Common metals can be removed from wastestreams by ____________ precipitation.

Common metals can be removed from wastestreams by ____________ precipitation.

• A. Hydroxide
• B. Sulfide
• C. Oxidation
• D. Ion exchange

Answer: (A)

Metal removal from wastestreams often uses hydroxide precipitation because many metals form insoluble hydroxide solids when the pH is raised. When alkaline reagents like lime or sodium hydroxide are added, metal ions such as Fe, Al, Zn, Cu, Ni, Pb, and others react with hydroxide to create M(OH)2 or M(OH)3 solids. These solid hydroxides are not soluble, so they precipitate out of solution and can be removed by settling in a clarifier or by filtration. This method is widely used because it’s simple, robust, and cost-effective for a broad range of metals, whereas oxidation doesn’t inherently remove metals, ion exchange uses resin beds, and sulfide precipitation is more selective and can introduce odor and toxicity concerns.

Metal removal from wastestreams often uses hydroxide precipitation because many metals form insoluble hydroxide solids when the pH is raised. When alkaline reagents like lime or sodium hydroxide are added, metal ions such as Fe, Al, Zn, Cu, Ni, Pb, and others react with hydroxide to create M(OH)2 or M(OH)3 solids. These solid hydroxides are not soluble, so they precipitate out of solution and can be removed by settling in a clarifier or by filtration. This method is widely used because it’s simple, robust, and cost-effective for a broad range of metals, whereas oxidation doesn’t inherently remove metals, ion exchange uses resin beds, and sulfide precipitation is more selective and can introduce odor and toxicity concerns.