Ammonia, anhydrous

Endpoint summary

Description of key information

Ammonia (NH3) is a highly reactive and soluble alkaline gas. It is naturally present in the environment: It is an intermediate in the global nitrogen cycle. It is essential for many biological processes and is a central compound in all living organisms.

Nitrogen is converted from atmospheric N2 to other forms by different processes. Nitrogen fixation (the process of converting atmospheric N2 to NH3) occurs naturally due to biological processes.

 

Ammonia is formed from nitrogen in air by the action of nitrogen-fixingbacteriathat exist in thesoilon the roots of certain plants like alfalfa.Nitrogen fixationcan also be accomplished by blue-greenalgaein the sea. These bacteria and algae possess anenzymecalled nitrogenase that permits them to convert nitrogen to ammonia at ambient conditions.

Much nitrogen is normally excreted by humans (and othermammals) asurea, a water soluble solid, butfishcan excrete ammonia directly. Urea eventually reacts with water to form ammonia, which therefore is usually present to some extent in waste water.

Furthermore, ammonia is naturally present in the environment as a consequence of the presence of decaying terrestrial and surface water plant or animal matter

In addition to natural processes, ammonia also originates from anthropogenic sources.

The current amount of nitrogen fixation that occurs by industrial processes equals that of natural, terrestrial nitrogen fixation. Both natural and anthropogenic sources produce a total of approximately 230–270 million metric tons of NH3 per year (ATSDR; Toxicological Profile for Ammonia. Atlanta, GA: Agency for Toxic Substances and Disease Registry, US Public Health Service (2004).

 

Although and basically, earth atmosphere is free from ammonia, because of its role in natural processes and cycles, ammonia is found at low concentrations in most environmental media. When ammonia is found at a local concentration that is higher than these background levels, it is often a result of human influence. Ammonia is hazardous only when exposure is to high levels.

 

Ammonia may be released to the atmosphere by volatilization from the following sources: decaying

organic matter; livestock excreta; fertilizers applied to soils; venting of gas, leaks, or spills during

commercial synthesis, production, transportation, or refrigeration equipment failure; sewage or waste water effluent; burning of coal, wood, and other natural products; and volcanic eruptions.

 

Ammonia may be released to water through effluent from sewage treatment plants, effluent from

industrial processes, runoff from fertilized fields, and runoff from areas of concentrated livestock. This usually occurs when the organic N compounds present in these sources enter the water and are converted microbiologically to ammonia.

 

Ammonia may be released to soils by natural or synthetic fertilizer application, animal (including

livestock) excrement degradation, decay of organic material from dead plants and animals, and indirectly from natural fixation of atmospheric nitrogen. In this latter case, ammonia releases can occur following nitrogen fixation by free-living microbes and plants (those that are symbiotic nitrogen-fixing bacteria), which subsequently die and release ammonia (or compounds that are converted to ammonia) to the soils.

 

Natural source of ammonia, e.g., the late summer decay of macrophytes in shallow lakes may cause transient very high concentrations of ammonia (Farnworth-Lee LA & Baker LA, Journal of Environmental Engineering 126 (3), March 2000).

Agricultural and industrial activity may potentially cause local and regional elevations in emission and atmospheric concentrations. Occupational exposure to ammonia may occur in industries involved in its synthesis, formulation, processing, transportation, and use. Occupational exposure to ammonia can also occur during the use of an extensive number of cleaning products that contain ammonia. Farmers may be exposed during the application of fertilizers containing anhydrous ammonia or liquid ammonia, or manures high in ammonia. Workers at cattle feedlots, poultry confinement buildings, or other industries that have a high concentration of animals may also be exposed. Exposure of the general population to elevated levels of ammonia is most commonly from the use of household cleaners that contain ammonia. People who live near farms or who visit farms during the application of fertilizer that contain or release ammonia may also be exposed. People living near cattle feedlots, poultry confinement buildings, or other areas where animal populations are concentrated can also be exposed to ammonia, in addition to other gases generated by putrefaction.

 

Ammonia does not absorb at wavelengths >290 nm and, therefore, is not expected to be susceptible to direct photolysis by sunlight. Ammonia may be photolytically dissociated at wavelengths of less than 222nm, resulting in the production of amino and ammonia radical species. The gas phase reaction of ammonia with photochemically produced hydroxyl radicals is thought to contribute about 10% to the overall atmospheric removal process.

In the atmosphere, ammonia can be removed by rain or snow washout. Reactions with acidic substances, such as H2SO4, HCl, HNO3, or N oxides (all produced in high concentrations from anthropogenic activities) produce ammonium aerosols, which can undergo dry or wet deposition. The gas phase reaction of ammonia with photochemically produced hydroxyl radicals is thought to contribute about 10% to the overall atmospheric removal process. The best estimate of the half-life of atmospheric ammonia is a few days (ATSDR; Toxicological Profile for Ammonia. Atlanta, GA: Agency for Toxic Substances and Disease Registry, US Public Health Service (2004)).

Ammonia will not hydrolyse.  The substance is highly soluble in water and will be present in an equilibrium as ammonia and the ammonium ion.  The balance of the equilibrium will be influenced by concentration and pH, however the ammonium ion will be predominant at relevant pH and low concentrations.

In the aqueous environment, ammonia will be present as ammonia (NH3) or ammonium ion (NH4+); the relative proportions of the two chemical species are dependent on pH and (to a lesser extent) temperature. At environmentally relevant pH values of 5- 8, the predominant form will be NH4+. At higher pH values the proportion of ammonia (NH3) increases. The background concentration of ammonia in surface water varies regionally and seasonally. Survey data for total ammonia have reported average concentrations of < 0.18 mg/litre in most surface waters, and around 0.5 mg/litre in waters near large metropolitan areas. In ground water, ammonia levels are usually low as a consequence of the strong adsorption of the ammonium ion on clay minerals, or bacterial oxidation to nitrate, both processes which limit mobility in soil. Ammonia in soil is in dynamic equilibrium with nitrate and other substrates in the nitrogen cycle.

Ammonium is readily converted by bacterial species to nitrate, via the process of nitrification. The primary stage of nitrification, the oxidation of ammonium to nitrite (NO2-) is performed by Nitrosomonas (among other) species. Other bacterial species (including Nitrobacter) are responsible for the subsequent oxidation of nitrite to nitrate (NO3-). Nitrification is important in preventing the persistence or accumulation of high ammonia levels in waters receiving sewage effluent or agricultural runoff. Other mechanisms may also act to limit the concentration of ammonia in natural waters: ammonia is readily assimilated by aquatic algae and macrophytes for use as a nitrogen source. Ammonia in the aqueous environment may also be transferred to sediments by adsorption on particulates, or to the atmosphere by volatilisation at the air-water interface. Both processes have been described as having measurable effects on ammonia levels in water; however, the relative significance of each will vary according to specific environmental conditions.

In soil, ammonia is readily converted by a variety of bacteria, actinomycetes and fungi to ammonium (NH4+) by the process of ammonification or mineralization. Ammonium is then rapidly converted to nitrate. Nitrate is subsequently taken up and utilised by plants or returned to the atmosphere following denitrification; the metabolic reduction of nitrate into nitrogen or nitrous oxide (N2O) gas. The most likely fate of ammonium ions in soils is conversion to nitrates by nitrification.