Cyanide, a deadly poisonous substance, exists in water as HCN, a weak acid with a Ka of 6×10−10. The cyanide ion has a strong affinity for many metal ions, forming relatively less-toxic ferrocyanide, Fe(CN)64−, with iron(II), for example. Volatile HCN is very toxic and has been used in gas-chamber executions and as a war gas. Cyanide is a very stable species and does not quickly decompose on its own or in the environment [1].
Cyanide is widely used in industry, especially for certain mineral-processing operations, metal cleaning and electroplating. It is also one of the main gas and coke scrubber effluent pollutants from gas works and coke ovens. Hydrogen cyanide besides carbon monoxide, plays a more significant role in the mechanism of deaths in fires in confined spaces.
Both hydrogen cyanide and cyanide salts are rapidly acting poisons; a dose of only 60–90 mg is sufficient to kill a human [2]. Level of HCN of 0.03% in the atmosphere is toxic. The consent condition for discharge of cyanide to sewers is 5–10 mg l−1[3] and the trigger concentration for cyanide could be found in the United Kingdom department of the environment (ICRCL, Interdepartmental Committee for the Redevelopment of Contaminated Land) (Table 1). A level as low as 0.2 mg l−1 is considered to be fatal to many fish species [4]. The US-EPA water quality criterion of total cyanide in waters is 5 μg l−1[5]. The former OSHA [6] limit for hydrogen cyanide was a 10 ppm 8 h TWA (Time Weighted Average: the average amount of a substance you can be exposed to over an 8 h day), and proposed a 10 min ceiling of 4.7 ppm for hydrogen cyanide. The ACGIH [7] has a 10 ppm ceiling limit. The data in ACGIH site (1986/Ex. 1-3) shows that exposure to levels of 45–54 ppm hydrogen cyanide can be tolerated for 1 h with no immediate or delayed effects, but that 18–36 ppm produces “slight” symptoms after several hours of exposure. NIOSH [8] (1976e/Ex. 1-240) has recommended that workplace exposures to hydrogen cyanide not exceed 4.7 ppm (5 mg/m3) as a 10 min ceiling.