________ is often added to provide _______ and facilitate iron precipitation

• A. lime, alkalinity
• B. soda ash, a higher pH
• C. caustic, basicity
• D. sodium bicarbonate, a higher pH

Answer: (A)

Iron removal hinges on creating conditions where dissolved iron shifts to a solid form that can be filtered out. Raising the water’s alkalinity with lime does exactly that: calcium hydroxide supplies hydroxide ions that boost both pH and buffering capacity. As the pH climbs toward the eight to nine range, iron tends to oxidize and precipitate as ferric hydroxide, which can then be settled out. The added alkalinity helps keep the pH from dropping back down, making the precipitation ongoing rather than temporary. Lime’s other advantage is that it introduces calcium, which can aid coagulation and solids formation, enhancing removal. While other chemicals can raise pH, lime is especially effective because it provides alkalinity and supports stable iron precipitation under typical plant conditions.