Question

When designing a coagulation system, if the zeta potential of suspended particles is highly negative, which type of metallic salt or coagulant is mathematically required to neutralize the charge?

Accepted Answer

When suspended particles possess a highly negative zeta potential, a coagulant with a high positive charge density is required to neutralize the electrical repulsion. Zeta potential is a scientific measure of the magnitude of electrostatic repulsion between particles in a liquid; a highly negative value indicates that particles are strongly repelled from one another, preventing them from colliding and aggregating. To achieve neutralization, you must mathematically introduce a coagulant containing polyvalent metallic cations, which are positively charged metal ions with a valence of three or higher. The most effective options are trivalent metallic salts, specifically aluminum sulfate (alum) or ferric chloride. These salts function because the positively charged metal ions are attracted to the negatively charged surfaces of the suspended particles. By reducing the zeta potential toward zero, which is the point of zero charge, the repulsive forces are minimized. Once the repulsive barrier is removed, Van der Waals forces—the natural attractive forces between particles—become dominant, allowing the particles to collide and clump together into larger masses called flocs, which can then be easily settled or filtered out of the water.

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When designing a coagulation system, if the zeta potential of suspended particles is highly negative, which type of metallic salt or coagulant is mathematically required to neutralize the charge?