I tried to research this but could only find that cyanide breaks down in water. What would happen to the cyanide if it leaked into a natural flowing river? Does it still break down? Would it kill all the wildlife before it broke down?
A gold mine has leaked cyanide ponds into a river and I want to understand the effects that will now take place. If there is a more appropriate thread for this question please let me know! Thanks 🙂
In: Chemistry
Gold-mine cyanide is usually sodium cyanide, NaCN. Its half-life in water is not known, but it dissolves very easily, creating free cyanide ions. The speed of this process is going to vary on a lot of factors like the salinity and pH of the water and the quantity, but in a free-moving river I’d imagine the NaCN would fully dissolve into Na+ and CN- ions rapidly, possibly essentially immediately if the quantity of NaCN is low enough. [A paper](https://www.journals.uchicago.edu/doi/pdf/10.1086/333858) found that NaCN saturating still water decomposes in a matter of days; in rapidly moving water like a river it would be much, much shorter.
These cyanide ions are, in turn, very very reactive, and are going to react with just about everything in the water. Since we’re in water and there’s hydrogen ions everywhere, the vast majority CN reacts with hydrogen to form hydrogen cyanide, a toxic gas with a high solubility in water. HCN can hang out in water (or air if it escapes) for 1 to 4 years before decomposing into CO, CO2, N2 and H2O.
It’s hard to give an exact answer because it will depend on initial conditions.
Cyanide chemistry is fairly complicated and it is removed from the water through several pathways:
Evaporation as hydrogen cyanide gas will pull much of it out within a few days – this is a function of temperature and water turbulence, hotter and more mobile water will discharge it faster than cold, still slag ponds.
Reaction with other chemicals will degrade some of it into insoluble metal complexes and sulfates. Those can persist for a long time in the soil, with varying degrees of harmfulness.
Some is consumed and metabolized by microbes. Cyanide may be incredibly toxic to us fancy multicellular oxygen-breathers, but primordial soil-dwelling bacteria that pre-date us all know how to use it for metabolic processes. Some environmental cyanide is consumed and converted into less-harmful chemicals by these bacteria.
Whether it causes a mass die off of wildlife before it’s removed from the system will depend on how high the initial concentrations are and how quickly it disperses downstream. Fish will tolerate a low dose for a while, but they won’t tolerate a huge dump.
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