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Environmental fate & pathways

Hydrolysis

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Description of key information

After adding AlN to water hydrolysis starts immediately. At room temperature (22 °C) the half life time was determined to be ca. 1320 min. Time-dependent conversion for a 3 % (w/w) AlN powder suspension in water at 22 °C, 50 °C and 90 °C was finally over 90 % at all temperatures, with different time-frames, respectively. The measured half-life time of AlN in aqueous solution does not directly result in free aluminium ions because the predominant resulting aluminium species was bayerite (Al(OH)3) which is not water-soluble.

Key value for chemical safety assessment

Half-life for hydrolysis:
1 320 min
at the temperature of:
22 °C

Additional information

Aluminium nitride (AlN) is hydrolytically degraded, and various poorly soluble aluminium salts, predominantly bayerite Al(OH)3 and boehmite AlO(OH) are formed. The nitride fraction is transformed into ammonia (NH3). Studies identifying and quantifying the degradation of aluminium nitride are summarised as follows:

Kocjan et al. (2011) found that the half-life time of AlN at room temperature (22 °C) was about 1320 min. More than 90 % of the test item was hydrolysed after 2000 min at this temperature.

Bowen et al. (1990) determined the hydrolysis rate of AlN in water: after 24 h ca. 80 % of the AlN powder was transformed.

The study by Oliveira et al. (2003) revealed that AlN powder/particles are rapidly transformed into intermediate transformation products and finally into Al(OH)3 and NH3. The presence of H3PO4 reduced the rate of transformation drastically due to the protective action of the insoluble aluminium phosphate formed at the surface.

The studies of Kocjan et al. (2007) and Krnel et al. (2004; 1999) confirmed that the main hydrolysis products of AlN are Al(OH)3, AlOOH and NH3. In addition, the study of Krnel et al. (1999) determined the reactivity of AlN powder in diluted inorganic acids by measuring pH and temperature during hydrolysis.

Fukumoto et al. (2000) found out that the formation of hydrolysis products from AlN is temperature dependent. Below 351 K (ca. 78 °C) a crystalline bayerite was produced on the surface of AlN particle, while crystalline boehmite was produced at temperatures higher than 351 K.

Under REACH (cf. ECHA guidance on CSA and information requirements, chapter R.7B), the term „hydrolysis“ refers to the decomposition or degradation of a chemical by reaction with water, as a function of pH where relevant (i.e. abiotic degradation). Aluminium can participate in hydrolysis reactions, thereby forming a number of monomeric and polymeric Al-hydroxides and this process is highly dependent on pH. However, aluminium persists in the environment irrespective of the chemical species formed as a result of hydrolysis, although this may result in insoluble aluminium hydroxides that precipitate from the solution. Characterisation of aluminium in environmental media is typically based on total aluminium concentrations (including all specific chemical forms or species). Since hydrolysis changes only the chemical form of aluminium but does not decompose the substance and since characterisation of total aluminium take into account all chemical forms, the concept of degradation of aluminium by hydrolysis is not relevant in the consideration of environmental fate.

The hydrolysis of AlN is assessed in a weight-of-evidence approach to illustrate the transformation processes of AlN in aqueous media. The different studies show that distinguishing between water solubility and hydrolysis is not possible for this test item. AlN hydrolysis results in several poorly soluble transformation products including boehmite and bayerite, which are practically insoluble in the neutral pH range (5–7). For this reason the transformation/dissolution test (T/D) performed for AlN (see Klawonn, 2014) was additionally included in the WoE. AlN was found to release low amounts of soluble Al during hydrolysis (Al concentrations at maximum 1 mg/L). These results in combination indicate that AlN is almost quantitatively transformed after exposure to water, but the concentration of free aluminium ions is low.