 Commonwealth of Australia 2010

23.1Overview of fate of sodium cyanide

The release of sodium cyanide in the relatively stable solid forms manufactured in Australia (e.g. briquettes) to dry land (e.g. through a spill event) is unlikely to result in migration in the short term from the point of release. However, when manufactured and supplied in the liquid form (~30% NaCN), or if the solid form comes into contact with water after an accidental release (e.g. released into a waterbody, rainfall, fire fighting water, and if left, absorption of water by deliquescence), dissociated sodium cyanide as free cyanide is likely and the potential for cyanide to migrate and to undergo further reactions with other chemicals, substances and biota is greatly enhanced.

The most commonly occurring forms of cyanide in soils following release of sodium cyanide include HCN, simple cyanides (e.g. inorganic salts), and iron-complexed cyanides (e.g. ferrocyanide and ferricyanide, also called hexacyanoferrate(II) and (III)). The iron-cyanide complexes occur in two oxidation states, with ferricyanide reduced to ferrocyanide only under reducing conditions (Kjeldsen, 1999). Nitriles, organic material with an R-CN composition, where R refers to the organic radical, and thiocyanates (-SCN), may also be present (Kjeldsen, 1999). The following section provides a summary of the general environmental fate of cyanide for the main environmental compartments into which it is released.

23.2General degradation pathways

Volatilisation	Free cyanide volatilisation to the atmosphere (i.e. as HCN gas) increases, particularly as pH decreases and the proportion of HCN increases: CN- + H2O  HCN + OH-
Complexation	Cyanide can potentially form complexes with ~28 elements including Cd, Co, Cu, Au, Fe, Hg, Ni, Ag, Zn (~72 metal complexes) (Ford Smith, 1964). e.g. Cu+ + 3CN-  Cu(CN)32-
Adsorption	Adsorption of free and complexed cyanide forms onto solid phases.
Precipitation	Cyanide complexes forming solid metallocyanide precipitates.
Formation of thiocyanate	Reaction of cyanide with various forms of sulphur (e.g. polysulphides and thiosulphate): Sx2- + CN-  [S(x-1)]2- + SCN- and S2O33- + CN-  SO32- + SCN-
Oxidation	Oxidation to various reaction products, such as cyanate and/or cyanogens, ammonia and water: 2HCN + O2  2HOCN (hydrogen cyanate); 2CN- + O2 + catalyst  2OCN- (cyanate ion); 2Cu2+ + 2CN-  2Cu + (CN)2 (cyanogen) HCN + 0.5O2 + H2O  CO2 + NH3 (CN)2 + 2OH-  OCN- + CN- + H2O Formation of cyanate occurs when cyanide is in the presence of strong oxidisers (e.g. ozone, hydrogen peroxide, hypochlorite).
Photolysis	Photolysis of stable iron complex forms (e.g. ferrocyanides) to free cyanide: Fe(CN)63- + uv  Fe(CN)52- + CN-, and Fe(CN)64- + uv  Fe(CN)53- + CN- With continuing UV exposure, similar reactions can continue through successive steps to release all the contained iron and cyanide, e.g. Fe(CN)64- ultimately forming Fe2+ + 6CN- Photodecomposition reactions in the presence of sunlight: HCN + uv  H+ + CN- HCN + uv + catalyst  + OCN-
Hydrolysis	As solution pH falls, the proportion of cyanide present as HCN increases. HCN may be volatilised from the water surface, or may be hydrolysed to formate, either as formic acid or ammonium formate: HCN + 2H2O  NH4COOH (ammonium formate), or HCN + 2H2O  NH3 + HCOOH (formic acid) Hydrolysis of cyanate: HOCN + H3O+  NH4+ + CO2 OCN- + NH4+  (NH2)2CO
Biodegradation	Aerobic: CN- + HCO3- + NH3  NO2- + NO3- 2HCN + O2 + enzyme  2HOCN	Anaerobic: CN- + H2S(aq)  HCNS + H+ HCN + HS-  HCNS + H+