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.
Nitrogen gas does not react with water. It does dissolve in water.
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.
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 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.
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.
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.
2 NH4+ + 3 O2 -> 2 NO2- + 2 H2O + 4 H+ - by nitrosomonas
6 NO3- + 5 CH3OH -> 3 N2 + 5 CO2 + 7 H2O + 6 OH-