Which chemical is commonly added to provide alkalinity and facilitate precipitation of iron in water treatment?

• A. Lime
• B. Soda ash
• C. Caustic soda
• D. Sodium bicarbonate

Answer: (A)

Raising alkalinity and pH to drive iron out of solution as an insoluble material is the idea here. Lime, which is calcium hydroxide, does this effectively: it dissolves to provide hydroxide ions that raise pH and add alkalinity (as Ca(OH)2). When the water’s pH is higher, dissolved iron (Fe2+) oxidizes and hydrolyzes to form iron hydroxide (Fe(OH)3), a precipitate that can be removed by filtration. Lime also contributes calcium, which can help with coagulation and overall softening, making it a common choice for iron removal.

Other options can raise alkalinity or pH, but they don’t offer the same combination of strong alkalinity provision and the practical benefits of lime softening. Soda ash adds carbonate without calcium, caustic soda is a very strong base used less for iron precipitation in typical plant operations, and sodium bicarbonate is a milder base with limited effectiveness for precipitating iron at practical dosages.