What chemical is commonly used as a coagulant in water treatment to remove fine solids and colloids and is highly corrosive?

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Coagulation in water treatment works by adding a positively charged chemical to neutralize the negative surface charges on fine particles and colloids. Once these charges are neutralized, the particles can come together to form larger flocs that settle out or get filtered.

Ferric chloride fits this role well because it hydrolyzes to ferric hydroxide rapidly, producing dense, strong flocs that effectively capture very fine solids and colloids. Its performance is robust across a range of conditions, making it a common choice for removing small particles that resist settling. However, it is highly corrosive to skin and metals, so handling requires corrosion-resistant equipment and proper protective gear.

Calcium hypochlorite is used for disinfection, not coagulation, and sodium chloride has no coagulating action. Aluminum sulfate is also a coagulant, but the question’s emphasis on removing fine solids and colloids and the noted corrosiveness points toward ferric chloride as the best fit.