Massachusetts Wastewater Practice Exam

Chemical precipitation of metal waste is often accomplished using:

1. Hydroxide.

2. Chlorine.

3. Alum.

4. Acid.

The correct answer is: Hydroxide.

Chemical precipitation of metal waste is most commonly accomplished using hydroxide. This process involves adding a source of hydroxide ions to a solution containing metal ions. The addition of hydroxide increases the pH of the solution, leading to the formation of insoluble metal hydroxides, which can then be easily removed from the solution through sedimentation or filtration. Hydroxides are particularly effective for precipitating a variety of metals, including lead, copper, and zinc, as these metals typically form stable hydroxide compounds that are not soluble in water. This characteristic is essential in wastewater treatment, where the goal is to reduce the concentration of hazardous metals before discharge or further processing. While other chemicals like alum can be used in certain contexts—often in the coagulation of solids—hydroxide specifically targets the precipitation of metal ions, making it the most applicable choice in the context of removing metal waste from water. Chlorine is typically used for disinfection rather than precipitation, and acids are usually employed to dissolve metals rather than remove them from solution.