The Coagulation Mechanisms of Polymeric Aluminum Chloride (PAC)

The effectiveness of Polymeric Aluminum Chloride (PAC) as a coagulant in water treatment arises from a combination of four primary physical-chemical mechanisms:

Double Layer Compression
Suspended particles in water often carry negative surface charges, causing them to repel each other. Upon addition, PAC increases the ionic strength of the water. This compresses the electrostatic double layer surrounding the particles, reducing the repulsive forces. As this repulsion weakens, attractive van der Waals forces can dominate, allowing particles to come closer together and aggregate.

Charge Neutralization
PAC hydrolyzes in water to form a range of cationic (positively charged) aluminum species. These highly charged polymers adsorb onto the surfaces of negatively charged colloidal particles. This adsorption effectively neutralizes the particles' surface charge, diminishing electrostatic repulsion and facilitating collision and aggregation.

Adsorption and Bridging
Polymer-based or high-basicity PAC products contain long-chain, pre-formed aluminum hydroxide polymers. These macromolecules can adsorb onto the surface of one particle while extended segments of the chain reach out into the solution. These free segments subsequently attach to other particles, creating physical bridges that bind multiple particles into larger, settleable aggregates or flocs.

 Sweep Flocculation (Enmeshment)