Organic Matter in Water

1
Chapter 12

• Organic Matter in Water

2
Introduction Organic matters (OM) are found in
oceans and fresh water of all types. Organic
matters have a yellow-brown color. The lowest
concentration of OM found in deep lakes is in the
range 1 to 3 mg/L. The concentration of OM is
determined by measuring carbon element (of
organic origin) content. organic matters (OM)
are present in high amounts in soil (as solid
materials) and in oceans as dissolved or
particulate materials. Also note that the amount
of OM is higher is soil than in oceans.
Carbon masses in the global environment.
Carbon reservoir Mass of carbon (1015g)
Carbon in Atmosphere 720
Carbon in terrestrial environment (soil) -Plants -Soil organic matter () -Peat 830 1400 500
Carbon in aquatic environment (oceans) -Dissolved organic matter () -Dissolved inorganic matter (carbonate species) 1000 37 000
Sedimentary carbonate material 20 000 000
3

• Organic matter (OM) can be classified according
  to their sources to
• Natural organic matter They derived from
  natural sources.
• Anthropogenic organic matter produced from
  industry (Human Effect).
• Organic matter (OM) can be classified according
  to their solubility to
• Dissolved Organic Matter (DOM). Particle
  diameter less than 0.45 µm
• Colloidal Organic Matter (COM). Particle diameter
  in the range 0.01 µm10 µm
• Particulate Organic Matter (POM). Particle
  diameter grater than 10 µm.

Origins of OM in Hydrosphere OM of natural
origin is derived from plant and microbial
residues. In fact dead leaves and roots of
plants finally produce OM. Also, litters
decompose to give OM. OM also produces from
secretions of growing micro and
macro-organisms. After production in soil, the
produced OM is ready to be transferred from soil
to hydrosphere by the action of rainfall or
strong winds.
4
Origins of OM in Hydrosphere On a global scale,
10 of photosynthesis activity in water goes to
the production of DOM. Human input can
contribute to organic matter in water, these
inputs include domestic sewage effluent and
industrial products. Many species of OM have
both natural as well as anthropogenic origins.
For example, 1500 organochlorines species have
identified as natural products found in living
organisms.

• The chemistry of OM.
• Organic matters can be classified according to
  their chemical structures to
• Discrete small molecules which have low molar
  mass. Example organic acids and pesticides.
• Macromolecules which have a large skeleton and
  have certain structural properties and chemical
  reactivity.
• In fact, many of OM have macro-structures.

5
Effect of organic matters (OM) on the
environment. OM can be toxic (in varying
degrees) to living organisms. Polyaromatic
hydrocarbons (PAHs), polychlorinated biphenyls,
dioxins, and pesticides are all typical examples
for toxic organic matters.   OM may be toxic
through their interactions with other organic or
inorganic species that present in water. An
example for this case is the alkylation of Sn4
by CH3-. The products of alkylation are CH3Sn3
and (CH3)2Sn2. These products are more toxic
compare to Sn4 and CH3-. It important to mention
here that the toxicity of the heavy metals may
increase or decrease after interaction with OM.
  OM that present in water can be oxidized by
dissolve oxygen to produce water system of
O2-poor state. This will change the chemistry of
the entire water system. Loss of O2 creates
stress on aquatic species, including fish.  
6

• Types of Humic Materials
• Fulvic acid (FA) FA is the fraction of HM
  that is soluble in aqueous solutions at all pH
  values. This due to the fact that it contains
  both acidic and basic functional groups.
• Humic acid (HA) HA is the fraction of HM that
  is insoluble under acidic pH conditions (pH 2)
  but soluble at pH gt 2. This due to the fact HA
  contains only acidic groups.
•  Humin material (Hu) Hu is the fraction of HM
  that is insoluble at all pH values. This due to
  the fact that Hu dose not contain any type of
  surface functional groups.

7

• Formation of Humic Materials
• Humic material is formed via complex sequences of
  only partially understood reactions. There are
  two mechanisms that can explain the formation of
  HM
• Degradative theory This theory assume that after
  death of plant, the carbohydrates and proteins
  will be degraded and lost by the action of
  microbial agents. The other remaining refractory
  biopolymers (like lignin, paraffinic compounds,
  melanins, and cutin) are transformed to produce
  Humin. The degradative mechanism can be
  presented in the following scheme
• Plant Material ? Humin ? Humic
  acid ? Fulvic cid ? Small molecules
• Polymerization theory This theory assumes that
  plant biopolymers are initially degraded into
  small molecules. The small organic molecules
  repolymerized to form humic substances. As a
  result of these complex re-polymerization
  processes, fulvic acid produces then humic acid
  and finally humin material. This mechanism is
  the reverse of the mechanism of degradative
  theory. The polymerization mechanism can be
  presented in the following scheme
• Plant Material ? Small molecules ?
  Fulvic cid ? Humic acid ? Humin
• - In wet-sediments and in aquatic environments
  the formation of HM is occurred via degradative
  theory because of the presence of oxygen.
  However, the production of HM in soil is occurred
  via polymerization mechanism because of the harsh
  conditions in the soil which converts the plant
  into small molecules immediately after death.

8
Composition and structure of humic
materials Humic substances obtained from
different locations have an identical chemical
composition. Typical chemical composition of
most humic substances
C O H N Ash
45-60 25-45 4-7 2-5 0.5-5
Humic substances (and organic matter) have a high
carbon content (up to 60). Ash comes from the
inorganic materials that present in most soils.
The relation between humic matter content (or
organic matter) and organic carbon content The
mass of HM can be obtained from mass of organic
carbon and vice versa. The relation is mass of
organic carbon 0.6 mass of humic matter (or
organic matter) Or mass of humic
matter (or organic matter) 1.7 mass of
organic carbon The conversion
factor (0.6 or 1.7) is adopted assuming that the
mass percent of carbon in any humic or organic
matter is approximately 60
9
Example The concentration of dissolved organic
carbon (DOC) in a certain natural water system is
2.3 mg/L. Calculate the concentration of organic
matter in this water system. Solution
Concentration of organic matter (in mg/L) 1.7
concentration of organic carbon (in mg/L)
Concentration of DOM 1.7 2.3 mg/L
3.9 mg/L. The carbon content of humic
substances increases in the trend Fulvic acid lt
Humic acid lt Humin The oxygen content of humic
substances increases in the trend Fulvic acid gt
Humic acid gt Humin The chemical analysis made
for humic materials showed that they contain
Several surface functional groups. The
identification of these chemical groups was
carried out using IR and NMR techniques.
10
Chemical functional groups present in HM (or in
OM)
Functional group Chemical structure Content (mmol/gHM)
carboxyl 2-6
Phenolic 1-4
Alcoholic 1-4
Carbonyl (ketones and quinones) 2-6
Methoxyl 0.2-1
The chemical structure of HM is not known
exactly, however, the enclosed Figure depicts a
generic structure for a humic substance.
11

• Forms of Humic Materials
• Humic substances are found in the aqueous and
  terrestrial environment in the following forms
• Free HM Consist of soluble or insoluble forms.
  If particle diameter is less than 0.45 µm then
  the HM is soluble. If particle diameter is more
  than 0.45 µm then the HM is insoluble.
• Complexed HM Humic matter that is bonded to
  metal cations (Mn), PO43-, or organic molecules.
• Surface-bonded HM HM that is bonded to solid
  surface such as clay minerals or Iron and
  aluminum oxides

12
Calculation of the molar concentration of
surface functional groups present in humic
matter HM contains many functional groups that
can make complexation with various organic and
inorganic molecules that present in solution.
Because surface functional groups are important
for complexation, it is important to calculate
their molar concentration in any given sample of
HM. Example Calculate the concentration of
carboxyl groups (in µmol/L) that present in 10
mg/L humic material in a certain water
system. Solution As can be seen in previous
Table, the typical concentration of carboxyl
groups in HM is in the range 2-6 mmol/gHM. The
average concentration of carboxyl groups is 4
mmol/gHM. The concentration of carboxyl groups
in µmol/L 10 mgHM/L 4 mmol/gHM (but
mmol/gHM µmol/mgHM) 10 mgHM/L 4 µmol/mgHM
40 µmol/L
13
Aqueous Humic Materials as Acidifying
Agents. Free HM are an acidic compounds because
of the presence of carboxylic groups (pKa
2.55) and phenolic groups (pKa 910). In
water systems that are lacking natural buffer
(hydrogen carbonate HCO3-), HM can acidify water
to pH 5.56.5 and achieve the electrical
neutrality for water system.
Example A sample of natural water containing 8
mg/L dissolved humic material and the following
ions
Cation Concentration Anion Concentration
NH4 3.6 µg/L (as N) Cl- 0.138 mg/L
Na 75.9 µg/L NO3- 7.0 µg/L (as N)
K 50.8 µg/L HCO3- 14.4 µg/L (as C)
Mg2 0.124 mg/L SO42- 59.4 µg/L (as S)
Ca2 0.569 mg/L
H (pH 5.88) 1.32 10-6 M
Calculate the concentration of carboxylate groups
in HM that will achieve elctroneutrality for the
water system.
14
Solution The total positive and negative charges
should be calculated using molar concentrations
Total positive charge 44.7 µmol/L
Total negative charge 9.4 µmol/L
This water system is not neutral, it contains
more positive charges. Excess positive charge
44.7 9.4 35.3 µmol/L This amount of
positive charge should be balanced by negative
charge to produce a neutral water system. In
fact the only source of negative charges is only
HM. At pH gt 5 (pKa of HM), the humic material
will be fully deprotonated and carry a net
negative charge. pH of this water system is 5.88
and it is higher than pKa of HM. To achieve
elctroneutrality, the HM should provide 35.4
µmol/L of negative charges. It is known that
the concentration of carboxylic acid (or
carboxylate groups) 2 6 mmol/gHM. The
equivalent concentration of carboxylate groups in
(µmol L-1) is 16-48. In fact HM contains enough
amount of negative charge to balance the extra
positive charge in solution (35.4 µmol/L).
15
mechanisms of interaction of organic molecules
with hm. Mechanism 1 Electrostatic Attraction
Interaction between positively charged molecules
and negatively charged HM (electrostatic
attraction). Example for this mechanism is the
interaction between HM and atrazine at pH 8.
At this pH, atrazine is positively charged and HM
is negatively charged.
16
Mechanism 2 Hydrogen Bonding. The hydrogen
bonding reactions are possible, involving oxygen,
and nitrogen-containing functional groups of both
HM and organic solute. Example for this
mechanism is the interaction between carbaryl (an
insecticide) and
17
Mechanism 3 Salt linkage or salt bridge In
this mechanism a salt bridge is formed between
the negative surface charge of HM and the
negatively charged organic solute. Example on
this mechanism is the interaction between
2,4-dichlorophenoxyacetic acid (2,4-D) and HM
molecules at pH 6-8 . At this pH range, both HM
and 2,4-D are both negatively charged.
18
Mechanism 4 Hydrophobic interactions The
interaction of non-polar molecules with HM is
occurred via hydrophobic hydrophobic
interactions. The non-polar molecules will
interact with the non-polar part (or hydrophobic
part) of HM. An example on this mechanism is the
interaction between DDT (dichlorodiphenyltrichloro
ethane) with HM molecules .
19

• Interaction of HM with clay minerals.
• There are three mechanisms of interaction between
  clay minerals and HM molecules
• Specific adsorption between Al3 Fe2 that
  present on clay surface and HM molecule.
• HM and clay minerals can be bonded via a salt
  bridge (Mechanism 3). Ca2 and Al3 can work as
  a salt bridge.
• Hydrogen binding. Clay minerals contain surface
  hydroxyl groups (-OH) these polar groups can
  make hydrogen bonding with polar groups present
  in HM molecule.

Specific adsorption M Al3 or Fe3
Hydrogen bonding
Salt bridge M Al3 or Ca2
20
Chapter 13

• Metals in the Hydrosphere

21
Introduction Humic materials, whether it is
dissolved in water or present as part of the
solid phase in soils and sediments, have
functional groups that are capable of acting as
ligands in forming complexes with metals. Most
metals can interact with HM. The functional
groups available for complexation reactions are
presented below.
A possible reaction between Pb2 ions and a
portion of HM is given the following reaction
22

• Factors that affecting the complexation of HM
  with metal cations
• Nature of the bond formed Covalent versus Ionic
  bond
• Alkaline earth metals form a weak covalent bond
  with the negative sites on the HM. While, the
  other heavy metals have large stability constants
  with HM, so, they form very stable covalent
  bonds. For example Cu2 and Pb2 tend to be
  more strongly bonded with HM than Ca2 and Mg2
  ions. This because Cu2 and Pb2 are able to
  make covalent bonds with HM, while Ca2 and Mg2
  are not able to make covalent bonds with HM and
  prefer to make ionic bonds.
• b) pH-value
• At acidic solutions (low pH), H will be in high
  concentrations, and so, H ions will compete
  metals for active sites on HM and decrease metals
  complexation.
• c) Ionic strength (IS)
• IS can be generated from salts like NaCl, KCl,
  NaHCO3 and Na2SO4. Ionic strength of the solution
  is inversely related to the complexation of metal
  cations with HM. This can be attributed into two
  reasons1) a competition of (Naand K) with
  metal cations for the negative active site on HM
  at high IS 2) the high concentration of anions
  (Cl-, SO42-, and HCO3-) will increase the
  complexation of these anions with metal cations
  and hence reduce their complexation with HM.

23
d) Availability of functional groups The
maximum complexation between metal cations and HM
functional groups occurs at 11 ratio. This
indicates that the complexation process is a
one-step process.
24
Complexation reactions of metal cations with HM.
The complexation reaction between the
functional groups in HM and dissolved metal
cation (Mn) can be presented as

The H-HM complex should be stable only at a)
lower ionic strength solution, b) moderate to
high pH values, c) Mn prefers to make covalent
bond, and d) 11 complexation occurs. The
value of Kf is changing with these variables
(a-d). Therefore, Kf values always calculated at
fixed conditions of pH, ionic strength, and
temperature (operationally defined). The pH
value has a significant effect on Kf values In
most calculations, a conditional formation
constant is used (Kf' ), which is the formation
constant (Kf), that is calculated at certain pH
value Kf' Kf C Kf' the
conditional formation constant. Kf the formation
constant. C constant which is calculated at
certain pH.
25
Conditional formation constants (Kf') at pH 5.0
for soluble fulvic acid (as a form of HM) with
some metals.
Kf' Mg2 Ca2 Mn2 Co2 Ni2 Cu2 Zn2 Pb2
Kf' 1.4102 1.2103 5.0103 1.4104 1.6104 1.0104 4.0103 1.1104
Example A natural water sample (at pH 5)
containing 85 µg/L (1.45 µmol/L) of Ni2 ions and
8 mg/L of soluble fulvic acid (a form of HM).
Assume that the concentration of functional
groups in fulvic acid that are capable of binding
with Ni2 is 5.0 mmol/g. Calculate the
concentration of complexed nickel and the
un-complexed nickel. The interaction of Ni2 by
the functional groups present in fulvic acid can
be imagined as
Ni2 FA ? Ni-FA
Kf' 1.6104
26
SOLUTION The concentration of functional groups
should be calculated in (µmol/L or
mol/L) Concentration of functional groups in FA
8.0 mg/L 5 mmol/g (or 5 µmol/mg)

40 µmol/L (concentration of binding sites)
Ni2
FA ?
Ni---FA Initial (M) 1.4510-6
4010-6
zero Change
-1.4510-6 -1.4510-6
1.4510-6
(M) net
zero
3.910-5
1.4510-6
(M) Change X
X
-X (X in M) Equilibrium
X 3.910-5 X
1.4510-6-X
(M)
At equilibrium
27
SOLUTION
The concentration of un-complexed Ni2 X
8.9310-7 mol/L. The concentration of
complexed Ni2 cinitial cequilibrium

1.4510-6 - 8.9310-7
5.610-7 M The percentage of
un-complexed Ni2
The percentage of complexed Ni2
Note The complexed nickel (39) stays in the
environment for a long time because the
complexation with HM keeps Ni2 ions in solution
for a long time.
28

• Metal complexes with ligands of anthropogenic
  origin
• There are many complexing agents that have
  anthropogenic sources
• NH3 result from nitrogen-containing organic
  waste.
• SO32- and SO42- result from pulp and paper
  mills.
• PO43- result from detergents industry.
• CN- result from gold industry.
• EDTA result from paper, detergent industry.
• NTA (nitrilotriacetic acid, H3T) result from
  detergent industry.
• Last two complexation agents are strong
  complexing agents for most metals in solution. A
  tetrahedral complex that is formed between NTA
  and Cu2 ions in solution.

29
Chapter 14

• Environmental Chemistry of Colloids and Surfaces

30
Introduction There is a clear connection between
the hydrosphere and terrestrial (solid)
environment. This connection contains suspended
solids of very small particle size that can
interact with ions in water. These suspended
matters (or sediments) are called colloids and
they come from the solid part of the environment.

some of natural colloids present in natural
water.
Colloid
1. SiO2
2. MnO2
3. Fe2O3
4. Al2O3
5. Humic material (humic and fulvic acid)
6. Montmorillonite (clay type)
7. Kaolinite (clay type)
sediment/water system and soil/water system.
31
the size-range distribution of some components of
natural water system including colloidal
materials.

1. The diameter of colloidal particles falls in the
   range 0.01 µm to10 µm.
2. Particle size less than 0.45 µm is considered to
   be soluble in water (true solution)
3. Particle size higher than 0.45 µm is considered
   to be insoluble(i.e., colloidal solution) .

32
Why do colloidal materials adsorb different
solutes (metals and organics) in solution?
33
Adsorption Process. Adsorption is a physical or
a chemical process in which a solute (adsorbate)
is transferred from solution to the adsorbent
surface. Adsorption finished when the active
sites (adsorption sites) filled with adsorbate
molecules.
Chemical or physical bond between adsorbate
molecule and surface site
The adsorption of an adsorbate onto a colloid
surface.
34
Factors control adsorption process Colloidal
materials adsorb metals and organics because they
have large specific surface area and contain
surface functional groups. Specific surface
area Most colloids have a large specific surface
area. As surface area increases the amount of
adsorbed ions increase.
The specific surface area of some colloids.
Colloid Specific surface area (m2/g)
Montmorillonite 5-20
Kaolinite 700-800
Humic and fulvic aid 700-1000
35
Example Calculate the particle diameter (in m)
for Montmorillonite assuming that the density of
this material is 1.7 g/ml.
Solution The average specific surface area of
Montmorillonite is 12.5 m2/g. Particle diameter
can be calculated from the following
relation Specific surface area (m2/g)
12.5 m2/g
dp (particle diameter) 310-7 m (0.3 µm)
Surface charge Most colloidal substances have
surface functional groups. These groups played
an important role for attracting the adsorbate
molecules from water. Adsorption of adsorbate
molecules on colloids decreases the concentration
of adsorbate in solution.
36
Types of Adsorption Processes 1.Electrostatic
Adsorption Colloidal materials can acquire
surface charge in solution. The nature of this
surface charge depends on the pH of the solution
in which they present (i.e., the pH of
surrounding solution). One can determine the net
surface charge (positive, negative or neutral) on
a colloid surface by comparing the solution pH
and the pHo of the colloid. pHo (pH at which the
net surface charge of colloidal material is zero)
is constant and can be determined experimentally
for any colloidal material. If solution pH gt pHo
then the colloid surface charge is negative and
if pH lt pHo then the net surface charge is
positive and if pH pHo then the net surface
charge is zero (neutral surface).
pHo values of various natural colloids
Colloid pH0
SiO2 2.0
MnO2 4.0
Fe2O3 (Hydrated) 7.0
Fe2O3 (Goethite) 7.5
Colloid pH0
Fe2O3 (Haematite) 8.5
Al2O3 (Hydrated) 7.0
Humic material 4-5
37
Humic material has a variably charged surface.
Negative surface charge is due to deprotonation
of carboxyl groups, and positive charge is due to
amino groups protonation. Generally speaking, the
pH of natural water is about 8.7 therefore, the
majority of natural colloids have a net negative
surface charge in solution. This negative charge
is balanced by the positive ions (like Na, K)
present in water system. Based on that it can be
imagined the colloidal particles in natural water
as
The ion-exchange reactions (electrostatic
adsorption) of colloidal materials with cations
is one of the suggested mechanism in solution
and can be presented as Colloid-O-Na K
? Colloid-OK Na In soil and sediments, the
importance of cations for ion-exchange with
colloidal materials is Ca2 gt Mg2 gt K gt Na
38
H competition In lower pH solutions, H can
displace the cations that present on the colloid
surface as following Colloid-O-Na H ?
Colloid-OH Na This is known as a
competition of H with the cations for the
binding sites of humic substances. The number
of negative exchange sites, which equal to the
number of positive charge, on a colloid material
is the called cation exchange capacity (CEC).
39
2. Specific adsorption In this type of
adsorption, a strong interaction between the
colloid and solute is established. This type of
adsorption occurs at pH pHo of the colloid.
The adsorption of organic acids (like fatty acid)
by colloidal iron oxides surface is a known
example for specific adsorption
Metal cations are also able to form specific
bonds with iron-oxide
40
The electrical double layer When a colloid
material is added to water, an electrical
potential is developed around the colloid surface
and this occurs due to the balancing of the
negative charge on the colloid with positive
charge that come from surrounding solution.
The electrical potential is highest at the
colloid surface and decrease to zero at the bulk
of the solution. The colloidal system is
stabilized in solution because the small charged
particles repel each other in solution.
Electrical double layer.
41


Electrostatic repulsion between colloid particles


These particles are unable to come close to
aggregate into settleable particles, and this due
to electrical repulsion. In fact adding strong
electrolyte (HCL or NaCl) will reduce the
thickness of the double layer, the surface
potential will decrease and coagulation
(aggregation) started.
The process of coagulation is called some times
sedimentation and it is usually occurred when
river colloids encounter with the high-salt sea
water.
Electrical double layer in a salty solution.
42

• Quantitative description of Adsorption
• Adsorption can be described quantitatively by two
  mathematical equations
• Langmuir equation
• This relation assumes the following hypothesis
• Number of active site that is capable of reacting
  with adsorbate molecules is limited.
• No adsorption after filling these sites
• No interaction between adsorbed molecules
• Langmuir equation is

Where Cs concentration of adsorbate
molecules on colloid surface (mol/g) Caq
concentration of adsorbate molecules remaining is
solution (mol/L) b binding constant
(L/mol) Csm maximum quantity of adsorbate
adsorbed on the colloid surface (mol/g)
43
The linear form of Langmuir equation is
(2) A
linear plot between (1/Cs versus 1/Caq) will
generate a straight line with (1/bCsm) as a slope
and (1/Csm) as an intercept.
Example Phosphate adsorption by a sedimentary
material can be presented by Langmuir equation.
The following data were obtained form phosphate
adsorption Equilibrium "P" concentration
in water Caq
(mol/L) in solid (sediment) Cs (mol/g)
Sample 1 4.0 10-7
2.0 10-7 Sample 2
1.3 10-7
1.0 10-7
44
Solution Applying Langmuir equation to sample 1
Applying Langmuir equation to sample 2
Solving the above linear equations, b and Csm can
be obtained b 2.7106 L/mol Csm 3.810-7 mol
P/gsediment (or 12.8 µg P/gsediment). The last
number indicate that 12.8 µg of P is adsorbed on
one gram sediment.
45
The Freundlich Equation Adsorption on colloids
or soil can be presented by Freundlich empirical
equation
Where Cs concentration of adsorbate molecules
on colloid (mol/g). Caq concentration of
adsorbate molecules remaining is solution
(mol/L). KF Freundlich constant (L/g). n
dimensionless number (n is usually less than
1.0). The linear from of Freundlich equation
is LogCs LogKF nLogCaq A linear
plot between LogCs versus LogCaq will generate a
straight line with (n) as a slope and (LogKF) as
an intercept.
46

• Notes on Freundlich equation
• Adsorption of adsorbate on solid surface becomes
  more difficult as more and more adsorbate
  accumulate (i.e. non-linear adsorption).
• 2. No maximum capacity can be estimated from
  Freundlich relation. This equation is valid only
  at lower concentration ranges.

Example For a frost soil, it has been reported
that KF 0.0324 L/g, n 0.82 for Cd metal
adsorption. Calculate Cs if Caq 4
µg/L. Solution
Cs (0.0324 L/g)(4 µg/L)0.82 Cs 0.1 µg Cd/g
47
Partitioning of small organic solutes between
water and soil (or colloid surface) 1. The
distribution Coefficient Kd At very dilute
concentration, n in Freundlich equation can be
approximated to unity, therefore, Freundlich
equation becomes
Usual units of Kd is (L/kg or mL/g) Cs
concentration of adsorbate molecules on colloid
or soil (mol/kgsoil) Caq concentration of
adsorbate molecules remaining is water
(mol/L) Notes If Kd gtgt 1, then the pollutant
is mainly adsorbed on the soil If Kd ltlt 1, then
the pollutant did not adsorb on the soil and
remain in water. The value of Kd depends on
Organic solute itself, the chemical and physical
nature of the solid phase (soil), temperature,
ionic strength of the solution. Because these
variables are different from soil to soil, then
it is impossible to tabulate values of Kd for
pollutants. In fact, KD value can be correlated
to a number of distribution coefficients KOM,
KOC, and KOW. These coefficients can predict the
movement of organic pollutants (like pesticides)
and inorganic pollutants (like metals) within
soil.
48
2 . Sorption of organic and inorganic pollutants
by soil The effective parts of the soil that
can interact with pollutants are only organic
matters and clay minerals. The surface
functional groups present in clay and organic
matter that can interact with pollutants Clay
(-Si-OH). Organic matter (OM-COOH, OM-OH,
OM-R-O-R, OM-R-NH2, OM-R-CO-R, and OM-R-COH).
The type of interactions of various pollutants
with clay minerals and organic matters are
mainly Van der waals, induced dipole-dipole,
dipole-dipole, H-bonding forces, and
hydrophopic-hydrophopic interactions.
49
2 . Sorption of organic and inorganic pollutants
by soil
Compared to clay minerals, organic matters (OM)
are more effective to adsorb pollutants from
solution. This because of the presence of many
functional groups in OM compare to clay
minerals. Most soils contain organic matters
(OM). The percentage of OM in soil is in the
range (13). Clay minerals also present in soil
in small quantities.
The concentration of a solute in the soil can be
presented as Cs fOMCOM fMMCMM Where
fOM and fMM are the fractions of organic
matter and mineral matter in the soil. COM
Concentration of solute, or adsorbate, or
pollutant in the organic matter component of the
soil or sediment (mol/gOM or mol/KgOM). CMM
Concentration of solute, or adsorbate, or
pollutant in the mineral matter component of the
soil or sediment (mol/gMM or mol/KgMM) Assuming
that CMM is so small, then Cs fOMCOM
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3. Organic Matter-Water Partition Coefficient
(KOM) and Organic Carbon-Water Partition
Coefficient (KOC) The distribution of
pollutants in water-soil systems can be described
by calculating KOM and KOC Organic Matter-Water
Partition Coefficient
Organic Matter-Water Partition Coefficient
Where Caq Concentration of solute, or
adsorbate, or pollutant in water
(mol/L). COMConcentration of solute, or
adsorbate, or pollutant in the organic matter
component of the soil or sediment (mol/gO.M or
mol/KgO.M). COCConcentration of solute, or
adsorbate, or pollutant in the organic carbon
component of the soil or sediment (mol/gO.C or
mol/KgO.C). Note that if KOM and KOC values are
greater that 1, then the pollutant is mainly
immobilized (adsorbed) on the surface. If the
values of KOM and KOC are less that 1, then the
pollutant is present in solution and has low
affinity for soil.
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4. The relation between Kd, KOM, and KOC
coefficients
52
Octanol-Water Partition Coefficient (KOW)
The experimental determination of Kd, KOM, and
KOC values for pollutants is not practical
because of the variations in the chemical and
physical properties of soils. Based on that,
environmental scientists developed a test to
predict the movement of various pollutants in
soil without determination of Kd, KOM, and KOC
values. n-octanol was selected because this
compound can perfectly function as organic matter
that present in the soil. n-octanol has
hydrophilic and hydrophobic components
KOW value can be used as a guide to predict the
movement of pollutants in soils.
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Octanol-Water Partition Coefficient (KOW)
The distribution of a pollutant between water and
octanol can be carried out by adding a certain
amount of the pollutant to a mixture of water and
n-octanol. After shaking the mixture, the
equilibrium concentration of the pollutant is
determined in each layer. The n-octanol/water
distribution coefficient can be calculated as
follows
Where KOW is the n-octanol/water distribution
coefficient (Lwater/Loctanol) Coctanol
Concentration of solute, or adsorbate, or
pollutant in the octanol layer (mol/Loctanol) Caq
Concentration of solute, or adsorbate, or
pollutant in aqueous solution (water)
(mol/Lwater)
Note if KOW lt 1, then the solute remains in the
aqueous phase and has a low molar mass and also
has high oxygen content. If KOW gt 1, then the
solute prefer to stay in octanol layer and this
also indicates that the pollutant has a high
molar mass and a large carbon to oxygen ratio.
Solutes of lower values of KOW do not adsorb on
soil and remain in solution. Solutes of higher
values of KOW are highly adsorbed on the soil.
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Solutes of higher molar masses have higher KOW
55
The relation between KOW and KOM In fact there
is a high correlation between KOW and KOM as
shown below
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Clay minerals Clay minerals are natural type
of colloids that distributed throughout the word.
Clay minerals are composed of "aluminosilicate
minerals" with a layered lattice structure. Clay
minerals are of terrestrial origin and carried to
water (hydrosphere) in run-off and by winds.
Clay minerals have SiO42 tetrahedral units
linked together in a planer structure by oxygen
atoms. There are many types of clay minerals
kaolinite, halloysite, smectite, vermiculite, and
chlorite. Each one of these minerals has its own
CEC capacity. CEC values of some natural clay
minerals and some other natural colloids.
Material Molecular formula CEC cm()/kg
Kaolinite Al2Si2O5(OH)4 8
Halloysite Al2Si2O5(OH)4.2H2O 8
Smectite (Na,Ca)0,3(Al,Mg)2Si4O10(OH)2 nH2O 100
Vermiculite (Mg,Fe,Al)3(Al,Si)4O10(OH)10.4H2O 125
Chlorite (Fe, Mg, Al)6(Si, Al)4O10(OH)8 25
Hematite Fe2O3 4
Feldspar NaAlSi3O8 2
Quartz SiO2 2
Organic matter See Figure (1) chapter 12 200
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Cation Exchange Capacity of Clay Minerals The
total cation exchange capacity of clays measures
its ability to exchange with cations in solution.
Clay minerals can undergo simple ion-exchange
reaction with cations present in solution as
following
If the amount of released Na is determined, then
the total cation exchange capacity of the clay or
soil-containing clay can be determined from the
following relation
Note that K is strongly adsorbed compare to Na
on the clay surface.
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Example 1.0 gram of a clay sample (in Na-form)
was mixed with 100.0 mL of 1.0 M of KNO3
solution. After filtering the colloid clay
sample, Na ions in the extract was analyzed
using atomic absorption spectroscopy and found to
be 250 ppm. Calculate the CEC for this clay
sample in cmol()/kgclay. Solution
0.1087 cmol()
We assume that all Na ions released from the
clay sample were replaced by K. Total amount
of Na released 250 mg/L 0.1 L 25 mg Na (or
0.025 g Na) Moles of Na
Mole of positive charge 1.087 10-3 mol 1
(Na contains one positive charge)
1.087 10-3 mol ()
charge (note that cmol 10-2 mol or mol
102 cmol) 1.087 10-3 mol ()
0.1087 cmol()
Based on that, we can write 0.1087 cmol() ?
1.0g ? ? 1000.0 g (1.0 kg)
CEC 109 cmol()/kg
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Example A sedimentary material containing 8
organic matter, 41 clay minerals 70 Kaolinite
and 30 chlorite. Calculate the total CEC (cmol
()/kg) of this sedimentary material. Solution
Using previous table CEC 0.08 200 (0.41)
(0.70) (8) (0.41) (0.3) (25) 21 cmol
()/kg (CEC for organic matter) (CEC
for kaolinite) (CEC for chlorite)