7/30/2019 Soil Physics Lecture 5[1]

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SOIL PHYSICS

The Solid Phase: Nature and Behavior of Clay Clay Minerals

• Clay minerals by definition consist of particles less than 2μmin equivalent diameter.

• Clay minerals are built up of silica and alumina sheets andclassified into types:

2:1 clays with 2 tetrahedral and 1 octahedral sheets

1:1 clays with 1 tetrahedral and 1 octahedral sheets

• The silicon tetrahedron: Si surrounded by 4 O atoms.

• The aluminum octahedron: Al surrounded by 6 OH- .

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SOIL PHYSICS

The Solid Phase: Nature and Behavior of Clay Clay Minerals

Si

4+

 Al3+

O-

Tetrahedral 

Octahedral 

Tetrahedral 

1:1 Clay 2:1 Clay

7/30/2019 Soil Physics Lecture 5[1]

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SOIL PHYSICS

The Solid Phase: Nature and Behavior of Clay Clay Minerals

1:1 Clay

2:1 Clay

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SOIL PHYSICS

The Solid Phase: Nature and Behavior of Clay Clay Minerals

• Examples of clay minerals are kaolinite, micas, illite, chlorite, vermiculite, and montmorillonite.

• Most significant is montmorillonite due to its high degree of shrinkage and swelling.

• Clays are also characterized by high specific surface ranging

from 5-100 m2 kg-1 for kaolinite to 700-800-to m2 kg-1 formontmorillonite.

• The negative charge of clays arises from isomorphoussubstitution.

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SOIL PHYSICS

The Solid Phase: Nature and Behavior of Clay Clay Minerals

Isomorphous substitution

• Refers to the substitution of an ion by another of similar size:

 Al3+ for Si4+ in the tetrahedral sheet

Mg2+ or Fe2+ for Al3+ in the octahedral sheet

Isomorphous substitution is the main cause of the negative

charge in clays.  Another source of negative charge is the broken edges.

The unbalanced charge must be compensated externally by theadsorption of ions (mostly cations) from surrounding solution.

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SOIL PHYSICS

The Solid Phase: Nature and Behavior of Clay Clay Minerals

Coulomb's Law 

F = force of attraction; q 1 and q 2 = the quantity of charge on twoobjects (Coulombs); d = the distance of separation between thetwo objects (m); k is Coulomb's constant.

• Clay minerals with negative charge at the tetrahedral layerhave higher attraction.

• This affects the hardness, ease of fracture of the minerals,swelling, crystal growth and mineral internal surface area.

2

21

d 

qqk  F 

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SOIL PHYSICS

The Solid Phase: Nature and Behavior of Clay Clay Minerals

Swelling and Shrinkage of Clay Minerals

• The amount of swelling is related to the strength of 

interlayer bond: if strong, you have non-expanding clay minerals

if weak, you have swelling clay minerals

• Clay dispersion occur when particles are separated to the extentthat the attractive forces are no longer able to pull them together.

+ + + + + + + + +

H2O

Interlayer spacing

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SOIL PHYSICS

The Solid Phase: Nature and Behavior of Clay Clay Minerals

Kaolinite

•It is a 1:1 clay mineral

• Has very little or no isomorphic substitution

• Has very little layer charge

•  Very few counterions (low CEC: 3-15 meq/100 g clay)

• Layers are held together by H-bonding

• External surface area only ≈ 25 m2/g of clay 

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SOIL PHYSICS

The Solid Phase: Nature and Behavior of Clay Clay Minerals

Montmorillonite

•It is a 2:1 clay mineral

• Fair amount of isomorphic substitution in octahedral sheet

• High CEC: 80-120 meq/100 g clay 

• Layers are held electrostatically by the covalent ions.

• The bond is relatively weak because of the distance.

• Large specific surface: 700-800 m2/g of clay 

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SOIL PHYSICS

The Solid Phase: Nature and Behavior of Clay Clay Minerals

Illite

•It is a 2:1 clay mineral

• Has isomorphic substitution in tetrahedral sheet

• Force of attraction is strong so that K+ is fixed

•  Water molecules do no enter in interlayer space

• The CEC is relatively low (20-40 meq/100 g clay)

K+ K+ K+ K+

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SOIL PHYSICS

The Solid Phase: Nature and Behavior of Clay Clay Minerals

Chlorite

•It is a 2:1:1 clay mineral

• Has isomorphous substitution in tetrahedral sheet

• Brucite-like layer (with some Al) is sandwiched betweentetrahedral layers

• Like illite, water molecules do no enter in interlayer space

• The CEC is relatively low (20-40 meq/100 g clay)

brucite [Mg2 Al(OH)6]+1

[Mg3 AlSi3O10(OH)2]-1

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SOIL PHYSICS

The Solid Phase: Nature and Behavior of Clay Clay Minerals

The Diffuse Electrical Double Layer

•Since overall electrical neutrality is always maintained innature, the net negative charge of the clay platelet iscompensated by an equivalent positive charge of cations.

• Since the cations counteract the negative charge of the clay 

platelet, they are called counterions.

• Counterions include Na+, K+, H+, Mg2+, and Ca2+, and areeither adsorbed at the surface or swarm in the surroundingsolution.

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SOIL PHYSICS

The Solid Phase: Nature and Behavior of Clay Clay Minerals

The Diffuse Electrical Double Layer

•Looking at the principle of minimum energy, cations try to beas close as possible to clay surface. Condition of minimumenergy .

• But due to their thermal motion (kinetic energy), the cations

 will tend to diffuse away from the clay surface and maintainsame concentration everywhere. This represent a condition of maximum entropy .

• The net result of these opposed forces is the condition of 

minimum free energy .

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SOIL PHYSICS

The Solid Phase: Nature and Behavior of Clay Clay Minerals

The Diffuse Electrical Double Layer

-----------

-----

+++++++++++

+++++ 

----------------

----------------

+

+

+

+

+

+

+

+

+

+

+

+

+

+

+

+

+

+

+

+

+

+

+

+

+

+

++

Distance from clay particle

Minimum Energy Maximum Entropy Minimum Free Energy

+

+

+

+

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SOIL PHYSICS

The Solid Phase: Nature and Behavior of Clay Clay Minerals

The Diffuse Electrical Double Layer

   I  o  n 

  c  o  n

  c  e  n   t  r  a   t   i  o  n 

Distance from clay particle

c+

 A-

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SOIL PHYSICS

The Solid Phase: Nature and Behavior of Clay Clay Minerals

The Diffuse Electrical Double Layer

Distance from clay particle

   I  o  n 

  c  o  n

  c  e  n   t  r  a   t   i  o  n 

n1

n2

n2 > n1

Effect of changing ion concentration 

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SOIL PHYSICS

The Solid Phase: Nature and Behavior of Clay Clay Minerals

The Diffuse Electrical Double Layer

Distance from clay particle

   I  o  n 

  c  o  n

  c  e  n   t  r  a   t   i  o  n 

Na+

Divalent (Ca2+) vs monovalent (Na+)cations 

Ca2+

7/30/2019 Soil Physics Lecture 5[1]

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SOIL PHYSICS

The Solid Phase: Nature and Behavior of Clay Clay Minerals

The Diffuse Electrical Double Layer

Distance from clay particle

   I  o  n 

  c  o  n

  c  e  n   t  r  a   t   i  o  n 

 A 

Effect of drying: Soil A is drier than soil B 

B 

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SOIL PHYSICS

The Solid Phase: Nature and Behavior of Clay Clay Minerals

Ion Exchange

• The cations in the double layer can be replaced or exchanged by 

other cations introduced into the solution.• This exchange is considered to be an intrinsic property of the

soil, called cation exchange capacity (CEC).

• Cation exchange depends on both clay content and clay type.

• Cation exchange affects retention of ions in the soil but alsoaffects transport of pollutants.

• Flocculation-dispersion processes of the soil colloids are affectedby the cation exchange.

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SOIL PHYSICS

The Solid Phase: Nature and Behavior of Clay Clay Minerals

Hydration and Swelling

•  Water is attached to clay surfaces by several mechanisms:

Electrostatic attraction Hydrogen bonding  Adsorbed cations

• When a confined body of clay sorbs water, swelling occurs.

• Swelling is greatest for monovalent cations.

• High salinity of the soil solution will suppress swelling.

• Swelling lowers soil permeability as larger pores get clogged.

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SOIL PHYSICS

The Solid Phase: Nature and Behavior of Clay Clay Minerals: Hydration and Swelling

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SOIL PHYSICS

The Solid Phase: Nature and Behavior of Clay Clay Minerals

Flocculation and Dispersion

• As clay particles interact with one another , we have: Repulsive forces

 Attractive forces

• When repulsive forces are dominant =dispersion.

•  When attractive forces dominate = flocculation.

Plate –condensation flocculation (tactoid)

Card-house structure flocculation

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SOIL PHYSICS

The Solid Phase: Nature and Behavior of Clay Clay Minerals

Flocculation and Dispersion