Fate of cyanide in landfills

26.4Summary of environmental fate

HCN is a highly volatile gas and is lighter than air, and also has high volatility from water despite being completely soluble (miscible) with water. The volatilisation rate is minimised if the solution is maintained at a high pH, as in NaCN manufactured in liquid form (~30% solution, pH ~13). However, under typical environmental conditions of pH<8 and temperature <25ºC, >94% of the cyanide is present as HCN. Thus volatilisation of HCN can be a significant removal process for free cyanide from aqueous solutions and soil to air.

Volatilising HCN in soil may reach the soil surface and pass into the atmosphere, or may be biodegraded and metabolised by micro-organisms, react with soil constituents in the subsurface, dissolve in soil moisture, or be entrapped in subsurface cavities. Volatilisation of HCN from groundwater is unlikely to be a significant transformation process. In the atmosphere, the lifetime of HCN is estimated to be a few months. Biomass burning is considered to be the main source, with release from the use of NaCN only a minor source. HCN may degrade via reactions with hydroxyl and oxygen radicals, but ocean uptake is considered to be the main sink for atmospheric HCN.

Cyanide is a highly reactive substance and once released into the environment may follow various degradation and reaction pathways. In addition to volatilisation, these include complexation with various metals, adsorption of free and complexed forms, formation of thiocyanate by reaction with various forms of sulphur, oxidation to cyanate and other products, hydrolysis of HCN to formate, and aerobic and anaerobic degradation reactions to form products such as cyanate, ammonia/ammonium and nitrites/nitrates. Metallocyanide complexes may form insoluble precipitates, and if exposed to light, iron-cyanide complexes may undergo photolysis reactions releasing HCN. Products such as thiocyanate, cyanate and formate are much less toxic than cyanide and continue to degrade, ultimately into oxides of carbon and nitrogen and simple forms of sulphur.

The fate of cyanide compounds in TSFs is complex and a range of reactions may occur, influenced by the general chemistry and geochemistry at the site. These result in various degradation and transformation processes, as well as atmospheric emissions. Cyanide compounds and products present and formed in tailings may potentially include free cyanide, a range of metallocyanide complexes (simple to strong complexes), thiocyanate, cyanate, nitrogenous compounds (e.g. ammonia, nitrite and nitrate), cyanogen and cyanogen chloride, formic acid/formate/ammonium formate, carbon dioxide and other simple compounds of carbon. Similar types of reactions may occur in heap leach piles.

There are various chemical processes available to detoxify cyanide in tailings, some of which are in regular use in Australia. These generally act to convert cyanide to cyanate. Conversion of cyanide to iron-cyanide complexes by adding ferrous sulphate is the main method used to initially deal with spills of NaCN. Water containing free cyanide is often recycled back to the tank leach process, commonly by draining from a TSF into a decant pond, or also by extracting water before discharge of the tailings, depositing them as a thickened slurry or paste. There have also been chemical processes developed to regenerate free cyanide from the tailings water so it can be re-used, but with limited commercial success in Australia.

Ultimately, cyanide may be lost from TSFs and heap leach piles by volatilisation of HCN, may degrade by various abiotic and biotic processes, may be fixed within the site by precipitation and adsorption of metallocyanides, and may migrate in seepage to underlying strata and groundwater. The extent to which seepage occurs varies widely between individual sites.

In landfills, cyanide residues are expected to undergo processes similar to those in soils and groundwater (volatilisation, complexation, biodegradation, adsorption, precipitation). Most sodium cyanide-derived waste from non-mining uses is expected to be treated to destroy free cyanide before delivery to landfill. Cyanide-resistant micro-organisms capable of biodegrading cyanide have been identified in aerobic sewerage treatment systems, and the small amounts of cyanide arising from industrial discharge into sewers are likely to be destroyed during secondary treatment.

Thus the overall fate of sodium cyanide (NaCN) and its products in the environment is complex and depends on factors such as concentration, chemical speciation, form released (solid, liquid), co-associated chemicals, pH, redox potential, temperature, and exposure to sunlight in the environment into which the cyanide is released.