Properties of Clay and Organic Colloids I

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Transcript Properties of Clay and Organic Colloids I

One half of the world’s population, about 3 billion people
on six continents, lives or works in buildings constructed
of clay - The New York Times
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Clays and Medicines
Kaolin and Pectin (Oral Route)
Kaolin is an adsorbent medicine used to treat diarrhoea.
It has the ability to adsorb some of the bacterial toxins
that often cause diarrhoea.
Kaolinite is an ingredient in "Kaopectate, Rolaids,
Di-gel, Mylanta, and Maalox."
Researchers have
discovered several
clays that kill—or
prevent from
including antibioticresistant strains.
Geophagy (Eating Clay)
Eating Dirt: It Might Be Good for You
Experts Claim the Habit of Eating Clay May Be Beneficial for Pregnant Women
Oct. 3, 2005
It melts in your mouth like chocolate, says Ruth Anne T. Joiner, describing her favorite treat.
"The good stuff is real smooth," she adds. "It's just like a piece of candy."
Joiner is describing the delectable taste of dirt -- specifically, clay from the region around her home in Montezuma, Ga.
“about two pounds of "Georgia Grown White Dirt" can
be purchased for a little less than $10”
Properties of Clay and
Organic Colloids
Properties of Colloids
Types of Colloids
Structure of Colloids
Sources of Charge on Colloids
Reactions of Soil Colloids
Clay and Organic Colloids
Soil colloids are organic and inorganic matter with very
small particle size and a correspondingly large surface
Organic materials =humus colloids
colloidal fraction
Inorganic materials =clay colloids
Their small size, large surface area, and electrically
charged surface give them the advantage of being highly
Their presence in soil give soil very large surface area.
General Properties of Clay Colloids
Size- Colloids are generally defined to be
less than 2 m in diameter.
Surface area- The smaller the particle size
in a given mass of soil, the greater the
surface area.
Surface Charge- Soil colloids carry
positive or negative electrostatic charges.
General Properties of Colloids (contd)
Adsorption of ions- Negatively charged soil
colloids attract positively charged ions
(cations e.g Al3+, Ca2+, Mg2+, K+, Na+, H+,
etc) while positively charged colloids attract
negatively charged ions (anions e.g NO3-,
SO42-, Cl-, etc).
Adsorption of water- Colloids attract and
hold large number of water molecules due to
the polar nature of water molecules.
Clay Crystal (Micelle)
•A clay unit has different
•Each Layer is made up of
•Each colloid particle
(micelle) attracts thousands
of cations to the colloid
surface by electrostatic
• Some cations will break
away from the swarm on the
surface and be replaced by
other cations of equal charge
in a process called “Cation
Exchange” process.
• The cations and anions
involved in the process are
called exchangeable ions
Classes of Soil Colloids
Soils contain different types of colloids, each
with its particular composition, structure,
and properties. The four main groups of
colloids are as follows:
Crystalline silicate clays (eg. kaolinte, mica, smectites, vermiculites)
Noncrystalline silicate clays (eg. Allophane)
Iron and Aluminum oxide clays (eg. Oxides)
Organic Colloids
1. Crystalline silicate clays
The predominant type of colloids in most
soils is the crystalline silicate type
These colloids have a layered structure
(like pages of a book) -phyllosilicates.
Each layer consists of two to four sheets
of closely packed and tightly bonded O2,
Si, and Al atoms (making them negatively charged).
Crystalline colloids differ in particle
shape, and adsorption of water and ions.
Examples = Kaolinite, smectite, mica, etc
General Structure of Crystalline silicate
Clays (2 main building blocks)
Single silica tetrahedral structure
Silica tetrahedral sheet
Single Al/Mg Octahedral structure
Al octahedral sheet
Figure 1. Silicon tetrahedron structure showing
a silicon ion in coordination with four oxygen
ions to form a tetrahedral structure.
Figure 3. Silicon tetrahedron sheet
in figure 2 turned upside down
toward figure 3 (the Aluminum
octahedral sheet)
Figure 2. Silicon tetrahedron sheet showing one
plane of oxygen ions bonded to two silicon ions in
two directions to form a sheet of silicon tetrahedrons
with unbalanced charges on the apical O ions.
Figure 4. The structure of kaolinite (~0.7
nm thick from the bottom oxygen to the top
oxygen) showing apical oxygen ions of a
silicon tetrahedral sheet bonded with the
octahedral sheet to form a 1:1 layer
Figure 3. Aluminum octahedral structure
showing aluminum in coordination with
six oxygen ions.
Figure 5. The basic structure of 2:1 clay
minerals showing two silicon tetrahedral
layers on top and bottom and one
aluminum octahedral layer in the middle of
the structure
Connecting the tetra and octa building blocks to form planes of
Si and Al (Mg) ions that alternate with planes of O2 and OH ions.
Arrangement of Sheets in Clays
Kaolinite (Si4Al4O10(OH)8)- One sheet of tetrahedron
bonded to one sheet of octahedron
Mica (muscovite)
2:1:1. Chlorite
2. Noncrystalline silicate
These clays consist of tightly bonded O2, Si,
and Al atoms, but they do not have ordered,
crystalline sheets.
Examples = Allophane, immogolite
These group of colloids are highly charged, and are
formed from volcanic ash.
3. Iron and Aluminum
These colloids are found in highly weathered
tropical environments.
They consist of Fe, Mn, and Al atoms in
coordination with O2 atoms.
Fe and Al oxides group of colloids consist of
crystalline sheets. But there are some members
in the group that may not be crystalline.
Their net charge range from slightly –ve to
moderately +ve.
Structure of Oxide colloids
These are octahedral sheets
with either Fe or Al in the
cation positions.
They do not have tetrahedron
sheets, and they do not have Si
in their structure.
They do not have
isormorphous substitution. Eg.
Gibbsite [Al(OH)3]shown here.
Other examples are Goethite
(FeOOH), Hematite [Fe2O3],
4. Organic Colloids
These colloids are not minerals
They are not crystalline
They consist of rings and chains of
C atoms bonded to H, O2, and N.
Organic colloids have a net –ve
Humus particles are the smallest
colloids and exhibit very high
water adsorbing capacity.
Organic Colloid
Structure of Organic Colloid
Structure of Humic Acid
 Consists of large organic
molecules whose chemical
composition varies.
 Structure contains complex
series of C chains and ring
structures with many
functional groups- carboxyl,
phenolic, and alcoholic
 -Ve or +ve charges on the
humus colloid develop as H+
ions are either lost or gained
by these groups.
Sources of Charge on Clay
Isomorphic Substitution - during weathering, primary
minerals dissolve and recrystalize as secondary minerals
During this process, one element may become substituted
for another element of similar size in the crystal structure
without changing the shape of the crystal.
If the two elements do not have the same ionic charge,
then an unsatisfied net charge remains at that point in the
crystal. Common substitutions are Al+3 for Si+4, Mg+2 for
Al+3, and Fe+2 for Al+3, each leaving a net negative charge
on the crystal. This charge is permanent charge or constant
Sources of Charge on Clay Colloids
Exposed hydroxyl groups (-OH-_ on the surfaces of clay
crystals. This accounts for most of the net negative charge in
Kaolinite and some of the charge in Montmorillonite,
Vermiculite and Illite.
Broken oxygen bonds at the edges of crystals. At the
broken edges of crystals, the small Al3+ and Si4+ ions are
exposed to weathering and may be lost. The remaining
oxygen ions have an unsatisfied net negative charge. This is
an important source of charge in all clays. E.g.
>SiOH2+ <------> >SiOH <--------------> >SiO- + H+
>AlOH2+ <------> >AlOH <--------------> >AlO- + H+
Adsorption of Cations
- Cation Exchange
Cation exchange is the exchange of cations
between the soil and the soil solution..
2 Na+ + Ca-clay <-------------> Na-clay + Ca2+
Cation Exchange Capacity (CEC)- sum total of
exchangeable cations that a soil can absorb
CEC is used as a measure of fertility, nutrient retention
capacity, and the capacity to protect groundwater from
cation contamination.
Cation Exchange Capacity is a function of:
Type and amount of clay
Humus content
Adsorption of Anions
- Anion Exchange
Some common anions in the soil include: Cl-, HCO3-,
CO32-, NO3-, SO42-, HPO42-, OH-, F-, H2BO3-, MoO42-,
etc. There are ways that anions are retained
against leaching:
Attraction by exposed cations along the edges of
clay crystals and exposed cations in humus
Adsorption of H2PO4-, SO42-, and MoO42- by Fe,
and Al oxides at low pH
Tropical soils, with lots of Fe, Al oxides, can have net AEC.
Soil Properties affected by Adsorption of ions
nutrient availability- Exch cations are available for
leaching of electrolytes- retention of substances
prevents their movement through the soil.
soil pH- CEC increases with increase in pH
(high OH-).
The colloids in soils are both organic and inorganic.
The size of colloids, structure of colloids (high surface
area), and Charges of soil colloids make them the
center of chemical and physical activity in soils.
The –ve and +ve charge sites they have attract ions
and molecules of opposite charge.
The replacement of one ion for another on the colloid
surface is called cation or anion exchange reaction
The total number of –ve colloid charges per unit mass
is termed CEC
That capacity influences sorption of contaminants,
nutrient availability, and pH of soils.