Which of the following is a chemical used for dechlorination?

• A. Aluminum dioxide
• B. Sulfur dioxide
• C. Ferric chloride
• D. Gaseous chlorine

Answer: (B)

Dechlorination relies on a chemical that neutralizes any remaining chlorine before the water is discharged. Sulfur dioxide acts as a reducing agent and reacts with residual chlorine species (like hypochlorous acid and hypochlorite) to form chloride ions, effectively removing chlorine from the effluent. This makes it a fast, controllable, and commonly used method to protect aquatic life after disinfection. The other options don’t perform this role: gaseous chlorine would add chlorine back into the water, ferric chloride is used as a coagulant for solids removal, and aluminum oxide/dioxide isn’t a standard dechlorination agent.