Soil Sorption and Desorption

PRESENTATION ON-
SORPTION AND DESORPTION
PHENOMENON IN SOIL
COURSE NO- SOILS-503
COURE TITLE- SOIL CHEMISTRY
COURSE INCHARGE-
Dr. T. Anajaih
(Dept. Of soil science and
agricultural chemistry )
PRESENTED BY-
PUJA PRIYADARSINI NAIK
RAM/19-97

• Sorption is a physical and chemical process by which
one substance becomes attached to another.
• Specific cases of sorption are –
A. ABSORPTION
B. ADSORPTION
C. ION EXCHANGE
Sorption
Source
https://www.nature.com/scitable/knowled
ge/library/introduction-to-the-sorption-of-
chemical-constituents

A. Absorption – The incorporation of a substance in one state
into another of a different state (E.g. Liquid being absorbed
by solid or gases being absorbed by a liquid)
B. Adsorption -The physical adherence or bonding of ions and
molecules onto the surface of another phase. (E.g. Reagents
adsorbed to a solid catalyst surface)
C. Ion exchange -An exchange of ions between two electrolytes
or between an electrolyte solution and a complex.
The reverse of sorption is desorption.
Sorption

SORPTIVE: Ions or molecules in solution that could potentially participate in a
sorption reaction.
SORBATE - the substance which is sorbed on the surface
SORBENT - the substance on which surface the sorbate is sorbed
Source-

Most relevant chemicals can be grouped into three sorptive categories:
(a) Anionic Sorptives
• These are negatively charged
• e.g., orthophosphate, PO4
3-
(b) Cationic Sorptives
• These are positively charged
• e.g., the divalent cations Ca2+ and Pb2+
(c) Uncharged Organic Sorptives
• it exhibits a range of polarities (non-polar to polar) based on the distribution
of electrons across the molecule
• e.g., the aromatic ring compound, benzene, C6H6, has uniform electron
distribution and hence is non-polar
• Sucrose, C12H22O11, has a non-uniform distribution of electrons and is highly
polar.
Sorptives
Source

The major solid phase materials (sorbents) in soils are
1. Layer silicate clays,
2. Metal-(oxyhydr)oxides, and
3. Soil organic matter (SOM).
Sorbents
Source

• Layer silicate clays are primarily negatively charged
because their stacks of aluminum-oxygen and silicon-
oxygen sheets are often chemically substituted by ions of
lower valance.
• In many soils, they represent the largest source of negative
charge.
1. Layer silicate clays
Sorbents
Source

• Metal-(oxyhydr)oxides are variably charged because their
surfaces become hydroxylated when exposed to water (Liu et al.
1998) and assume anionic, neutral, or cationic forms based on the
degree of protonation (≡M-O-, ≡MOH or ≡MOH2
+, where ≡M
represents a metal bound at the edge of a crystal structure), which
varies as a function of solution pH.
• Thus, these variably charged minerals adopt a net positive surface
charge at low pH and a net negative surface charge at high pH
(Qafoku et al. 2004 ).
2. Metal-(oxyhydr)oxides
Sorbents
Source

• SOM includes living and partially decayed (non-living) materials as well
as assemblages of biomolecules and transformation products of organic
residue decay known as humic substances.
• SOM contains many reactive sites ranging from potentially anionic
hydroxyls (R-OH) and carboxylic groups (R-COOH) to cationic
sulfhydryl (R-SH) and amino groups (R-NH2), as well as aromatic (-Ar-)
and aliphatic ([-CH2-]n) group.
• Variations in the abundance, surface area and chemical composition of
these three sorbents significantly influence the sorption characteristics
of a given soil (Johnston & Tombácz 2002).
3. Soil organic matter (SOM).
Sorbents
Source

Factors Affecting Sorption
1. Sorptive concentration
2. Sorptive and sorbent charge
3. Solution pH
4. Sorbate size
Source

1. Sorptive concentration
• At low sorptive concentration, Le Chatelier's Principle of
chemical equilibrium predicts that increasing (or decreasing)
sorptive concentration will result in an increase (or decrease)
in sorbate concentration.
• For instance, adding fertilizer to a soil will increase the solution
potassium (K+) concentration and subsequently increase the
amount of K sorbed by the solid phase.
• Conversely, as growing plants uptake K+ from the soil solution,
this will drive desorption of the sorbate K+ from the soil. In this
way, the soil serves as a nutrient reserve for plants and soil
organisms.
Source

• A common method for assessing sorption characteristics of a soil is to measure
the relationship between the equilibrium concentration of the sorptive and
the sorbate across a range of sorptive concentrations while holding
temperature and other parameters constant.
• The resulting dataset is termed a sorption isotherm and is often used in a
predictive fashion to describe sorption behavior. Source

2. Sorptive and sorbent charge
• For ionic sorption, the sign and magnitude of electrical charge on
the sorptive and sorbent are important determinates of sorptive fate.
• Anions will be attracted to positively charged sorbents and cations
will be attracted to negatively charged sorbents.
• Ionic sorptives are attracted to oppositely charged sorbents by long-
range electrostatic forces.
• For example, on negatively charged sorbents, these forces result in a
preferential accumulation of cations relative to anions near the
sorbent surface.
• However, diffusive and dispersive forces act to homogenize this ion
distribution such that the ratio of cations to anions decreases
exponentially with distance from the sorbent until the bulk solution
ion balance is obtained.
Source

• A precise description of the resulting ion distribution is complex
and remains incomplete, but is commonly approximated by concepts
involving an inner layer of cations immediately adjacent to the
sorbent surface, often termed the Stern layer and a diffuse
collection of cations and anions, termed the diffuse layer which
undergo diffusive exchange with the bulk solution.
• Sorbates within the Stern layer that shed one or more of their
surrounding water molecules (i.e., waters of hydration) and form
direct ionic and/or covalent (electron sharing) bonds with the
sorbent are considered inner-sphere adsorption complexes, while
those ions retaining their hydration spheres and maintaining
proximity to the surface solely through electrostatic interactions
are considered outer-sphere adsorption complexes (Sposito 2004).
Source

• To accommodate a negatively charged sorbent, cations accumulate closer to
the surface than anions.
• Inner-sphere and outer-sphere complexes occupy the first layer of sorbates,
with the remainder of the sorbent charge balanced by preferential cation
accumulation in the diffuse layer.
Source

3. Solution pH
• Solution pH can have a pronounced effect on sorption by influencing
both the magnitude and sign of sorptive and sorbent charge.
• As the solution pH increases, sorbent hydroxyl and carboxyl
functional groups of metal-(oxyhydr)oxides and SOM deprontonate.
• In turn, this increases negative charge density on the sorbent thus
facilitating cation adsorption while decreasing anion adsorption.
Source

4. Sorptive Size
• Many isovalent series cations exhibit decreasing sorption affinity with
decreasing ionic radius:
Cs+ > Rb+ > K+ > Na+ > Li+
Ba2+ > Sr2+ > Ca2+ > Mg2+
Hg2+ > Cd2+ > Zn2+
• Ionic potential: defined as the charge over the radius (Z/r).
• More is the (Z/r), more is the hydration energy of an ion, so more is the
thickness of hydration shell
• The more is the hydration, more is the effective distance between the
colloidal charge sites to ions and lesser is the attraction.
Source

ADSORPTION IN SOIL
• Adsorption is one of the reactions attributed to the surface
chemistry of soil colloid.
• The adsorption process deals with the adsorption of molecules
or ions from the soil solution on the surfaces of the soil solids.
• The material adsorbed is called- Adsorbate
• The material on which adsorption occurs is called adsorbent.
Source
PRINCIPLES OF SOIL CHEMISTRY
Fourth Edition
Kim H. Tan

• The amount of material adsorbed per unit mass of the
adsorbent depends largely upon the nature of the adsorbate
and the adsorbent, also on the concentration of the
adsorbate and temperature
where, x is the amount of adsorbate adsorbed
m mass of the adsorbent,
C is the equilibrium concentration of the adsorbate
T is the temperature
𝒙
𝒎
= 𝒇(𝐧𝐚𝐭𝐮𝐫𝐞 𝐨𝐟 𝐚𝐝𝐬𝐨𝐫𝐛𝐚𝐭𝐞 and adsorbent, C, T)
ADSORPTION IN SOIL
Source
Fourth Edition
Kim H. Tan

• if the concentration of
adsorbate is more on the
colloidal surface as
compared to its
concentration in bulk phase.
• The solute usually
decreases surface tension.
• If the concentration of the
adsorbate is less on the
colloidal surface than its
concentration in bulk phase.
• In this case, surface tension
is increased.
Bailey and White (1970) believe that perhaps the surface acidity
of the adsorbent may play an important role in the rate and
degree of negative or positive adsorption. The term surface
acidity refers to the H+ ion concentration (acidity) in the
interface.
Positive Adsorption Negative Adsoption
Source
Fourth Edition

•
•
Specific
Adsorption
Non-specific
Adsorption
• Adsorption reaction related to
covalent bonding of solutes by the
adsorbents.
• This type of adsorption is also
called chemisorption by
Schwertmann and Taylor (1977).
• Adsorption reaction related to
electrostatic bonding of solutes by
clay is known as nonspecific
adsorption.
• Sposito (1989) refers specific
adsorption to complexation of
solutes by innersphere surfaces of
clay minerals.
• Nonspecific adsorption is related to
complexation of solutes by
outersphere surfaces of clay
minerals.
Source
Fourth Edition
Kim H. Tan

Adsorption Characteristics
1. SURFACE AREA
• Adsorption is dependent not only on the surface charge, but also on
the surface area.
• The amount of material adsorbed is directly proportional to the
specific surface.
Clay minerals Surface area
Kaolinite 7-30 m2/g
Smectite, vermiculite 600-800 m2/g
Illite 60-100m2/g
Source
Fourth Edition
Kim H. Tan

2. The surface charge density
• It is defined as the number of charges per formula weight divided by the
specific surface area:
σo = e/S
• Where, σo is the surface charge density , e is the number of charges per
unit formula, and S is the specific surface (Fripiat, 1965).
• Due to its small specific surface, the surface charge density of kaolinite
may be larger than smectite; hence, kaolinite would be expected to have a
larger adsorption capacity than smectite.
• However, Bailey and White (1970) believe that the total charge is more
important than the surface charge density in adsorption processes.
Source
Fourth Edition
Kim H. Tan

3. Adsorption reactions are reversible and equilibrium
reactions.
• Sometimes an adsorption process results in chemical
changes of the adsorbed material.
• The changes are of such a nature that desorption is
inhibited; hence, the process is neither reversible nor in
equilibrium. This type of adsorption is called pseudo-
adsorption.
Source
Fourth Edition
Kim H. Tan

4. Adsorption is characterized by a positive heat of
adsorption, meaning that energy is released during the
adsorption process.
• The amount of heat produced because of adsorption of gas
by solid surfaces can be expressed by the following
equation (Gortner and Gortner, 1949):
log h = log a + b log x
in which h is the total heat released, x is the amount of gas
adsorbed in cm3, and a and b are constants.
Source
Fourth Edition
Kim H. Tan

5. Temperature
• Adsorption generally decreases as temperature
increases; in other words, adsorption is less at elevated
temperatures.
• This is caused by an increased kinetic energy of the
molecules at higher temperature, which interferes with
the concentrating process.
Source
Fourth Edition
Kim H. Tan

FORCES OF ADSORPTION
Forces responsible for adsorption reactions include
(1)physical forces,
(2)chemical forces,
(3) hydrogen bonding
(4)hydrophobic bonding,
(5)electrostatic bonding,
(6) coordination reactions, and
(7) ligand exchange.
Source
Fourth Edition
Kim H. Tan

Adsorption Isotherms
• The capacity for a soil or mineral to adsorb a solute from
solution can be determined by an experiment called a batch test.
• In a batch test, a known mass of solid (S m) is mixed and allowed
to equilibrate with a known volume of solution (V ) containing a
known initial concentration of a solute (C i).
• The solid and solution are then separated and the concentration
(C ) of the solute remaining is measured.
• The difference C i - C is the concentration of solute adsorbed.

• The mass of solute adsorbed per mass of dry solid is given
by
where S m is the mass of the solid.
• The test is repeated at constant temperature but varying
values of C i.
• A relationship between C and S can be graphed
• . Such a graph is known as an isotherm and is usually
non-linear.

• Adsorption isotherm is a plot between the amount adsorbed per
unit mass of the adsorbent and the equilibrium concentration of
the adsorptive , at a constant temperature.
• Various equations have been developed to predict precisely the
shapes of the experimental adsorption isotherms between
different adsorbates and adsorbents.
• The most commonly used are due to Langmuir(1918),
Freundlich(1926), Brunauer et al.,(1938)
Source-ISSS

(1) Langmuir Adsorption Theory
• The basic theory of adsorption of gases on soilds is due to
Langmuir.
ASSUMPTION –
• The surface of solid is made up of adsorption sites, each of
which can adsorb one gas molecules.
• All the adsortion sites are identical in their affinity for that gas
molecule.
• The presence of a gas molecules on one site does not affect the
properties of the neighbouring sites.
• The rate of adsorption is proportional to the number of
adsorption sites and the pressure of the gas. The rate of
desorption is proportional to the number of occupied sites.
Source-ISSS

LINEAR EQUATION-
𝑪
𝑿
=
𝑪
𝑿𝒎
+
𝟏
𝑿𝒎𝑲
Where, C= concentration of the adsorbate at equilibrium.
X =Amount adsorbed per unit mass of adsorbent at any instant
Xm= maximum amount adsorbed per unit mass of the adsorbent as a monolayer
K = Equilibrium constant
Source-ISSS

USES
• The use of langmuir equation provides a measure of the
adsorption maximum(Xm) of soils.
• It describes the nutrient (phosphate) retention process in
soil, especially at higher adsorbate concentration.
• From the value of Xm, the surface area (A)of soil or clay can
be esimated ,
• A=
𝑋𝑚 𝑁∏𝑟²
𝑀
Source-ISSS

LIMITATION
• Normally, the langmuir equation is not used to calculate the surface
area of solids because it does not define total adsorption process but
covers only a certain fraction of adsorption site.
• This isotherm refers to gaseous adsorption on non-ionic solid,while
adsorption in soil is essentially an ionic process.
• The soils are mixture of inorganic and organic substances with each
substances reacting towards competing cations differently. Means, soil
exchangers are essentially polyfunctional ion exchangers.
• It does not take consideration the competition between the cationic
species
NOTES-Langmuir isotherm is for monolayer adsorption, but more than a
monolayer of adsorbed molecules can formed at higher pressure.
Source-ISSS

(2) The Freundlich isotherm
• This is one of the most widely adsoption equations to describe
the experimental data on adsorption of ionic or molecular
species in soil.
• It implies that the bonding energy of the adsorbate on a
given adsorbent decreases with the fractional coverage of
surface area of the adsorbent.
• It does not predict any adsorption maximum.
Source-ISSS

• Expressed by equation –
Where, S=amount adsorbed per unit mass of adsorbent
C= equilibrium concentration of the adsorbate
K =partition coefficient and n  1
The linear form of this equation-
n
KCS 
Log x = log K + n log C
Source-ISSS

C (mg L
-1
)
0 10 20 30 40
S(mgg
-1
)
0
10
20
30
40
50
60
S = 1.5C
1.0
S = 5.0C
0.5
FREUNDLICH ISOTHERMS
• When n < 1, the plot is concave with respect to the X- axis. When n = 1, the plot is
linear. In this case, K is called the distribution coefficient (Kd ).
• The partition coefficient K is a measure of the degree to which a adsorbate partitions
between the surface and the solution. The higher the value of K, the greater affinity the
adsorbate has for the surface
Source-ISSS

(3)Brunauer, Emmett, Teller (BET) Adsorption
isotherm
• This theory extends Langmuir derivations to obtain an
equation for multilayer adsorption.
ASSUMPTION –
• Heat of adsorption in the second, third,…nth layers are the
same as the heat of adsorption in the monolayer, i.e., heat of
liquification or condensation of the gas
• If the adsorption take place on a free surface then at
p˚(saturation pressure of the gas), an infinity number of layers
can be built up on the adsorbent.
Source-ISSS

isotherm
• The BET equation used to define multiple layer adsorption at
any equilibrium pressure, p,
LINEAR EQUATION –
• Where, v=volume of gas adsorbed per unit mass of adsorbent
Vm=volume of gas required for monolayer on the entire adsorbent
surface of unit mass
C’ =constant at a given temp
𝑝
𝑣(𝑝˚−𝑝)
=
𝐶′−1 𝑝
𝑉𝑚𝐶′ 𝑝˚
+
1
𝑉𝑚𝐶′
Source-ISSS

isotherm
• ADVANTAGES-
I. It describes the multilayer adsorption.
II. It yields the volume of gas required for a monolayer.
III. It can be used to calculate the surface area of the soil.
Source-ISSS

isotherm
• Surface area can be calculated by multiplying Vm with
the cross-sectional area of the adsorbate molecules.
• To determined the surface area of soils and minerals,
adsorbate molecules such as nitrogen, ethane, water,
ammonia or other gases may be used.
• Nitrogen is a weakly adsorbed molecules, may not
penetrate in to all the interlayer surfaces, but use of
strongly adsorbed molecules, like ammonia, which
penetrate in to the internal surface also, leads to the
measured surface area close to the total surface area.
Source-ISSS

IONS EXCHAGE
1. Cation exchange
2. Anion exchange
Cation exchange capacity-
The capacity of the solids(clay) to adsorb and exchange cations
is called as CEC.
Or the total of the exchangeable cations that can be adsorbed by
a soil colloidal surface
Anion exchange capacity-
The total amount of anions held exchangeably by a unit mass of
soil, is termed as AEC
Source-ISSS

THE SPECIFIC ADSORPTION OF
COPPER BY SOILS
• Very small amounts of solution-plus-exchangeable copper are
present in soil and it has been suggested that these forms of
copper are in equilibrium with specifically adsorbed forms
which constitute the bulk of the ‘available’ copper reserves
(McLaren and Crawford, 1973).
• In their paper, they examine the relative importance of different
types of adsorbing surfaces in soils in relation to the adsorption
of copper.
R. G. McLaren and D. V. Crawford,1973

Materials and Method
• Twenty-four different topsoil samples (air dried) were used
in these studies.
• They were selected to include ranges of pH, organic matter,
free oxides, clay, and total copper contents since these
properties were expected to exert the greatest influence on the
behaviour of copper in soils.
• As well as different soil constituents like
A. Organic matter
B. Clay minerals
C. Soil oxide material
i. Concretionary material (ferro-manganese concretions)
ii. Iron oxide
iii. Manganese oxide

1 gram samples of soil were equilibrated with
200 ml solution(known amount of copper and
0.05M CaCl2) at 30˚C in 250 ml centrifuge bottle
After 24 hours, the samples were then
centrifuged
The supernatant liquid analyzed for copper by
atomic absorption spectrophotometry
Copper adsorbed was calculated from the initial
and final copper concentrations in the solution

Results and Discussion
experiments showed that pH had a
considerable effect on the amount
of copper adsorbed by soils and
individual soil constituents
Adsorption maxima for soil
constituents followed the order
manganese oxides > organic matter >
iron oxides > clay minerals,

In order to compare satisfactorily
different materials, adsorption was
studied in single pH
A pH of 5.5 was selected because most
types of soil surface showed
considerable adsorption of copper at this
pH from a solution of concentration 5
μg Cu 𝒎𝒍−𝟏

Where
x =the amount of copper adsorbed by unit weight
of soil
c = equilibrium copper concentration in solution
a = the Langmuir adsorption maximum
b = the Langmuir ‘bonding term’-related to
bonding energy
Replotting the data in accordance with the
Langmuir adsorption equation produced
linear graphs, indicating satisfactory
agreement with this theoretically derived
equation
𝒄
𝒙
=
𝟏
𝒂𝒃
+
𝒄
𝒂

At low solution concentrations of copper, specific adsorption appears to exceed non-
specific adsorption

Conclusion
• Soils were found to have specific adsorption maxima at pH 5.5 of
between 340 and 5780 μg 𝒈−𝟏
, and a multiple regression analysis
revealed that organic matter and free manganese oxides were the
dominant constituents contributing towards specific adsorption.
• Adsorption maxima for soil constituents followed the order
manganese oxides > organic matter > iron oxides > clay
minerals.
• Clay minerals and free iron oxides show only weak specific
adsorption of copper.
• The link between the amount of solution-plus-exchangeable copper
and the pool of specifically adsorbed copper ,commonly associated
with soil organic fraction, has been well established in certain
condition.

Removal of Lead from Water by
Adsorption on a Kaolinite Clay
• The possible use of kaolinite clay from Nigeria as an
adsorbent for the removal of lead from water was investigated.
• Studies were carried out as a function of contact times,
concentrations, temperatures and pH.
• The kinetics of adsorption as well as adsorption isotherms at
different temperatures were equally studied.
Nigeria Faraday F. 0. Orumwense, 1995

Standard lead solution was prepared by dissolving about
160 mg of lead nitrate in 1000 𝑐𝑚3
deionised- distilled
water (lead concentration is 100 mg 𝑑𝑚−3
).
From the stock of standard lead solution prepared as
described above, the experimental solutions of desired
concentrations were prepared by appropriate dilutions
The pH of each solution was adjusted to the desired value
ranging from pH 3 to 9.5 with solutions of either 0.5 M
NaOH or HCl.
To follow the adsorption of lead by the clay, 1.0 g of kaolinite clay
was introduced into different test tubes containing 50 𝑐𝑚3
of
aqueous solutions of [Pb(NO3)2] of the desired concentrations and
pH.

The stoppered test tubes were agitated in a thermostat
shaker bath at 25°C for different time intervals.
The speed of agitation was kept constant for each run
throughout the experiment to ensure equal mixing.
After different periods of time, the adsorbates were
separated by filtration using filter paper
and the supernatant liquid was analysed using the
atomic absorption spectrophotometer for residual lead
content.

• Figure shows the variation of the amount of lead adsorbed as a function of
time.
• It may be observed from the figure that the amount of lead adsorbed increases
with time and the concentration of lead and then attains a constant value after
120 mins

Langmuir equation has been used to determine
the equilibrium distribution of lead at the solid-
liquid interface and its given by:
Where, C= equilibrium concentration of lead in solution,
q=the amount of lead adsorbed at equilibrium (mg g-')
Qo and b are the Langmuir constants showing the
adsorption capacity of the adsorbent and energy of the
adsorption process, respectively.
It is clear from Table and Fig that the
adsorption capacity of the clay for the
removal of lead increases with an increase
in temperature of the solution, that is, the
equilibrium amount of lead adsorbed
decreases with an increase in
temperature
𝐶𝑒
𝑞𝑒
=
1
𝑄°𝑏
+
𝐶𝑒
𝑄°

• The pH of the aqueous solution is an
important variable which controls the
adsorption of the metal at the clay-water
interfaces.
• Hence, the adsorption of lead on the clay
was examined from solution at different pH
values covering a range of 3 to 9.5.
• The results are presented in figure, which
reveals that the adsorption of lead
decreases from 9.30 mg g-' (93%) to 8.20
mg g-' (82%) with an increase in the pH of
the solution

On the basis of the above results and discussion, the following conclusions
can be drawn.
1. The adsorption process does exhibit a Langmuir type behaviour which is
affected by temperature.
2. The presence of a negative charge on the silica surface sites of the
adsorbent is responsible for the adsorption of lead.
3. Lead removal is favoured by low concentration, high temperature and
acidic pH.
4. The high lead adsorption capacity of the clay at high temperature is
suggestive of an endothermic reaction. The +∆H value at high temperature
(40°C) further confirms that the adsorption process is endothermic, which
is an indication of the existence of a strong interaction between adsorbate
and adsorbent.
5. The results obtained show that the kaolinitic clay is a good adsorbing
medium for lead ions.
Conclusion

Soil Sorption and Desorption

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