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Type of Mechanisms - Coggle Diagram
Type of Mechanisms
Charge neutralization
Adsorption of counterions on particles to neutralize the charges on the surface eliminates electrostatic repulsions among colloids making aggregation possible.
Electrostatic attraction occurs when surfaces are oppositely charged, which is promoted by the adsorption of specific ions on the surface of the particle.
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Above ZPC, surface charge is –ve (anionic)
Below ZPC, surface charge is +ve
Double layer compression
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Increasing the ionic strength will compress the double layer, causing a decrease in its thickness
Destabilization of colloidal dispersion by adding counterions in the form of chemical agents is achieved by reducing the volume of the diffuse layer needed in order to maintain electrical neutrality of the colloidal dispersion.
The compression of the double layer enhances the likelihood of aggregation among colliding particles because the attractive forces (van der Waals forces) between particles are short-distance forces and operate only during collisions or near misses.
Inter-particle bridging
Long-chain polymers carrying –ve charges can form bridges between particle, thus destabilizing the suspension
This mechanism was shown to be the major mechanism controlling the aggregation of bacterial and alga suspensions
Chemical groups of the polymer chains bond to the sites of colloidal particulates forming particle-polymer-particle aggregates (hence the name bridging in reference to the mechanism)
Reduction of surface charge can be achieved by: pH alteration & addition of specifically adsorbed ions
Sweep coagulation
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Al or Fe salts are used at concentrations exceeding saturation with respect to amorphous metal hydroxide solid that is formed
In this mechanism, finely divided particles are entrapped in the amorphous precipitate formed