Answer
Historically, sodium cyanide has been used to leach gold from its ores. But this resulted in cyanide pollution.
Work Step by Step
The gold can be separated out of its ores by selectively dissolving it into solution, a process called leaching . In this process, solid gold reacts with sodium cyanide to form a soluble gold complex.
$4 Au(s) + 8 CN^{-}(aq) + O_{2}(g) + 2 H_{2}O(l) \rightarrow4 Au(CN)_{2}^{-}(aq) + 4 OH^{-}(aq)$
The impurities are filtered out of the solution and the gold is reduced back to elemental gold with a reactive metal such as zinc.
$2 Au(CN)_{2}^{-}(aq) + Zn(s) \rightarrow Zn(CN)_{4}^{2-}(aq) + 2 Au(s)$
Leaching often results in the contamination of streams and rivers with cyanide. New alternatives, using the thiosulfate ion ($ S_{2}O_{3}^{-}$) , are replacing it.
