Soil Colloids

Soil Colloids

The colloidal state refers to a two-phase system in which one material in a very finely divided state is dispersed through second phase.
 
The examples are:

Solid in liquid – Clay in water (dispersion of clay in water)
Liquid in gas -Fog or clouds in atmosphere

The clay fraction of the soil contains particles less than 0.002 mm in size. Particles less than 0.001 mm size possess colloidal properties and are known as soil colloids.

General Properties of Soil Colloids

1. Size: The most important common property of inorganic and organic colloids is their extremely small size. They are too small to be seen with an ordinary light microscope. Only with an electron microscope they can be seen. Most are smaller than 2 micrometers in diameter.

2. Surface area: Because of their small size, all soil colloids expose a large external surface per unit mass. The external surface area of 1 g of colloidal clay is at least 1000 times that of 1 g of coarse sand. Some colloids, especially certain silicate clays have extensive internal surfaces as well. These internal surfaces occur between plate like crystal units that make up each particle and often greatly exceed the external surface area. The total surface area of soil colloids ranges from 10 m2/g for clays with only external surfaces to more than 800 m2/g for clays with extensive internal surfaces. The colloid surface area in the upper 15 cm of a hectare of a clay soil could be as high 700,000 km2/g

3. Surface charges: Soil colloidal surfaces, both external and internal characteristically carry negative and/or positive charges. For most soil colloids, electro negative charges predominate. Soil colloids both organic and inorganic when suspended in water, carry a negative electric charge. When an electric current is passed through a suspension of soil colloidal particles they migrate to anode, the positive electrode indicating that they carry a negative charge. The magnitude of the charge is known as zeta potential. The presence and intensity of the particle charge influence the attraction and repulsion of the particles towards each other, there by influencing both physical and chemical properties.

The negative electrical charge on clays comes from
i) Ionizable hydrogen ions and
ii) Isomorphism substitution.
 
i) Ionizable hydrogen ions: Ionizable hydrogen ions are hydrogen from hydroxyl ions on clay surfaces. The -Al-OH or -Si-OH portion of the clay ionizes the H and leaves an unneutralized negative charge on the oxygen (-Al-O- or – Si-O). The extent of ionized hydrogen depends on solution pH; more ionization occurs in more alkaline (basic) solutions.
 
ii) Isomorphous substitution: The second source of charge on clay particles is due to the substitution of one ion for another of similar size and often with lower positive valence. In clay structures, certain ions fit into certain mineral lattice sites because of their convenient size and charge. Dominantly, clays have Si4+ in tetrahedral sites and A13+ in octahedral sites. Other ions present in large amounts during clay crystallization can replace some of the A13+ and Si4+ cations. Substitutions that are common are the Si4+ replaced by A13+, and even more extensive replacement of A13+ by one or more of these: Fe3+, Fe2+, Mg2+ or Zn2+ Since the total negative charge from the anions (the oxygen) remains unchanged, the lower positive charge because of substitution results in an excess negative charge at that location in the structure.

4. Adsorption of cations: As soil colloids possess negative charge they attract the ions of an opposite charge to the colloidal surfaces. They attract hundreds of positively charged ions or cation such as H+, A13+ Ca2+ , and Mg2+. This gives rise to an ionic double layer.

The process, called Isomorphous substitution and the colloidal particle constitutes the inner ionic layer, being essentially huge anions; with both, external and internal layers that are negative in charge. The outer layer is made up of a swarm of rather loosely held (adsorbed) cations attracted to the negatively charged surfaces. Thus a colloidal particle is accompanied by a swarm of cations that are adsorbed or held on the particle surfaces.

5. Adsorption of water: In addition to the adsorbed cations, a large number of water molecules are associated with soil colloidal particles. Some are attracted to the adsorbed cations, each of which is hydrated; others are held in the internal surfaces of the colloidal particles. These water molecules play a critical role in determining both the physical and chemical properties of soil.

5. Cohesion: Cohesion is the phenomenon of sticking together of colloidal particles that are of similar nature. Cohesion indicates the tendency of clay particles to stick together. This tendency is primarily due to the attraction of the clay particles for the water molecules held between them. When colloidal substances are wetted, water first adheres to the particles and then brings about cohesion between two or more adjacent colloidal particles.

6. Adhesion: Adhesion refers to the phenomenon of colloidal particles sticking to other substances. It is the sticking of colloida1 materials to the surface of any other body or substance with which it comes in contact.

7. Swelling and shrinkage: Some clay (soil colloids) such as smectites swell when wet and shrink when dry. After a prolonged dry spell, soils high in smectites (e.g. Vertisols) often are crises-crossed by wide, deep cracks, which at first allow rain to penetrate rapidly. Later, because of swelling, such soil is likely to close up and become much more impervious than one dominated by kaolinite, chlorite, or fine grained micas. Vermiculite is intermediate in its swelling and shrinking characteristics.

8. Dispersion and flocculation: As long as the colloidal particles remain charged, they repel each other and the suspension remains stable. If on any account they loose their charge, or if the magnitude of the charge is reduced, the particles coalesce, form flocs or loose aggregates, and settle out. This phenomenon of coalescence and formation of flocs is known as flocculation. The reverse process of the breaking up of flocs into individual particles is known as deflocculation or dispersion.

9. Brownian movement: When a suspension of colloidal particles is examined under a microscope the particles seem to oscillate. The oscillation is due to the collision of colloidal particles or molecules with those of the liquid in which they are suspended. Soil colloidal particles with those of water in which they are suspended are always in a constant state of motion. The smaller the particle, the more rapid is its movement.

10. Non permeability: Colloids, as opposed to crystalloid, are unable to pass through a semi-permeable membrane. Even though the colloidal particles are extremely small, they are bigger than molecules of crystalloid dissolved in water. The membrane allows the passage of water and of the dissolved substance through its pores, but retains the colloidal particles.