Any material that is reasonably insoluble in the dispersion medium can be caused to form a colloidal solution. Certain organic substances and compounds that are considered soluble in water do not form true solutions instead they form colloidal solutions e.g. soap, starch, gelatin, agar-agar.
Dispersions binding strongly with the liquid are generally more stable and difficult to separate from the liquids. Colloids that bind strongly with water are termed hydrophilic (water loving) and those who do not are termed as hydrophobic (water hating).
Turbidity imparted by Settleable suspended solids is apparent turbidity whereas turbidity imparted by Colloids is a true turbidity.
Properties of Colloids:-
1) Large, specific surface area
2) Negatively Charged due to adsorption of OH-
3) Repel each other and hence, there will not be settling
4) Brownian motion
5) Smaller mass, no settling due to gravity.
6) Scattering of light
Turbidity:-
The term turbid is applied to waters containing suspended matter that interferes with the passage of light through the water or in which visual depth is restricted.
Turbidity has important considerations in Public water supplies due to the following reasons.
1) Aesthetic: -Consumers of Public water supplies have a right to demand a clean water. Any turbidity in the drinking water is associated with a water pollution and health hazards.
2) Filterability: - Filtration of water is rendered more difficult and costly when turbidity increases. The turbid water shortens the cycle of sand filter and increase the cost of cleaning.
3) Disinfection: -The degree of disinfection depends on the contact between organism and disinfectant. In turbid waters, the organisms may be encased in the particles and get shelter from disinfectant.
Electro-kinetic properties: -
The stability of hydrophobic colloids depends upon the electrical charge that they possess. This primary charge may result from charged groups within the particle surface or may be gained by adsorption of a layer of ions from the surrounding medium.
Because of the primary charge on the particle, an electric potential exists between the surface of the particle and the bulk of the solution the charge is maximum at the particle surface and decrease with distance from the surface. When two particles of similar primary charge approach each other, their diffuse layer layers begin to interact. As they come closer, the similar primary charges they possess repulsive forces. The closer the particles approach, the stronger the repulsive force.
The repulsive forces which keep particles from aggregating are counteracted to some
degree by an attractive force termed as van-der walls force. All colloidal particles possess this attractive force regardless of charge and composition and density of the colloid, but independent of the composition of the aqueous phase. The van der Walls force decreases rapidly with increasing distance between the particles.
As two similar particles approach each other, the repulsive electrostatic forces increase to keep them apart. If the particles can be brought sufficiently close together to get past this energy barrier the attractive van-der walls force will predominate, and the particles, will remain together. If it is desired to coagulate colloidal particles, then they must be given sufficient kinetic energy to overcome the energy barrier that exists, or else the energy barrier must be lowered by some means.
The surface potential or primary charge on a colloid cannot be measured directly. It is the potential at slip plane or bound layer and loose layer.
Measurement of zeta potential gives an indication of the effectiveness of added electrolyte in lowering the energy barrier between colloids and thus serves to guide the selection of optimum condition for coagulation.
There are four basic mechanisms by which colloids can be coagulated.
1) Double layer compression
2) Charge neutralisation
3) Entrapment in precipitate
4) Interparticle bridging
1) Double layer compression:-
If a high concentration of electrolyte is added to a colloidal solution, the concentration of ions within a diffused layer will increase and hence the thickness of this layer will decrease. This will result in greater decrease in charge with distance from the particle interface resulting in a decrease of the potential barrier. With reduced energy barrier particles can come together and coagulate.
2) Charge neutralisation :-
The charge on a colloid can sometimes be neutralised by addition of molecules of opposite charge which have the ability to adsorb onto the colloid. The opposite charges of the organic and the turbidity particle cancel each other out and the coagulation results.
3) Entrapment in precipitate:-
When sufficient metal salts of Al3+ and Fe3+ are added to solution, they may combine with OH- to rapidly form hydroxide precipitates. Colloidal particles may provide condensation sites where the precipitates may form and the turbidity becomes entrapped in the precipitate and settles with it. The settling precipitates can also entrap colloids which it passes bringing them down.
4) Interparticle bridging:-
Long-chained charged synthetic and natural polymers (Polyelectrolytes) can act to destabilise the colloids by forming a bridge between one colloid and another. One charge site on the long polymer can attach to site on one colloid while the remainder of the polymer molecule extends out into the solution. If the extended portion becomes attached to another colloid then the two colloids are effectively tied together.