1.
ADSORPTION
ISOTHERMS
1

2.
Adsorption
Mechanism of adsorption
Types of adsorption
Applications of adsorption
Adsorption isotherms
Different types of isotherms
Applications of isotherms
Summary
References
2

3.
Adsorption
3
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.

4.
Some Important Terms
4
Adsorbent-The solid
that takes up a gas,
vapour or a solute from
a solution, e.g. silica
gel, charcoal, clay, etc.
Adsorbate-The
gas, vapour or the
solute, which is
held to surface of
the solid. e.g.
poisonous gases
Desorption-The
process of removal
of adsorbed
substances from
the surface on
which it is
absorbed.

5.
silica
Activated
charcoal
Alumina
Bentonite clay Zeolites Bagasse
Some commonly used adsorbents
5

6.
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.
Examples of adsorption
6

7.
Difference between adsorption and
absorption
7
 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.
 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
Absorption

8.
continued……
8
Absorption Adsorption

9.
Mechanism of Adsorption
9
 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.

10.
Continued…..
10
Exothermic
process
Spontaneous
∆𝐺
= ∆𝐻 − 𝑇∆𝑆

11.
Types of Adsorption
Physisorption Chemisorption
11

12.
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
12

13.
CONTINUED……
13
Lenard-Jones model for physisorption and
chemisorption of molecules.

14.
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.
14

15.
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
15

16.
Adsorption equilibria
16
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, P-
diagrams at
constant
temperature

17.
Continued….
Isobars: m,
T-
diagrams
at constant
Pressure
Isosteres:
In(𝑝 𝑝𝑜),
T-
diagrams
at
constant
m
17

18.
Adsorption isotherm
18
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.

19.
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.
19

20.
Basic Adsorption isotherm
20
After saturation pressure Ps, adsorption
does not occur anymore.
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.

21.
Types of isotherms
Langmuir
isotherm
Freundlich
isotherm
BET isotherm
Dubinin–
Radushkevich
isotherm
Temkin isotherm
21

22.
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
22

23.
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
23

24.
Langmuir Adsorption isotherm
The linear form of the Langmuir isotherm
qe =
bqmax
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) 24

25.
Continued….
Plot of original equation
25

26.
Continued….
The rearranged form of the Langmuir isotherm
1
qe
=
1
Cebqmax
+
1
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)
26

27.
Continued….
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.
27
y = 0.2597x + 0.0018
R² = 0.9637
0
0.01
0.02
0.03
0.04
0.05
0.06
0 0.05 0.1 0.15 0.2 0.25
1/qe
1/Ce

28.
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.
28

29.
Freundlich Adsorption Isotherm
This isotherm is
an empirical
expression that
accounts for
surface
heterogeneity.
Estimate the
sorption
intensity of
the adsorbent
towards the
adsorbate.
1909, Dr.
Herbert
Freundlich
29

30.
Assumptions of Freundlich Isotherm
30
Surface roughness
Inhomogeneity
Adsorbate-
adsorbate
interactions

31.
Freundlich Adsorption Isotherm
31
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.

32.
Continued…
The linear form of Freundlich isotherm is
logqe = log Kf +
1
n
logPe
or
logqe = log Kf +
1
n
logCe
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
32

33.
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.
33

34.
Continued…
The parameters of the Freundlich isotherm can be
determined by plotting logqe versus logCe.
34
y = 0.9452x + 1.3982
R² = 0.9637
2
2.5
3
3.5
4
4.5
5
1 2 3 4
log
qe
log Ce

35.
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. 35

36.
Differences between Freundlich and
Langmuir adsorption isotherms
36
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 = 𝐾𝑃
1+ KP
• Homogeneity of
adsorption site.
• Based on adsorption
and desorption
equilibrium.

37.
BRUNAUER–EMMETT–TELLER ISOTHERM
In 1938, Stephen
Brunauer, Paul Hugh
Emmett, and Edward
Teller
Applies to
systems of
multilayer
adsorption
Utilizes probing
gases that do not
chemically react
with material
surfaces to
quantify specific
surface area 37

38.
CONTINUED…
38
• the most commonly used
gaseous adsorbate used for
surface probing
Nitrogen
• At boiling temperature of
N2 (77 K)
BET
analysis
conducted
• Argon, carbon dioxide and
water
Other
probing
adsorbents

39.
CONTINUED….
39
 The concept of the theory is an
extension of the Langmuir
theory, which is a theory
for monolayer molecular
adsorption, to multilayer
adsorption with the following
hypotheses
 Gas molecules physically
adsorb on a solid in layers
infinitely
 Gas molecules only interact with
adjacent layers
 Langmuir theory can be applied
to each layer

40.
CONTINUED…
The BET equation
(P/Po)/Q ((P/Po)-1)= 1/kQm + (k-1/Qmk)(P/Po)
Where
P and Po are the equilibrium and the saturation
pressure of adsorbates at the temperature of adsorption,
Q (in moles) is the amount adsorbed on 1g of adsorbent
and Qm is the monolayer adsorption capacity.
k is the BET constant.
40

41.
CONTINUED…
Plot of BET isotherm
41

42.
Drawbacks of BET adsorption theory
42
Drawbacks
• Surface is assumed to be homogenous
Drawbacks
• Interaction between the adsorbed
molecules in neglected
Drawbacks
• Heat of adsorption from second layer
onwards to be equal to subsequent
layers which is not usually true

43.
Applications of BET isotherm
43
The inner surface
of hardened cement
paste determination
solid
catalysis
Specific
surface area
calculation

44.
TYPES OF BET ADSORPTION ISOTHERMS
Six types of isotherm are
characteristic of adsorbents
Microporous (type I)
Nonporous or macroporous
(types II, III, and VI)
Mesoporous (types IV and V)
44

45.
TYPE I BET ADSORPTION ISOTHERM
Described by Langmuir
equation
Horizontal plateau
Microporous materials
Organic vapors on zeolites and
molecular sieves
Chemisorption systems often
also show AIs of Type I
No multilayer adsorption 45

46.
TYPE II ADSORPTION ISOTHERM
46
Sigmoid type graph
Multilayer adsorption
Pore condensation but
no hysteresis.
Nonporous or
macroporous solids.
(pore diameter > 50
nm).
Described by the BET
equation
Vapors of non-polar
organic substances on
mesoporous activated
carbons.`

47.
TYPE III ADSORPTION ISOTHERM
47
Hyperbolic graph
Occur in systems where the
adsorbate-sorbent (a-s)
interaction is small
compared to the adsorbate-
adsorbate (a-a) interaction
Water on activated carbon
and hydrophobic zeolites

48.
TYPE IV BET ADSORPTION ISOTHERM
48
Water vapor from humid air on
special types of activated carbons
and hydrophilic zeolites.
Pore condensation with hysteresis
behavior between the adsorption and
the desorption branch.
Adsorption behavior of special
mesoporous materials

49.
TYPE V BET ADSORPTION ISOTHERM
49
Elongated S shaped graph
Perpendicular middle portions
deviate from Type IV curves
Mesoporous materials
Pore condensation may occur.
Water on special activated
carbons and carbon molecular
sieves.

50.
TYPE VI BET ADSORPTION ISOTHERM
50
Stepwise multilayer adsorption
Pronounced at low
temperatures
Nonpolar, spherical molecules
on planar graphite surfaces
Butanol on aluminum silicate

51.
DIFFERENCES BETWEEN LANGMUIR
AND BET ADSORPTION ISOTHERM
Langmuir
• monolayer
• More surface area
• (P/Po)/Q = 1/kQm +
(1/Qm)(P/Po)
BET
• Monolayer +Multilayer
• Less surface area
• (P/Po)/Q ((P/Po)-1)=
1/kQm + (k-1/Qmk)(P/Po) 51

52.
BET ADSORPTION ISOTHERM
Why is
Langmuir
surface area
always higher
than BET
surface area?
52

53.
DUBININ RADUHKEVITCH (D-R)
ADSORPTION ISOTHERM:
Dubinin and his co-workers conceived this
equation for micropore solids
Langmuir and Freundlich isotherm constant
do not suggest anything regarding
adsorption mechanism
D-R isotherm helps to determine the
adsorption mechanism
Distinguishing between the chemical
adsorption and physical adsorption
53

54.
CONTINUED…….
The linear form of D-R model
lnQe = lnQm − β2
Where
Qe=amount of adsorbate adsorbed per unit weight of
adsorbent at equilibrium (mgg-1)
Qm= Maximum adsorption capacity of adsorbent (mgg1)
β = constant related to adsorption energy
 = polanyi potential (kJ2mol-2)
54

55.
CONTINUED…….
 can be acquired from the following relation
Where,
R is gas constant
T is the temperature in (K)
Ce is the concentration of adsorbate at equilibrium in solution
after adsorption(mgl-1).
The experimental data can be evaluated by plotting lnQe against
2.
The value of Qm and β are estimated from the intercept and
slope respectively. 55

56.
CONTINUED…….
y = -0.0076x + 6.5814
R² = 0.8041
4
4.5
5
5.5
6
6.5
7
7.5
0 50 100 150 200 250
ln
qe
Ԑ2 ( x 103)
D-R adsorption isotherm
56
The experimental data is evaluated by plotting lnqe against
2.

57.
CONTINUED…….
The value of Qm and β are estimated from the
intercept and slope respectively.
The mean adsorption energy is calculated with
the equation
E =
1
2β
57

58.
CONTINUED…….
This parameter
tells the nature of
adsorption process
such as chemical
ion exchange or
physical adsorption
The value smaller
than 8 kJmol-1 then
the reaction
continues
physically
The value between
8-16 kJmol-1 then
the process will
proceeds
chemically
58

59.
USES OF D-R ISOTHERMS
Describe
adsorption by
microporous
solids
carbonaceous
sorbents or
activated
carbons
applicable for
microporous
solids with
uniform
microporous
structures
59

60.
DRAWBACKS OF D-R ISOTHERM
The D-R isotherm does not
reduce to Henry's law at low
pressures
Not act as thermodynamically
consistent adsorption
isotherm
60

61.
TEMKIN ADSORPTION ISOTHERM
History
• At first, Temkin
equation was
proposed to
describe adsorption
of hydrogen on
platinum electrodes
within acidic
solutions
Basic assumptions
• The adsorption heat
of all molecules
decreases linearly
with the increase in
coverage of the
adsorbent surface
• Uniform distribution
of binding energies
• Useful for
chemisorption 61

62.
CONTINUED…….
Tempkin model was:
qe = BTlnKT + BTlnCe
Where
KT = equilibrium binding constant (lmg-1)
BT = heat of adsorption (Tempkin constant) (Jmol-1)
bT = Tempkin isotherm constant related to variation of
adsorption energy (Jmol-1),
BT =
RT
bT
R is the gas constant (8.314 JK-1mol-1),
T = temperature (K) 62

63.
CONTINUED…….
y = 483.54x - 61.548
R² = 0.8841
0
200
400
600
800
1000
1200
1 1.5 2 2.5 3 3.5
qe
ln Ce
63
Plot of amount adsorbed qe against lnCe

64.
DRAWBACKS OF TEMKIN ISOTHERM
Temkin isotherm
model can’t be used
for complex systems
64

65.
APPLICATIONS OF ADSORPTION ISOTHERMS
65
Spontaneity
Exothermicity
Percentage removal of adsorbate
Langmuir parameters- maximum adsorbent
uptake and affinity between adsorbent and
adsorbate

66.
CONTINUED…….
Fruendlich parameters-
adsorption capacity of adsobents.
BET isotherms- specific surface
area, pore size distribution curves
D-R parameters- adsorption
mechanism
Temkin parameters- adsorbent-
adsobate interactions
66

67.
SUMMARY
 Adsorption-The phenomenon of higher concentration of
any species of solid or liquid or gas at the surface than
in the bulk of a material is known as adsorption.
 Desorption-The process of removal of adsorbed
substances from the surface on which it is absorbed.
 Some commonly used adsorbents- Silica, Activated
charcoal, Alumina, Bentonite clay, Zeolites, Bagasse
 Absorption is the phenomenon in which the particles of
gas or liquid uniformly distributed throughout the bulk
and it occurs at uniform rate.
 While, adsorption is surface phenomenon. It arises
because of unbalanced forces on the surface of solids
and liquids.
67

68.
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
68

69.
CONTINUED…….
 The Langmuir adsorption assumed that fix number of
accessible sites are available on the adsorbent surface,
with same energy. Surface is homogeneous and
monolayer adsorption takes place.
 Fruendlich isotherm- This isotherm was an empirical
expression that accounts for surface heterogeneity by
multilayer adsorption, exponential distribution of active
sites of adsorbent and their energies towards adsorbate.
 Freundlich adsorption isotherm failed at higher pressure.
 The BET theory applies to systems of multilayer
adsorption and usually utilizes probing gases that do not
chemically react with material surfaces as adsorbates to
quantify specific surface area.
69

70.
CONTINUED…….
 The concept of the theory is an extension of
the Langmuir theory, which is a theory for monolayer
molecular adsorption, to multilayer adsorption.
 Surface is assumed to be homogenous which the case
is not always. Lateral interaction between the adsorbed
molecules in neglected.
 The D-R equation is an adaptation of the earlier Polanyi
potential theory of adsorption.
 D-R isotherm helps to determine the adsorption
mechanism i.e. helps in distinguishing between the
chemical adsorption and physical adsorption
 The D-R isotherm does not reduce to Henry's law at low
pressures, which is a necessity for a thermodynamically
consistent. 70

71.
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