyes like methylene blue by
charcoal
Protection from poisonous gases
Solution of raw sugar become colorless when
passed over bed of animal charcoal.
Air becomes dry in the presence of silica gel.
Difference between adsorption and
absorption
Absorption




It is the phenomenon in
which the particles of gas or
liquid uniformly distributed
throughout.
Bulk phenomenon
Occurs at uniform rate
Anhydrous CaCl2 absorbs
water.
Adsorption




It is the phenomenon in
which the higher conc. of
particles of gas or liquid on
surface than in bulk.
Surface phenomenon
Rapid in beginning but
slowly decreases.
Silica gel absorbs water
vapour.
continued……
Absorption
Adsorption
Mechanism of Adsorption
Adsorption
is
surface
phenomenon.
 It arises because of unbalanced
forces on the surface of solids
and liquids.
 The surface is under tension
due to unbalanced forces.
 The surface of the solid or
liquid tends to satisfy their
residual forces by attracting
and retaining the molecules of
other species when brought in
contact with them.

Continued…..
Exothermic
process
∆𝐺
= ∆𝐻 − 𝑇∆𝑆
Spontaneous
Types of Adsorption
Physisorption
Chemisorption
continued……
PROPERTIES
PHYSISORPTION
CHEMISORPTION
Bonding
Weak, long range forces,
van der Waals interactions.
Strong, short range
forces, Chemical bonding
involving orbital overlap
and charge transfer.
Enthalpy
20-40 kJ mol-1
80-240 kJ mol-1
Saturation
Multi-layer
Mono-layer
Nature
reversible
irreversible
Effect of temperature
Occurs at low temperature
and decreases with increase
in temperature.
Occurs at high
temperature and
increases with increase in
temperature.
Effect of pressure
Increases with increase in
pressure of adsorbate.
Decreases with increase
of pressure
Activation energy
Not needed
High activation energy is
needed.
Surface specificity
No
Yes
Factors affecting adsorption
Nature of gas - easily liquefiable gases easily gets
adsorbed.
Nature of adsorbent- More the surface area per unit mass
of the adsorbent, more will be the adsorption
Temperature- adsorption increases as temperature
decreases.
Pressure- magnitude of adsorption increases with increase
in pressure.
APPLICATIONS OF ADSORPTION
Gas masks
Adsorption indicators
Chromatographic separation
Removal of coloring matter
Heterogeneous catalysis
Controlling humidity
Curing diseases
Froth flotation process
Production of high vacuum
Purification
Adsorption equilibria
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.
Isotherms:
m, Pdiagrams at
constant
temperature
Adsorption isotherm
The process of adsorption
is studied through graphs
know as adsorption
isotherms.
Graph between the
amounts of adsorbate (x)
adsorbed on the surface of
adsorbent (m) and
pressure (P) at constant
temperature.
Continued…
 In
adsorption, adsorbate gets adsorbed on adsorbent.
 The direction of equilibrium would shift in that
direction where the tension can be relieved.
 In
excess of pressure to the equilibrium system, the
equilibrium will shift in the direction where the
number of molecules decreases.
 Number of molecules decreases in forward direction.
Therefore, forward direction of equilibrium will be
favored.
Basic Adsorption isotherm
Limited numbers of vacancies on the surface
of the adsorbent.
After saturation pressure Ps, adsorption
does not occur anymore.
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.
Types of isotherms
Temkin
isotherm
Langmuir
isotherm
Dubinin–
Radushkevich
isotherm
Freundlich
isotherm
BET isotherm
Langmuir Adsorption isotherm
History
In 1916, Dr. Irving
Langmuir presented this
model.
Langmuir was awarded the
Nobel Prize in 1932
Adsorption can be
physisorption or
chemisorption
Estimate the adsorption capacity
of adsorbent used
Basic assumptions of Langmuir Model
The surface is homogeneous
All sites are equivalent
Mono-layer adsorption only
No interactions between adsorbate
molecules on adjacent sites
Heat of adsorption is constant and
equivalent for all sites
Adsorbate molecules have tendency
to get adsorb and desorb from surface
Langmuir Adsorption isotherm
The linear form of the Langmuir isotherm
bqmax
qe =
1 + bCe
Where
qe= Quantity of adsorbate adsorbed per unit weight of
adsorbent at equilibrium (mgg-1)
Ce = Concentration of adsorbate at equilibrium in
solution after adsorption (mgl-1)
qmax = Maximum adsorption capacity(mgg-1)
b = Langmuir adsorption equilibrium constant (lmg-1)
Continued….
Plot of original equation
Continued….
The rearranged form of the Langmuir isotherm
1
1
1
=
+
qe Ce bqmax qmax
Where
qe= Quantity of adsorbate adsorbed per unit weight of
adsorbent at equilibrium (mgg-1)
Ce = Concentration of adsorbate at equilibrium in
solution after adsorption (mgl-1)
qmax = Maximum adsorption capacity(mgg-1)
b = Langmuir adsorption equilibrium constant (lmg-1)
Continued….
1/qe
Isotherm is plotted between 1/qe versus 1/Ce and the slope
of the graph gives the value of qmax and intercept gives the
value of b.
0.06
0.05
0.04
0.03
0.02
0.01
0
y = 0.2597x + 0.0018
R² = 0.9637
0
0.05
0.1
0.15
1/Ce
0.2
0.25
Limitations of Langmuir isotherm
Holds only at low pressure.
Multilayer adsorption is possible.
Ignores adsorbate-adsorbate interactions.
Fails to account for the surface roughness of the
adsorbent.
Saturation value of adsorption depends upon temperature.
Freundlich Adsorption Isotherm
1909, Dr.
Herbert
Freundlich
Estimate the
sorption
intensity of
the adsorbent
towards the
adsorbate.
This isotherm is
an empirical
expression that
accounts for
surface
heterogeneity.
Assumptions of Freundlich Isotherm
Surface roughness
Adsorbateadsorbate
interactions
Inhomogeneity
Freundlich Adsorption Isotherm
The linear equation
x/m = k.P1/n (n > 1)
Or
x/m = k.C1/n (n > 1)
where
x is the mass of the gas adsorbed
on mass m of the adsorbent
P is pressure,
C is equilibrium concentration of
adsorbate in solution,
k and n are constants.
Continued…
The linear form of Freundlich isotherm is
1
logqe = log K f + logPe
n
or
1
logqe = log K f + logCe
n
Where
qe = Extent of adsorbate adsorbed per unit weight of
adsorbent at equilibrium (mgg-1)
Pe = Equilibrium pressure of adsorbent in solution after
adsorption
Ce = Equilibrium concentration of adsorbent in solution
after adsorption (mgl-1)
Kf = Freundlich constant indicating adsorption capacity
n = Empirical constant
Continued….
Kf approximately indicates adsorption capacity
1/n is a function of the strength of adsorption
n = 1, the boundary between the two phases is
independent of the concentration.
1/n below 1 indicates a normal adsorption.
n varies with the heterogeneity of the adsorbent and
range for favorable adsorption is of 1-10.
1/n = 0, x/m = constant, the adsorption is independent of
pressure.
1/n = 1, x/m = k P, i.e. x/m ∝ P, the adsorption varies directly
with pressure.
Continued…
The parameters of the Freundlich isotherm can be determined by
plotting logqe versus logCe.
5
log qe
4.5
4
3.5
3
y = 0.9452x + 1.3982
R² = 0.9637
2.5
2
1
2
log Ce
3
4
Limitations of Freundlich Isotherm
Freundlich equation is purely empirical and
has no theoretical basis.
The equation is valid only upto a certain
pressure and invalid at higher pressure.
The constants K and n vary with
temperature.
Frendilich’s adsorption isotherm fails at high
concentration of the adsorbate.
Differences between Freundlich and
Langmuir adsorption isotherms
FRENDLICH
ADSORPTION
ISOTHERM
• Tells about the
quantity of gas
adsorbed by unit mass
of solid adsorbent with
pressure.
• Represented by
formula
x/m = KP1/n
• Heterogeneity of
adsorption site.
• No information about
adsorption and
desorption.
LANGMUIR
ADSORPTION
ISOTHERM
• Tells about the
number of active site
undergoing
adsorption and
pressure.
• Represented by
formula θ = 𝐾𝑃ൗ1+ KP
• Homogeneity of
adsorption site.
• Based on adsorption
and desorption
equilibrium.
APPLICATIONS OF ADSORPTION ISOTHERMS
Spontaneity
Exothermicity
Percentage removal of adsorbate
Langmuir parameters- maximum adsorbent
uptake and affinity between adsorbent and
adsorbate
CONTINUED…….





The extent of adsorption increases with the increase
of surface area per unit mass of adsorbent at given
temperature and pressure.
Types of adsorption physisorption and chemisorption
Factors affecting adsorption temperature, pressure,
nature of adsorbent and adsorbate
Adsorption isotherm- It is the graph between the
amounts of adsorbate (x) adsorbed on the surface of
adsorbent (m) and pressure (P) at constant
temperature.
Types of adsorption isotherms-Langmuir isotherm,
Freundlich isotherm, BET isotherm, DR isotherm,
Temkin isotherm
ACTIVATED CARBON
 Water
contains dissolved organic matter that
cannot be removed with floc formation, floc
removal or sand filtration. These dissolved
organic compounds are:
➢ Odor, taste and color-producing compounds
➢ Organic
micropollutants
(pesticides,
hydrocarbon compounds)
 Activated carbon adsorbs (part of) the organic
matter and is mainly used to treat drinking
water produced from surface water.
ACTIVATED CARBON
Activated carbon is a substance with a high
carbon concentration
➢ The activated carbon filters of the drinking
water production plant have the objective
➢
❖ to
improve the taste of the water,
❖ to reduce the regrowth of bacteria in the piped
network, and
❖ to remove toxic substances from the water.
➢
The carbon filters are placed after the floc
formation, sedimentation and rapid sand
filtration to avoid rapid clogging
ACTIVATED CARBON
Under high temperatures this material becomes
carbonated, meaning that the carbon partly
transforms into carbon monoxide and water.
➢ This is how the carbon gets its open structure
➢ The internal surface area of the activated carbon
is several times larger than the external surface
area
➢
ACTIVATED CARBON
macro pores > 25 nm
meso pores 1 - 25 nm
micro pores < 1 nm
The open structure of activated carbon
ACTIVATED CARBON

Depending on the size of activated carbon, they are
divided into
Granular
➢ Activated powder
➢
Students should discuss the problems associated with the use of
Powder activated carbon (PAC) and granular activated carbon
(GAC) during adsorption
TUTORIAL


The following data were gathered in a laboratory study of
the adsorption of trichlorophenol (MW= 197.5 g/mol) on a
commercial powdered activated carbon for the following
conditions: Initial concentration of solute: 470 mg/L;
Volume of water: 500 mL
Fit these data to the Langmuir and Freundlich isotherms, and determine
the constants of both the models. Find the maximum adsorption capacity