Soil Colloids: Properties and Unit Cell

Soil Colloids

• A colloid is a two-phase system with one finely divided phase dispersed in another.
• Colloids are derived from the Greek words "colla" (glue) and "eidos" (like).
• Colloidal state is a two-phase system where one phase is finely divided and dispersed through a second continuous medium.
• Examples of colloids include: solid in liquid (clay in water) and liquid in gas (clouds in the atmosphere).
• Soil particles with a size range of 1nm to 1µm are called soil colloids.
• Soil colloids are chemically active and greatly influence ecosystem functions.

Properties of Soil Colloids

• Size
• Surface area
• Colloids have predominantly negative charges
• Adsorption of cations and anions
• Swelling and Shrinkage
• Plasticity
• Cohesion
• Flocculation and Deflocculation
• Brownian movement
• Tyndall effect

Unit Cell

• A unit cell is the simplest repeating unit in a crystal.
• The unit cell repeats itself in 3 dimensions to form the crystal lattice.
• Basal spacing plays a key role in identifying clay mineral species using X-ray diffraction.

Types of Soil Colloids

• Soil colloids can be classified into two main categories: inorganic and organic.
• Inorganic soil colloids include: crystalline silicate clays, noncrystalline silicate clays, and iron and aluminum oxides.
• Organic soil colloids consist of humus.

Inorganic Soil Colloids

• Layer aluminosilicates are inorganic colloids comprised of thin layers of repeated structural units.
• Layer aluminosilicates are dominant clay minerals found in temperate regions.
• Amorphous aluminosilicates form from volcanic ash.
• Aluminum and iron oxides can be crystalline or amorphous and are common in subtropical and tropical regions.

Organic Colloids

• Organic colloids are more chemically reactive and have a greater influence on soil properties.
• Negative charges of humus are associated with partially dissociated enolic (-OH), carboxyl (-COOH), and phenolic groups that are in turn associated with units of varying complexity.
• Humus colloids are composed of C, H, and O, unlike silicate clays which contain Al, Si, and O.
• Humus is amorphous and not as stable as clay, making it more dynamic by being formed and broken down faster.

Crystalline Silicate Clays

• Crystalline silicate clays have a layered structure consisting of two to four sheets per layer.
• Crystalline silicate clays are predominantly negatively charged and have diverse properties.

Noncrystalline Silicate Clays

• They do not have an ordered crystalline structure.
• These clays possess both positive and negative charges.
• Noncrystalline silicate clays exhibit high water-holding capacities and low stickiness.

Iron and Aluminum Oxides

• Found in highly weathered soils
• May be crystalline or amorphous
• Exhibit low plasticity and stickiness.

Organic Colloid (Humus)

• This is the smallest of all soil colloids.
• It consists of partially decomposed cell walls and biomolecules.
• Humus has a very high water adsorbing capacity.
• The net charge of humus is always negative.

Basic Structure of Silicate Clays

• Consist of silica tetrahedron and tetrahedral sheets
• Also feature alumina octahedron and octahedral sheets

Octahedral-Tetrahedral Linkage

• Minerals have a shorthand notation expressing the linkage of tetrahedral (tet) and octahedral (oct) sheets.

Dioctahedral Structure

• In dioctahedral structures, trivalent cations (Al3+, Fe3+) can occupy space in the structure.
• Dioctahedral structures are more stable than trioctahedral structures.
• Two-thirds of the octahedral positions are filled in dioctahedral sheets.
• Kaolinite (1:1) and Talc (2:1) are examples of dioctahedral structures.

Trioctahedral Structure

• Divalent cations such as (Mg2+, Fe2+) may occupy space in a trioctahedral structure.
• Trioctahedral sheets are less stable than dioctahedral sheets.
• All of the octahedral positions are filled in trioctahedral sheets.
• Serpentine (1:1) and Pyrophyllite (2:1) are examples of trioctahedral structures.

1:1-type or Dimorphic Clay Minerals (Kaolinite group)

• The simplest clay mineral type, consist of a 1:1 stacking of tetrahedral and octahedral units.
• The tetrahedral and octahedral sheets in a given layer of kaolinite are held together by oxygen anions shared by the 'Si' and 'Al' cations.
• Layers are held together by hydrogen bonding, where OH- groups on one unit layer share hydrogen atoms with the oxygen atoms on the adjacent layer.
• When wetted, the structure is fixed and no expansion occurs between layers.
• Kaolinite's effective surface is restricted to its outer faces.
• Due to limited isomorphous substitution, free charges occur only on outer faces or external surfaces due to broken or exposed crystal edges.
• Kaolinite crystals are usually hexagonal in shape.
• Kaolinite exhibits very little plasticity, cohesion, shrinkage, and swelling.
• Soils dominated by 1:1 clays are easy to cultivate for agriculture and productive with nutrient management.
• Kaolinite-containing soils are well-suited for roadbeds and building foundations.
• Kaolinite is used for making bricks and ceramics.
• The general compositionof the kaolinite: Si4Al4O10(OH)8

2:1-type or Trimorphic Clay Minerals

• These can be dioctahedral and trioctahedral.
• Consists of one octahedral sheet sandwiched between two tetrahedral sheets.
• Include Expanding (Smectite, Vermiculite) and Non-expanding types (Illite).
• Have higher SiO2 contents than dimorphic kaolinite.
• Isomorphous substitution of ions occurs frequently in the layers.

Smectite Group of Minerals: Expanding Minerals

• Consists of one octahedral sheet sandwiched between two tetrahedral sheets.
• Plastic and sticky in moist conditions, is higher in SiO2 than kaolinite.
• Swelling can occur under fluctuating wet and dry conditions..
• Weak oxygen-to-oxygen and cation-to-oxygen linkages hold the layers loosely together.
• Montmorillonite, beidellite, nontronite, and saponite are part of this group.
• In montmorillonite, magnesium replaces aluminum in octahedral sheets. Silicon atoms in the tetrahedral sheet may be replaced by aluminum, creating a negative charge.
• High cation exchange capacity (CEC) is 10-15 times greater than that of kaolinite.
• Smectite-dominated soils, like Vertisols, commonly form wide cracks when dried.

Vermiculite Group

• 2:1 layer-lattice structure like montmorillonite.
• Vermiculites can be dioctahedral and trioctahedral.
• Major isomorphous substitution takes place in the tetrahedral sheet.
• Aluminum replaces silicon, resulting in negative charge.
• Water molecules, along with magnesium and other ions, are strongly adsorbed in the interlayer space, replacing K+ ions.
• The molecules Primarily act as bridges, holding the units together.
• Exhibits limited expansion.
• Cation exchange capacity (CEC) of vermiculite is higher than all other silicate clays.
• Vermiculite crystals are larger than smectite but smaller than kaolinite.

Non-Expanding Group (Illite)

• The basic structure is similar to that of montmorillonite, particles are much larger than those of smectite.
• Some of the silicon ions are replaced by aluminum ions in the tetrahedral sheet (20%), forming a net negative charge which is compensated by potassium ions.
• Potassium acts as a binding agent to prevent crystal expansion.
• Properties such as hydration, cation adsorption, swelling, shrinkage and plasticity are less.

2:1:1 (Chlorite)

• Iron or magnesium are located in the octahedral sheets.
• Hydroxy octahedral sheets are found in the interlayer space.
• There is no water adsorption between the chlorite crystal units causing the non-expanding nature of the mineral.
• Exhibits restricted swelling due to a locked interlayer structure.
• Has low nutrient value and is common in sedimentary rocks and soils derived from them.

Clay Mineralogy and Weathering Pattern

• Shows which clay minerals result from other clay minerals that were exposed to increased weathering

Types of Charges

• Two types of charges arise in soil colloids: constant or permanent charges and dependent or variable charges.

Constant Charges:

• Arise due to isomorphous substitution
• Independent of pH
• The replacement of a cation by another cation of similar size to isomorphous substitution.
• It depends on the type and abundance of cations.
• It is common in 2:1 clays
• Negative charge arises when a lower charged cation replaces a higher charged cation.
• For example, the replacement of Al3+ with Mg2+ in octahedral sheets and replacement of Si4+ with Al3+ in tetrahedral sheets (fine grained mica).
• Positive charge arises when a higher charged cation replaces a lower charged one.
• The replacement of Al3+ replaces one Mg2+ in dioctahedral sheets is an example.

Variable Charges

• Are found mainly in 1:1 clays, such as kaolinite, and humus (exchange occurs in external surface only).
• May result in both positive and negative charges.
• Charge magnitude is dependent on pH of soil solution.
• The negative charge is caused by the -OH group present in the crystal edge.
• Variable charges arise under moderate to extreme acidic conditions, with edge oxygen associated with H+, hence a net positive charge.

Relationship Between Soil pH and Charges

• Cation Exchange Capacity increases with increasing soil pH.
• Anion Exchange Capacity decreases with increasing soil pH.
• In acid rich 1:1 clay soils of tropical regions, positive charges are more abundant, while negative charges predominate in 2:1 clays of temperate regions.

Adsorption of Cations and Anions

• Adsorption is the adhesion of gases, ions, or atoms to a surface.
• Adsorption may be physical or chemical.
• Charged colloid surfaces adsorb cations and anions.
• Two types of formations aid in adsorption: outer and inner sphere complexes.

Outer-Sphere Complex

• Ions are typically surrounded by water in soil solution.
• Water molecules act as a bridge between ions and colloid surfaces.
• A weak electrostatic force binds the ions to colloids, allowing ions to be easily replaced.

Inner-Sphere Complex

• Involves no water molecules between ions and colloids.
• Ions directly bond with the colloidal surface.
• It is difficult to remove ions in an inner-sphere complex.