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Environmental fate & pathways

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Most (98%) HCN from ground-level sources remains in the stratosphere. Tropospheric oxidation by hydroxyl radicals presents the main atmospheric reaction for HCN. This photo-oxidation occurs both in the troposphere (0 - 8 km) and stratosphere (up to 80 km). Another path for HCN oxidation, dominant in the higher stratosphere (> 34 km), was reportedly the reaction with singlet oxygen. An alternative theory is that the ocean is a major sink for atmospheric HCN concentrations. 

Cyanide discharged to into the air will remain in the air, and that which is discharged into the surface water will remain mainly in the water. The Henry's Law Constant coefficient is low, approximately 0.0054.HCN is not strongly partitioned into the sediments or suspended adsorbents, primarily due to its solubility in water. Cyanides are relatively mobile in the soil, indicating that adsorption is unlikely to be significant in most aquatic environments.

Abiotic hydrolysis, to formic acid and ammonia, is normally a very slow reaction and it does not play a role under environmental conditions. HCN hydrolysis depends on temperature and pH. The pKa value of the dissociation equilibrium of HCN  is 9.36 at 20°C. Typical pH values of natural waters range from pH 6 to 8, indicating that the nonionised HCN fraction will dominate in natural environmental conditions. 

 

Aerobic and anaerobic/anoxic micro-organisms and treatment systems degrade cyanide.   Non-toxic concentrations of cyanide can be readily biodegraded, both aerobically and anaerobically. Degradation of cyanides by bacteria in sewage treatment plants depends on the availability of micro-organisms adapted to the presence of cyanide for about two weeks. Sudden high levels of cyanide in these sewage plants may lead to a loss of viability, while fully adapted sludge may tolerate and degrade concentrations up to 100 to 150 mg CN/l with a high degree of efficiency.

 

With a low n-octanol-water partition coefficient of -0.25, HCN is not bioaccumulative.