Which of the following is a difficult-to-treat waste cyanide complex?

The correct answer identifies calcium as a difficult-to-treat waste cyanide complex. In wastewater treatment, cyanide can form strong complexes with various metals, and the stability of these complexes can vary. Calcium cyanide is one of the more challenging cyanide complexes to handle due to its tendency to preferentially form stable complexes with cyanide ions.

This stability can complicate treatment processes because standard methods may not effectively break these complexes down, requiring more advanced or specific treatment strategies to ensure that cyanide is safely treated and removed from the wastewater.

The other choices, such as sodium, potassium, and magnesium, do not form similarly robust complexes with cyanide, making them less problematic in wastewater treatment. The characteristics of calcium in this context highlight why it is recognized as a particularly difficult complex to manage, necessitating careful attention in treatment planning and execution.