7/29/2019 Coagulation - Flocculation Theory

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General Properties of Colloids

Coagulation and Flocculation

Electrical Double Layer

Destabilization of Colloidal Dispersion

General Properties of Colloids

Colloidal particles are so small that theirsurface area in relationto mass is very large.

1.

Electrical properties: All colloidal particles are electricallycharged. If electrodes from a D.C. source are placed in a

colloidal dispersion, the particles migrate towards the pole ofopposite charge.

2.

Colloidal particles are in constant motion because ofbombardment by molecules of dispersion medium. This motion iscalled Brownian motion (named after Robert Brown who firstnoticed it).

3.

Tyndall effect: Colloidal particles have dimensioThese arereversible upon heating. e.g. organics in water.

4.

Adsorption: Colloids have high surface area and hence have alot of active surface for adsorption to occur. The stability ofcolloids is mainly due to preferential adsorption of ions. Thereare two types of colloids:

5.

Lyophobic colloids: that are solvent hating. These areirreversible upon heating. e.g. inorganic colloids, metalhalides.

i.

Lyophilic colloids: that are solvent loving. These arereversible upon heating. e.g. organics in water.

ii.

Coagulation and Flocculation

Colloidal particles are difficult to separate from water becausethey do not settle by gravity and are so small that they passthrough the pores of filtration media.To be removed, the individual colloids must aggregate and grow

in size.The aggregation of colloidal particles can be considered as

involving two separate and distinct steps:

Particle transport to effect interparticle collision.1.Particle destabilization to permit attachment when contactoccurs.

2.

Transport step is known as flocculation whereas coagulationis the overall process involving destabilization and transport.

Electrical Double Layer

ulation - Flocculation Theory http://nptel.iitm.ac.in/courses/Webcourse-contents/IIT-KANPUR/w

3/19/2013

7/29/2019 Coagulation - Flocculation Theory

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Although individual hydrophobic colloids have an electrical charge, a

colloidal dispersion does not have a net electrical charge. The diffuselayer in a colloidal dispersion contains a quantity of counter ionssufficient to balance the electrical charge on the particle. The chargedistribution in the diffuse layer of a negatively charged colloid can berepresented by the curve ABCD in the figure. The ions involved in thiselectroneutrality are arranged in such a way as to constitute what iscalled electrical double layer.

Net repulsion force, which may be considered as energy barrier mustbe overcome before aggregation occurs. The magnitude of energybarrier depends on (1) charge on the particle, and (2) ioniccomposition of water.

Destabilization of Colloidal Dispersion

Particle destabilization can be achieved by four mechanisms:

Change characteristics of medium-Compression of double layer.Change characteristics of colloid particles-Adsorption and chargeneutralization.Provide bridges-

Enmeshment in a precipitate.1. Adsorption and interparticle bridging.2.

ulation - Flocculation Theory http://nptel.iitm.ac.in/courses/Webcourse-contents/IIT-KANPUR/w

3/19/2013