Aluminium nitride

Endpoint summary

Additional information

Aluminium nitride is an inorganic substance hence cannot undergo biological degradation. However, AlN is susceptible to hydrolysis, forming insoluble aluminium salts (boehmite, bayerite), low concentrations of free Al3+, and ammonia (NH3) at a reaction half-life of 1320 minutes.

Ammonia is a naturally occurring substance, and an integral part of local and global nitrogen cycles. Nature has developed many mechanisms for altering the distribution of ammonia across biological systems and it can be stated that all organisms contribute, either directly or indirectly, to ammonia related environmental fate processes. Ammonia occurs in the environment in the ionised (NH4+; ammonium) and unionised (NH3, ammonia) form, whereas the equilibrium is on the side of the ionised species. Organisms are regularly exposed to low levels of ammonia in air, soil and water. Natural levels of ammonia in air are between 1 and 5 ppb, whereas the levels in rivers and bays are usually less than 6 ppm, and soil typically contains about 1-5 ppm of ammonia. Naturally occurring levels of ammonia vary during the day and from season to season. Ammonia does not last long in the environment, because it is recycled and nature has many ways of incorporating and transforming ammonia. In soil or water, for example, plants and microorganisms rapidly take up ammonia. After application of ammonia containing fertilizer to soil the amount of ammonia in that soil decreases to low levels within a few days. In the air ammonia will persist for approximately one week. (ASTDR - Tox Profile of Ammonia, 2004).

Wastewater treatment plants can be seen as main point of ammonia release into surface water but it can enter surface water also by deposition from the atmosphere, and ammonium can be released upon death and decomposition of N2-fixing cyanobacteria. Ammonia enters soil via different human processes, including fertilizer applications and natural biological processes. Approximately 80 % of the U.S. production of ammonia is applied to soil in fertilizer formulations designed to release ammoniacal nitrogen. Natural processes of ammonia release into soil include the decomposition of animal wastes on cattle feed lots or other confinement areas.

Atmospheric ammonia can be readily removed from the air by rain or snow washout (Adamowicz 1979; Asman et al. 1998; Kumar 1985). Ammonia can dissolve in the water found in clouds (Asman et al. 1998; Brimblecombe and Dawson 1984; Sprenger and Bachmann 1987) or fog (Johnson et al. 1987). Ammonia can be removed from the atmosphere through direct absorption by surface waters in areas where the local atmospheric concentration is high (Hutchinson and Viets 1969) and by wet deposition onto soils and surface waters (Asman et al. 1998; Cuesta-Santos et al. 1998; Goulding et al. 1998). Uptake of atmospheric ammonia by different species of plants also occurs (Harper and Sharpe 1995; Nason et al. 1988; Rogers and Aneja 1980). Depending on the local atmospheric concentration, however, plants can also release ammonia to the atmosphere (Harper and Sharpe 1995; Lee et al. 1997; O'Deen and Porter 1986; Parton et al. 1988). In water, adsorption of ammonia on sediment and suspended organic material can be important under certain conditions (Ankley et al. 1990). Adsorption to sediment should increase with increasing organic content, increased metal ion content, and decreasing pH. Ammonia, however, can be produced in, and subsequently released from, sediment (Jones et al. 1982; Malcolm et al. 1986). The uptake of ammonia by fish can also occur under the certain conditions (Hargreaves 1998; Mitz and Giesy 1985). Because ammonia, as ammonium ion, is the nutrient of choice for many plants (Kramer 2000; Rosswall 1981), uptake of soil ammonia by living plants is an important fate process. Because of these processes, and because ammonia generally exists in soils as NH4+ (which binds to soils particles), ammonia does not leach readily through soil; thus, it is rarely found as a contaminant of groundwater (Barry et al. 1993). In soil, ammonia that results from the application of fertilizers is usually found in the top 10 inches of the soil (Beauchamp et al. 1982). However, nitrate derived from ammonia may leach to groundwater.

 

Aluminium is the most abundant metal in the earth's crust, but it is never found free in nature. All naturally occurring aluminium forms compounds with other elements or molecules, like oxygen, hydroxides or sulphates. More than 8 % of the earth's crust is composed of aluminium, and aluminium is present in more than 270 different minerals. Aluminium is characterised by a complex biogeochemical cycle, it can participate in hydrolysis reactions, thereby forming a number of monomeric and polymeric Al-hydroxides, and this process is highly dependent on pH. The transport and partitioning of aluminium in the environment is determined by its chemical properties, as well as the characteristics of the environmental matrix that affect its solubility. At a pH > 5.5, naturally occurring aluminium compounds exist predominantly in an undissolved form such as gibbsite, Al(OH)3.

As an element, aluminium cannot be degraded in the environment, but may undergo various precipitation or ligand exchange reactions. Aluminium in compounds has only one oxidation state (+3) and would not undergo oxidation-reduction reactions under environmental conditions. Aluminium can be complexed by various ligands present in the environment (e.g., fulvic and humic acids). The solubility of aluminium in the environment will depend on the ligands present and the pH.

 

Literature:

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