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Environmental fate & pathways

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Sodium azide is an inorganic substance and therefore biotic breakdown processes are not considered important when considering the environmental fate of the substance.

The abiotic breakdown of sodium azide in water, by dissociation and photolysis have been demonstrated to be important processes. Sodium azide is highly soluble in water and once dissolved it has been demonstrated to form hydrazoic acid. Two studies (Betterton, 2003 and US EPA, 1977) suggest that sodium azide is likely to undergo transformation in water when exposed to light, with the production of various reaction products, including nitrogen gas, metal nitrides and hydroxylamine.

In a study according to the OECD guideline 106 the mean Koc was determined to be 424 mL/g (Landsberg 2017). Also sodium azide has been shown to be hardly adsorbed by mineral soils, but is adsorbed by muck soils and by activated charcoal. It may be converted to hydrazoic acid in acid soils, and both the sodium azide and hydrazoic acid are readily leachable (WSSA 1983).

Additional information

Sodium azide does not contain any hydrolysable chemical groups. Therefore, the substance cannot undergo hydrolysis. Two review papers (Betterton 2003, and Ellenhorn 1997) on sodium azide are available that discuss the behaviour of sodium azide in water. Both of these studies suggest that sodium azide is highly soluble in water, and once dissolved will form hydrazoic acid, which is the corresponding acid in an acid-base equilibrium (which must not be confounded with hydrolysis).

According to Müller and Brous (1933), solid Sodium azide was found to be decomposed photochemically by wavelengths below approximately 405 nm. The rate of decomposition was directly proportional to the light intensity. Regarding wavelength of 254 nm no photolysis is reported by Betterton (2003). A phototransformation within the troposphere is not expected.

Two studies (Betterton 2003, and US EPA 1977) suggest that sodium azide is likely to undergo transformation in water when exposed to light, with the production of various reaction products, including nitrogen gas, metal nitrides and hydroxylamine.

Sodium azide has been shown to be hardly adsorbed by mineral soils, but is adsorbed by muck soils and by activated charcoal. It may be converted to hydrazoic acid in acid soils, and both the sodium azide and hydrazoic acid are readily leachable.

Since surface applied sodium azide is readily leached into soil, photodecomposition is not an important means of dissipation. In acid soils, sodium azide is readily converted to hydrazoic acid, which is highly volatile, moves readily through the soil, and can account for a major part of azide dissipation unless some type of vapor seal is employed (WSSA 1983).

The arithmetic mean Koc is calculated from five matrix-specific determinations as 424 mL/g.

Betterton (2003) report a Henry's law constant between 10 - 12.4 M/atm. Weiss (1996) state a volatility of hydrazoic acid (the protonated form of the azide anion) in acidified solution of 76 mm Hg /M, which is equal to 10.132 Pa/m^3/mol.