Metal finishing wastes would require the destruction of which contaminant?

• A. Cyanide
• B. Mercury
• C. Oil
• D. Chlorine

Answer: (A)

Cyanide contamination is the contaminant in metal finishing wastes that needs to be destroyed because cyanide ions are extremely toxic to aquatic life and can release hydrogen cyanide gas if the wastewater chemistry shifts. Plating baths often rely on cyanide complexes to keep metals dissolved, so the wastewater can contain CN− in forms that are both highly hazardous and persistent. Treatment aims to destroy cyanide through oxidation, converting CN− into less harmful species such as cyanate and eventually into carbon dioxide and nitrogen, so the effluent meets strict regulatory limits before discharge.

Oil is typically removed by separation and biodegradation rather than a dedicated destruction step. Chlorine, if present, is usually managed or neutralized as part of normal water treatment, not targeted for destruction in the same sense as cyanide. Mercury requires careful removal and stabilization due to its toxicity, but the specific destruction step emphasized for metal finishing wastes is focused on cyanide because of its distinct toxicity and the regulatory emphasis on converting it to nonhazardous forms.