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Nitrogen (N) and water

Nitrogen and water: reaction mechanisms, environmental impact and health effects

Seawater contains approximately 0.5 ppm nitrogen (dissolved inorganic nitrogen compounds without N2). The amount is clearly lower at the surface, being approximately 0.1 ppb. River water concentrations vary strongly, but are approximately 0.25 ppm in general.
Depending on water properties, various inorganic nitrogen compounds may be found. In aerobic waters nitrogen is mainly present as N2 and NO3-, and depending on environmental conditions it may also occur as N2O, NH3, NH4+, HNO2, NO2- or HNO3.
Water in coastal areas mainly contains elementary nitrogen gas (N2). This can be no surprise, as air consists of 78% nitrogen, and water comes in contact with air regularly in coastal regions because of low water depth and active currents.
Ammonium, nitrate and nitrite play the most important role in biochemical processes, but some organic nitrogen compounds in water may also be of significance. Total nitrogen represents the sum of organic and inorganic nitrogen compounds. For wastewater Kjeldahl-nitrogen is generally applied as a measure. The TKN value (Total Kjeldahl Nitrogen) represents a total nitrogen concentration, which is the sum of organic nitrogen compounds and ammonium nitrogen (TKN = org-N + NH4-N [mg/L]). Nitrogen mainly occurs in wastewater in this form. After biological wastewater treatment, it mainly occurs as oxidized nitrite.


In what way and in what form does nitrogen react with water?

Nitrogen gas does not react with water. It does dissolve in water.


Solubility of nitrogen and nitrogen compounds

Nitrogen (N2) solubility at 20oC and pressure = 1 bar is approximately 20 mg/L. Nitrogen solubility may differ between compounds. Nitrogen (I) oxide solubility is 12 g/L, and nitriloacetate (salt) solubility is 640 g/L, whereas nitrogen chloride is water insoluble. Nitrates and ammonia dissolve in water readily.


Why is nitrogen present in water?

Nitrogen ends up in the environment mainly through agricultural processes, and thereby also ends up in water. The main source of nitrogen compounds in water are fertilizers that mainly contain nitrate, but also ammonia, ammonium, urea and amines. The most widely applied nitrogen fertilizers are probably NaNO3 (sodium nitrate) and NH4NO3 (ammonium nitrate). After fertilization, crops take up a relatively small part of added nitrogen compounds, namely 25-30%. The residue ends up in groundwater and surface water through soils, because nitrates are water soluble. Organic fertilizers mainly contain nitrogen as proteins, urea or amines, which have different mechanisms of absorption. Guano is a natural fertilizer that contains sufficient amounts of nitrogen. Finally, various pesticides added to farmland contain nitrogen.
Nitrogen compounds are applied in several different industries. Most nitrogen is applied to synthesize ammonia by the Haber-Bosch process. Thereby other nitrogen compounds, such as nitrous oxide applied in anaesthetics, can be produced. Nitric acid, urea, hydrazine and amines are other products from nitrogen industries. Nitrogen compounds are by-products of colouring and synthetic agent production.
Liquid nitrogen is applied in large amounts to freeze food. Deep-freezing samples and chemicals is achieved by the same method. Liquid nitrogen is also an interesting agent for superconductor and ceramic development.
Nitrogen is applied as a protective gas in welding for semi-conductor production. It is also applied in sprays and fire extinguishers. N2O4 is a rocket fuel oxidator. The element is a constituent of explosives and is applied in mining.
A significant amount of nitrogen can be found in domestic wastewater. The exact concentration depends on protein application of the population. Usually about one-third of total nitrogen are organic nitrogen compounds, mostly urea. The remainder are ammonium salts. Domestic wastewater generally contains no more than 3% nitrates and nitrites. Decomposition products of the first two sewage water treatment steps are mainly ammonium and nitrate.
Nitrates and nitrites are applied as food additives to conserve the red colour of meat, and to prevent toxin formation. NTA (nitriloacetate) is a replacement of phosphate in detergents.
Nitrogen may end up in water and soils from landfills. The occurrence of nitrogen in soils and waters is largely explained by the nitrogen cycle.


What are the environmental effects of nitrogen in water?

Nitrogen is a dietary requirement for all organisms, because it is a constituent of all proteins and nucleic acids. Plants consists of approximately 7.5% nitrogen (dry mass). Nitrogen is essential for plants, and can be found in air in large amounts. This elementary nitrogen cannot be taken up directly. Nitrogen must first be bound and converted, for instance to nitrate. This so-called nitrification process is carried out by bacteria, which convert ammonia and ammonium to nitrate and nitrite. This releases energy, and establishes a nitrate stock in soils that can be applied by plants.
When nitrogen fertilizers are applied, the plant nitrogen amount increases. A number of crops, such as spinach, even accumulate nitrogen compounds. When nitrogen fertilizers are applied outside the growing season, this is completely useless and negatively affects the environment. The fertilizers cannot be taken up or immobilized, causing them to end up in groundwater and drinking water. Nitrogen has a high spreading potential. A number of plants are relatively susceptible to NO2.
Nitric acid is an important constituent of precipitation. Together with H2SO4 it causes acid rain, which negatively affects crops and soils.
Nitrogen is an essential protein constituent, and is therefore present in animal tissue in large amounts. Elementary nitrogen has no direct effect on warm-blooded organisms. High nitrogen concentrations in air may lead to asphyxiation, because in this case the oxygen concentration decreases.
Nitrogen itself is not hazardous when present in water, and therefore does not cause any environmental damage. In seawater nitrates, nitrites and ammonia are dietary requirements for plankton, causing nitrogen concentrations to be lower at the surface than in the deep. At increasing nitrogen concentrations in surface layers, plankton production increases, leading to algal blooms. This may occur in any type of surface water. Large amounts of nitrate may cause eutrophication, which means an excess of nutrients resulting in oxygen deprivation and fish deaths (see oxygen and water). Nitrogen does not limit algal growth, because phosphorus is generally a limiting factor in water bodies. This means that phosphorus is the determining factor of algal spreading through surface waters. Oxygen deficits in surface water generally result in nitrate reduction to elementary nitrogen or nitrous oxide. This so-called denitrification process causes oxygen reserve releases, when oxygen supplies decrease to zero. In some cases nitrate may even be biologically reduced to ammonia. Ammonium compounds decrease the water oxygen concentration, because these are oxidized from nitrite to nitrate. Small concentrations of free ammonia may be toxic to fish.
Nitrification may also play an important role in water. This process means ammonia oxidation to nitrite and nitrate. The nitrite concentration is decreased, which is positive for higher plants, because nitrite is toxic at low pH values.
NOx compounds react with water to soluble nitric acid. This means that oceans can reduce atmospheric nitrogen oxide concentrations. PAN compounds (Peroxy Acetyl Nitrate) are derived from terrestrial environmental pollution, but may also be transported in the troposphere and in oceans. Eventually, these compounds are decomposed to NOx. The reaction mechanism is as described above.
There are some examples of toxic nitrogen compounds. NTA, which is generally complexed with heavy metals, can disturb electrolyte metabolism. In rats it may damage kidneys at concentrations above 14 mg/kg body weight. The LD50 value is 1.5 g/kg for rats and 0.75 g/kg for rhesus monkeys. It may cause chromosome defects in the in vitro system. For nitro aniline the LD50 for rodents is 1-3.6 mg/kg. The non-toxic concentration for fish is approximately 10 mg/L (48 h).
Nitrogen naturally has two stable isotopes. There are also six instable isotopes.


What are the health effects of nitrogen in water?

The human body consists of approximately 2.6% nitrogen, which is a constituent of most proteins and nucleic acids. This means nitrogen is a dietary requirement. Nitrogen is the main constituent of the air we breathe. Increased nitrogen concentrations in air may cause asphyxiation, but mainly because it results in a lower oxygen concentration.
We mainly absorb nitrogen as proteins. These cannot be stored and are therefore directly converted to energy when not required. Nitrogen is excreted through the kidneys as urea. We also release nitrogen through the skin and the intestinal tract. When kidney failure occurs, one is incriminated with protein decomposition products. The calculation factor from nitrogen to protein in 6.25. This value does not represent protein digestibility.
Nitrates are not generally considered toxic, but at high concentrations the body may convert nitrate to nitrite. Nitrites are toxic salts that disrupt blood oxygen transport by disrupting haemoglobin to methemoglobin conversion. This causes nausea and stomach aches for adults. For young infants it may be extremely risky, because it rapidly causes blood oxygen deprivation.
The maximum recommended concentration for nitrate is 10 mg/L, and for nitrite the maximum level is 1 mg/L (EPA standards).
Nitrites and amines from protein-rich food form so-called nitrosamines, which are carcinogenic substances. This reaction may be prevented by the reducing and anti-oxidant properties of vitamin C.
Examples of toxic nitrogen compounds are PAN-compounds, which are fifty times more toxic than the nitrogen compounds these are converted from (nitriles and nitrilo compounds). NTA is not absorbed in the stomach, because it is complexed with heavy metals. It may however still disrupt electrolyte metabolism.
Nitrogen oxides play a more significant role in air than in water. These can cause breathing disorders. Nitrogen hydrogen acid fumes may cause irritations, heart problems and collapsing.


Which water purification technologies can be applied to remove nitrogen from water?

In wastewater treatment plants the first two treatment steps may remove only 50% of nitrogen concentrations. For further treatment, lime and HOCl addition were attempted. This however turned out not to be very effective. Consequently, the third step of wastewater treatment includes biological nitrogen removal. This means a combination of nitrification and denitrification processes, carried out by various micro organisms.
Nitrification means ammonium oxidation from protein decomposition processes by bacteria, and subsequent conversion to nitrates. This requires oxygen, which is added by aeration. The water must be aerated for a sufficient period of time. Ammonium is converted to nitrite, and subsequently to nitrate. The reaction mechanism is a follows:

2 NH4+ + 3 O2 -> 2 NO2- + 2 H2O + 4 H+ - by nitrosomonas
2 NO2- + O2 -> 2 NO3- - by nitrobacter

During the denitrification bacteria decompose nitrates to nitrogen. This does not require aeration, as it is an anaerobic process. Nitrogen is eventually released into air. A carbon source is often added to speed up the decomposition process. One example of a possible reaction mechanism is:

6 NO3- + 5 CH3OH -> 3 N2 + 5 CO2 + 7 H2O + 6 OH-

These processes exclude one another, because one requires oxygen and one does not. Consequently, wastewater treatment requires both aeration, and the presence of oxygen-pour spaces. When these processes are applied as a third water purification step, approximately 90% of nitrogen may be removed.
In countries such as Brazil, water hyacinths are applied as a third water purification step. These remove both nitrogen and phosphorus from water. Helophyte filters can be applied in water purification of small surface waters.
Ammonia can be removed from water by the so-called stripping process. This means removing ammonia from wastewater by means of air or steam, by gasifying it.
Other nitrogen compounds that generally occur in small amounts may be removed by various methods. For example, NTA can be decomposed under aerobic conditions in aeration tanks. Nitro aniline cannot be decomposed.
Regular ionic nitrogen compounds, such as NO3-, NO2- and NH4+, and amines, may be removed by means of ion exchange.

Literature and the other elements and their interaction with water







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