Cyanide (CN-) most commonly occurs as hydrogen cyanide (HCN) and its salts: sodium cyanide (NaCN) and potassium cyanide (KCN). The term free cyanide refers to the cyanide ion and hydrogen cyanide. Cyanides are widely spread in nature, arising from both natural and anthropogenic sources. Cyanides are produced by certain bacteria, fungi, and algae. Hydrogen cyanide is a colourless liquid/gas with a characteristic odour of bitter almonds.
Organic chemical industries as well as iron and steel production are major sources of cyanide releases to the aquatic environment, while most of atmospheric cyanide comes from car emissions. In air, cyanide is present as hydrogen cyanide, mainly in gas form, which is very stable (with a half-life of 1 to 3 years), but a small part forms dust particles which can settle over land or water. In water cyanide doesn't dissolve, but can mix. It weakly adsorbs to particulate matter or soils. In surface water most cyanide will form hydrogen cyanide and evaporate rapidly (half-life of 4 days), some can also be biodegraded. 
Cyanide concentrations above 100 µg/l can be acutely toxic for zooplankton. Acute toxicity occurs in fishes at exposure to cyanide concentrations above 150 µg/l. Molluscs tolerate concentrations up to 300 µg/l.
The mean concentration of cyanide in fresh surface water is 3,5 µg/l. Concentrations in waste waters can reach up to 120 mg/l.
Environmental standards and legislation
Please note that others may also have edited the contents of this article.