Cyanide's Toxicity

Cyanide in the Anthroposphere
Sodium cyanide, like most other cyanide salts is among the most rapidly acting of all known toxins. When NaCN enters the body, it will inhibit mitochondrial cytochrome oxidase, blocking electron transport, resulting in lethal lactic acidosis. A quantity as small as 200 mg of NaCN orally ingested is enough to be fatal (fig. 4) (e.g. Cooper and Brown, 2008).

Fig. 4 - Demonstration of how cyanide can inhibit enzyme functions.
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However, cyanide is a chemical. Most chemicals we use in science processes are dangerous and potentially deadly when ingested - as quoted by the German philosopher Paracelsus, "All things are poison and nothing is without poison; only the dose makes a thing not a poison". When handling chemicals like cyanide, extra precaution is required, which allows the use of cyanide to be considered 'safe' (Cyanide Safe Use Guidelines, 2013).

One step added in the gold cyanidation process is addition of soda (sodium hydroxide) or slaked lime (calcium hydroxide) to the slurry. This is to ensure the pH of the overall extracting solution never drops below 10.5, to prevent the creation of hydrogen cyanide (HCN). HCN is much more deadly than NaCN - as little as 100 -300 ppm of HCN in the air is enough to kill a human in 10-60 minutes (USDHHS, 2006), and at quantities of 2000 ppm, can kill within 1 minute of inhalation.


Cyanide in the Biosphere and Hydrosphere
Cyanide can be produced naturally; various bacteria, fungi, algae and many species of plants such as beans, fruit seeds, plant roots, and young bamboo shoots all produce some micro levels of cyanide (fig. 5).

Fig. 5 - Example of naturally occurring poisons,
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However, these levels of cyanide are extremely low compared to those which are used during gold cyanidation. If some sodium cyanide was spilled, or hydrogen cyanide was accidentally produced and got into the atmosphere, it can cause lots of damage to the biosphere. Cyanide is not very harmful to plants, it may affect the respiration (e.g. Solomos, 1997), but is most likely tolerated. However, its high level of toxicity can cause it to kill many animals (e.g. Eisler, 1991).
-Birds can experience an LD50 (Lethal Dose required to kill at least 50% of those tested) of around 1-10 mg per kg of bodyweight.
-Mammals can experience an LD50 2-20 mg per kg of bodyweight
-Fish and aquatic invertebrates; cyanide is dissolved in water, changing its lethality in comparison to non-marine animals. Effects include reduced swimming performance, inhibited ability to reproduce as well, many lesser (yet still negative) effects. LD50 of around 0.073 mg HCN at concentration of dissolved oxygen of 10 mg (e.g. Alabaster et al, 1983; Donato et al., 2007)

One such event, where safety precautions weren't followed, the 1989 cyanide-extraction gold mines in California, Nevada and Arizona resulted in many animal deaths. According to Clark and Hothem (1991), 519 animals were recorded to have died due to cyanide poisoning. Around 35% were rodents, and 34% bats. Since 1991, 10 mammal species that are listed as rare, threatened, protected or endangered are known to have cyanide-extraction mines in their geographic location.

Treatment
The best way to treat cyanide's release into the atmosphere and poisoning everything is simply to prevent its release. As I mentioned above, adding soda or slaked lime to the slurry will prevent the pH dropping below 10.5, which will prevent hydrogen cyanide from being produced.
Cyanide will not biomagnify; it will not be digested by multiple animals, then go on to cycle through the food chain with the top predators getting amplified doses of cyanide. This is due to its rapid detoxification; when absorbed the majority will react with thiosulfate in the presence of enzymes to form thiocyanate which is easily excreted (Eisler, 1991). Furthermore, if small amounts of cyanide is ingested and acute poisoning occurs it can be countered with medical doses of sodium thiosulfate, cyanokit and nithiodote (USAMRICD, 2007; USDHHS, 2006).