Ammonia

CAS RN: 7664-41-7

Environmental Fate

TERRESTRIAL FATE: In soil, ammonia may either volatilize to the atmosphere, adsorb to particulate matter, or undergo microbial transformation to nitrate or nitrite anions(1). Volatilization of ammonia from moist soil surfaces is expected to be an important fate process(SRC) given a Henry's Law constant of 1.61X10-5 atm-cu m/mole(2). Ammonia is a gas with a vapor pressure of 7500 mm Hg at 25 deg C and atmospheric pressure(3), and therefore, is expected to volatilize from dry soil surfaces(SRC). In soil, ammonia can serve as a nutrient source for plants, which can be taken up by plants and microorganisms and converted to organic-nitrogen compounds(1). Ammonia in soil can be rapidly transformed to nitrate by the microbial population through nitrification(1). The nitrate formed will either leach through the soil or be taken up by plants or other microorganisms(1). Ammonia at natural concentrations in soil is not believed to have a very long half-life. If ammonia is distributed to soil in large concentrations (such as following an ammonia-containing fertilizer application), the natural biological transformation processes can be overwhelmed, and the environmental fate of ammonia will become dependent upon the physical and chemical properties of ammonia, until the ammonia concentration returns to background levels(1).
AQUATIC FATE: Ammonia is lost from water by volatilization(1) and volatilization is expected(2) based upon a Henry's Law constant of 1.61X10-5 atm-cu m/mole(3). Using this Henry's Law constant and an estimation method(2), volatilization half-lives for a model river and model lake are 1.4 and 12 days, respectively(SRC). In water, ammonia is in equilibrium with the ammonium ion (NH4+), and the ammonia-ammonium ion equilibrium is dependent on the pH(4). The pKa of ammonia is 9.25(5). The proportion of un-ionized ammonia in water increases with increasing temperature and pH, but decreases with increasing salinity(1). At pH 8.5, the proportion of un-ionized ammonia is approximately 10 times that at pH 7.5 and, for every 9 deg C increase in temperature, the proportion of un-ionized ammonia approximately doubles(1). In surface water, groundwater, or sediment, ammonia can undergo sequential transformation by two processes in the nitrogen cycle, nitrification and denitrification, which would produce ionic nitrogen compounds, and from these, elemental nitrogen(4). The ionic nitrogen compounds formed from the aerobic process of nitrification (nitrate and nitrite anions) can leach through the sediment or be taken up by aquatic plants or other organisms(4). Removal of ammonium from water can also occur by adsorption to sediments or suspended organic material(4).
ATMOSPHERIC FATE: According to a model of gas/particle partitioning of semivolatile organic compounds in the atmosphere(1), ammonia, which has a vapor pressure of 7500 mm Hg at 25 deg C(2), is expected to exist solely as a gas in the ambient atmosphere. Gas-phase ammonia is degraded in the atmosphere by reaction with photochemically-produced hydroxyl radicals(SRC); the half-life for this reaction in air is estimated to be 100 days(SRC), calculated from its rate constant of 1.60X10-13 cu cm/molecule-sec at 25 deg C(3). Gas-phase ammonia is also degraded in the atmosphere by reaction with nitrate radicals(SRC); the half-life for this reaction in air is estimated to be 54 days(SRC), calculated from its rate constant of 5.99X10-16 cu cm/molecule-sec at 25 deg C(3). Ammonia reacts rapidly in the atmosphere with both sulfuric and nitric acids to form fine particles(4). In most of the US the majority of aerosol ammonium is associated with sulfate ion(4). Once released into the atmosphere, ammonia is returned to the surface as either gaseous ammonia or as an ammonium ion(4). The ammonium ion can be associated with nitrate, sulfate, or some other anion and incorporated into an aerosol or as part of the ionic mix found in cloud and raindrops(4). Ammonia can dissolve in the water in the atmosphere and form clouds or fog(5). The half-life for ammonia in the atmosphere has been estimated to be a few days; the reaction with acidic substances in the air results in the formation of ammonium aerosols that can be removed by wet or dry deposition(5). Vapor deposition of ammonia from air to surface (to vegetation, soil, etc) also occurs(4). Ammonia does not absorb at wavelengths >290 nm(6) and, therefore, is not expected to be susceptible to direct photolysis by sunlight(SRC).
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