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The chemistry of cyanide solutions is complicated because the cyanide ion forms compounds and complexes with many elements. Some cyanide species are highly toxic whereas others are relatively inert and harmless. Molecular hydrogen cyanide (HCN) is the most toxic form of cyanide. Under most conditions, HCN exists as a gas which readily dissipates or reacts with the environment to form less toxic or nontoxic forms of cyanide. Thus HCN is an ephemeral toxin, and many naturally occurring geochemical processes reduce the HCN concentration of a heap system with time.

As discussed below, free cyanide includes the two species, ionic cyanide (CN~) and molecular hydrogen cyanide. Free-cyanide toxicity in man, mammals, and aquatic species is well documented (Doudoroff, 1976; Ecological Analysts, 1979; Towill et al., 1978). The lethal doses for NaCN reported for human adults vary with the type of exposure as follows:

• One to three mg/kg body weight if ingested;
• One hundred to 300 ppm if inhaled; and
• One hundred mg/kg of body weight if absorbed.

Acute toxicity of free cyanide to freshwater invertebrates ranges from 0.028 to 2.295 mg/1, depending on species and test conditions. Generally, free-cyanide concentrations greater than 0.1 mg/1 are expected to kill sensitive species in freshwater or marine environments (Doudoroff, 1976; Cardwell et al., 1976). Concentrations of HCN as low as 0.05 mg/1 can be lethal to fish (Scott and Ingles, 1981). Several site specific variables including the solution pH and temperature, oxygen content of the water, ionic strength, species acclimation, and body size can affect the degree of free-cyanide toxicity.

Cyanide is, a generic term indicating the presence of the cyanide ion (CN”). Cyanide is a very common, naturally occurring compound produced by many biochemical reactions. Many plant species synthesize organic compounds which contain cyanide in the form of cyanogenic glycosides (Knowles, 1976). For example, trace amounts of cyanide are present in the seeds and leaves of many members of the Rosaceae (rose) family (Kingsbury, 1964).

Many common items such as lettuce, maize, sweet potatoes, kidney beans, (Oke, 1969) almonds, apple seeds and cigarette smoke contain cyanide. As examples, a recent chemical analysis of chocolate-covered almonds measured a total cyanide content of about two ppm (Steffen Robertson and Kirsten, 1987); and the U.S. Surgeon General (U.S. Department of Health, 1964) has shown that cigarette smoke contains up to 1,600 ppm total cyanide.

Reference on what the lethal dose of cyanide is: http://www.infomine.com/library/publications/docs/Smith1988.pdf