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Environmental fate & pathways

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Ammonia in water may be transferred to the atmosphere by volatilisation from the air–water interface; this process has a measurable effect on ammonia levels in water. The rate of volatilisation of ammonia from water will increase with increasing pH (generally only important for pH values >7) and temperature, and can be influenced by other environmental factors such as flow conditions and wind speed. Total ammonia in the environment comprises two principal forms: the ionised ammonium ion (NH4 +) and un-ionised ammonia (NH3). The proportion of ionised and un-ionised ammonia can be calculated from total ammonia, taking into account salinity, temperature and pH. The proportion of unionised ammonia increases with increasing temperature and pH, but decreases with increasing salinity. Of the three main factors affecting ammonia speciation, salinity appears to be relatively unimportant. Temperature and pH, however, have a much more significant effect. At pH 8.5, the proportion of un-ionised ammonia is approximately 10 times that at pH 7.5 and, for every 9°C increase in temperature, the proportion of un-ionised ammonia approximately doubles. At the pH of most biological systems, ammonia exists predominantly in the ionised form (NH4 +).

Concentrations of ammonia in water are controlled directly by volatilisation to and dissolution from the atmosphere. Ammonia dissolves readily in water where it ionises to form the ammonium ion. Under normal atmospheric conditions, ammonia does not undergo any primary photochemical reactions at wavelengths greater than 290 nm. In water, ammonia is not expected to adsorb to suspended solids and sediment to an appreciable degree. Ammonia at natural concentrations in soil is not believed to have a very long half-life. In soil, ammonia may either volatilise to the atmosphere or undergo microbial transformation to nitrate or nitrite anions. Uptake by plants can be a significant fate process. Based on its solubility, ammonia is not expected to adsorb to particulate matter to an appreciable degree.