Nitrogen

Typical wastewaters

• municipal sewage urea and protein hydrolysed to ammonia in the sewer; TKN 20 to 70 and ammonia 12 to 45 mg/L as N, nitrate nil in the raw water
• ammonia and fertiliser manufacture ammonia (NH₃ and NH₄⁺) as N in process wastewater; limits set per 1000 kg of product
• chemical sector effluent total nitrogen 5 to 25 and inorganic nitrogen 5 to 20 mg/L as N after biological treatment
• digester supernatant and landfill leachate (sidestreams) warm, ammonia rich liquor, NH₄⁺ at high concentration, treated by anammox, stripping or ion exchange
• agricultural runoff, manure and septic tank or pit latrine leachate nitrate NO₃⁻ leaching to groundwater from fertiliser and manure, wastewater disposal and the oxidation of nitrogenous waste in excreta
• textile dyeing and finishing and leather tanning ammonium nitrogen (textile 10, 1, 0.5; leather 15, 10, 1 mg/L as N by level) and total nitrogen

1 · Identity

Symbol, number

N, 7

Oxidation states in water

-3 as ammonia and ammonium (free ammonia NH₃ and the ammonium ion NH₄⁺, together called ammonia by WHO) and as organic nitrogen in urea, amino acids and proteins (Kjeldahl nitrogen is organic plus ammonia nitrogen); 0 as dissolved N₂, the end product of denitrification and anammox and an inert gas otherwise; +3 as nitrite NO₂⁻, the unstable intermediate of nitrification and denitrification; +5 as nitrate NO₃⁻, the stable oxidised form and the mobile groundwater contaminant. Chloramines (-3 nitrogen bound to +1 chlorine) and nitrogen trichloride are in the chlorine chapter.

Note

The element entry carries the Haber process, the fertiliser economy and the eutrophication narrative. This chapter is the nitrogen species in water, their limits and their treatment. Breakpoint chlorination and chloramination are written out in the chlorine chapter and only referenced; struvite is in the magnesium chapter; hydrogen driven denitrification is in the hydrogen chapter.

2 · Occurrence in water

Natural sources

Nitrate is part of the nitrogen cycle and an important plant nutrient; nitrite is not usually present in significant concentrations except under reducing conditions because nitrate is the more stable state (WHO fact sheet). Natural groundwater nitrate does not usually exceed 4 to 9 mg/L as nitrate ion and nitrite 0.3 mg/L; natural vegetation occasionally raises groundwater nitrate; rainwater carries up to 5 mg/L nitrate in industrial areas (WHO background document). Natural ammonia in groundwater and surface water is usually below 0.2 mg/L, up to 3 mg/L in anaerobic groundwater and strata rich in humic substances or iron (WHO ammonia document).

Anthropogenic sources

Excess application of inorganic nitrogen fertilisers and manures, wastewater disposal, oxidation of nitrogenous waste in human and animal excreta including septic tanks and pit latrines, and nitrification in source water or distribution systems (WHO); intensive rearing of farm animals gives much higher surface water ammonia; chloramination adds ammonia to drinking water and cement mortar linings release it (WHO ammonia document); ammonia plants and fertiliser works (US effluent guideline part 418), coke ovens, primary beryllium raffinate (299,400 mg/kg as N daily maximum in 40 CFR 421.152), landfill leachate and digester supernatant (Metcalf and Eddy).

3 · Speciation

Ammonia dissolves to give ammonium and hydroxide with a base constant of 1.78 x 10⁻5 (WHO), that is pKa 9.25 for NH₄⁺ at 25 C: below pH 8 nearly all ammonia is the ionised, non volatile, non toxic NH₄⁺, at pH 9.25 half is the free NH₃ that is toxic to fish, volatile and the species that stripping towers remove; the fraction rises with temperature. Nitrate and nitrite are fully dissociated anions at every pH, do not adsorb, do not precipitate and are removed only by membranes, resins or bacteria. Redox, not pH, moves nitrogen between the three: oxic water with nitrifiers turns ammonia to nitrite and nitrate within days, anoxic water with organic carbon turns nitrate back to nitrogen gas, and nitrite accumulates wherever either process is incomplete, in chloraminated mains, galvanised pipes with stagnant water, and overloaded nitrifying filters (WHO).

Solubility

Ammonia gas is very soluble, 421 g/L at 20 C (WHO); all ammonium, nitrite and nitrate salts of the common cations are soluble, so none of the three has a solubility control in water; nitrogen gas is sparingly soluble and supersaturates in denitrifying clarifiers, floating sludge.

Hydrolysis

Ammonium is a weak acid (pKa 9.25), ammonia a weak base; nitrate and nitrite are the anions of strong and moderately weak acids (nitrous acid pKa about 3.3) and are not hydrolysed at natural pH. Urea hydrolyses to ammonia and CO₂ in the sewer and in the soil.

Complexation

Ammonia complexes copper, nickel, silver and zinc as ammines in plating rinses and in chloraminated water, keeping them dissolved; nitrate complexes weakly and nitrite forms nitroso complexes with iron; none quantified here.

Precipitates

None of the three species alone. Magnesium ammonium phosphate (struvite) precipitates from digester liquors and can be made deliberately (magnesium chapter); ammonium ion exchanges onto clinoptilolite.

4 · Role in treatment

5 · Removal and control

6 · Analytics

Sampling pitfalls

Nitrogen species change in the bottle: ammonia is nitrified and nitrite oxidised in oxic samples, nitrate is denitrified in anoxic ones. Cool to 4 C and analyse within 48 hours, or acidify to pH below 2 with sulfuric acid for ammonia, Kjeldahl and nitrate plus nitrite together (acid destroys nitrite as a separate species). Quench chlorine or chloramine before ammonia analysis, and remember that monochloramine reads as ammonia after dechlorination. Report the units: nitrate as NO₃ (WHO, EU) is 4.43 times nitrate as N (US); nitrite as NO₂ is 3.29 times nitrite as N (WHO conversion factors 0.226 and 0.304).

7 · Regulatory limits

Limits change, and many are set locally. Treat these as the published values to start from, not as your compliance target: check the standard in force at your site and the numbers written into your own permit.

8 · Health and environmental effects

Toxicity

Nitrate is absorbed over 90 percent and about 25 percent is recirculated in saliva, where mouth bacteria convert about 20 percent to nitrite; the body itself makes about 62 mg nitrate a day. Nitrite oxidises haemoglobin to methaemoglobin, and above 10 percent methaemoglobin infants turn blue; 97 percent of clinical cases were above 44.3 mg/L nitrate and almost all under 3 months old with gastrointestinal infection. Nitrate competitively inhibits iodide uptake with weak evidence of thyroid effects above 50 mg/L. IARC: ingested nitrate or nitrite under conditions of endogenous nitrosation is probably carcinogenic (Group 2A), nitrate alone is not; the weight of evidence does not support an association between drinking water nitrate and cancer (WHO). Ammonia is a key metabolite made at 4000 mg/day in the intestine; environmental exposure is insignificant beside it (WHO ammonia document).

Bioaccumulation

Not applicable; nitrogen is cycled, not accumulated.

Ecotoxicity

US EPA 2013 freshwater ammonia criteria: 17 mg total ammonia nitrogen per litre acute and 1.9 mg/L chronic (30-day average, 4-day average not above 4.8 mg/L) at pH 7 and 20 C, falling steeply as pH and temperature rise because the toxic species is NH₃; based on sensitive freshwater mussels and snails. Nitrate and nitrite have no US aquatic criterion in the table read; nitrite is toxic to fish (brown blood disease) at mg/L levels (general, not from a source read). Nitrogen with phosphorus drives eutrophication, the reason for the UWWTD nutrient limits.

Flags

• Nitrification and denitrification stoichiometry, oxygen and alkalinity factors, pKa 9.25 and the wastewater composition are cited to Metcalf and Eddy chapters 2, 3, 7, 8 and 15 from memory of the text, not re-read this session.
• The anammox and stripping efficiencies are general figures, not from the sources read.
• WHO occurrence data are compilations of 1980s to 2010s surveys and vary strongly by region.
• The US ammonia effluent guideline values are production normalised, not concentrations.
• The UWWTD figures are from the 2024 recast; the 1991 Directive values (15 and 10 mg/L TN) were not re-read.
• Abu Dhabi values cover two media (marine outfall ammonia 2.0, nitrate 30, TKN 10 mg/L as N; sewer TKN 150 mg/L); other GCC states not read.

Gaps

• No survey of nitrogen in industrial effluents beyond the CWW AELs was read; fertiliser, coke oven and landfill leachate concentrations belong to the ledger.
• No ion exchange capacities, membrane rejections or denitrification filter loading rates are quoted.
• Nitrous oxide emissions from nitrification and denitrification, and organic nitrogen compounds (nitrosamines, which are in the WHO fact sheets), are not covered here.
• The pH and temperature dependence of the US ammonia criteria is not tabulated; only the pH 7, 20 C values are quoted.
• The 1991 UWWTD table and the US industrial category nitrogen limits other than part 418 and 421 were not read.
• Other GCC discharge standards were not read.

Sources

Identity

Name and symbol

Nitrogen, N

Atomic number

7 protons

Position

group 15 · period 2 · p-block · diatomic nonmetal

CAS number

17778-88-0

Atomic structure

Atomic mass

14.0067 u

Electron configuration

1s² 2s² 2p³
[He] 2s² 2p³

Electrons per shell

2, 5

Valence electrons

5 outer shell

Physical properties

State at room temperature

Gas

Melting point

63.15 K (-210 °C)

Boiling point

77.36 K (-195.79 °C)

Density

0.0013 g/cm3 (gas at STP, so 1.2506 g/L)

Appearance

colorless gas, liquid or solid

Thermal conductivity

25.83×10-3 W/(m·K)

Electrical resistivity

not in sources

Electrical conductivity

not in sources

Crystal structure

hexagonal

Molar heat capacity

not in sources

Chemical properties

Oxidation states

+5, +4, +3, +2, +1, -1, -2, -3

Electronegativity

3.04 (Pauling Scale)

Ionisation energy

14.534 eV
1st 1,402.3, 2nd 2,856, 3rd 4,578.1 kJ/mol

Electron affinity

0 eV

Atomic radius

empirical 71, covalent 71, van der Waals 155 pm

Ionic radius

N³⁻ 146 (4-coordinate); N³⁺ 16; N⁵⁺ 13 pm

Reactivity

Dinitrogen is mostly unreactive at room temperature because its triple bond is very strong (about 945 kJ/mol); it combines with lithium and a few transition metal complexes in the cold, and with hydrogen, oxygen and metals only under forcing conditions.

with water

Does not react.

with oxygen, air

No reaction under ordinary conditions; in lightning or an electric discharge the gases combine to nitric oxide, which then oxidises to brown nitrogen dioxide: $\ce{N2 + O2 -> 2 NO}$

with acids

Does not react.

with halogens

Does not react directly except with fluorine in an electric discharge at elevated temperature, giving the stable gas nitrogen trifluoride; the other trihalides, such as the explosive NCl3, are made indirectly: $\ce{N2 + 3 F2 -> 2 NF3}$

Typical compounds

• NH₃ ammonia Haber process; fertiliser feedstock, 150 million tonnes a year
• HNO₃ nitric acid Ostwald oxidation of ammonia; fertilisers and explosives
• NO₂ nitrogen dioxide acrid brown gas; gives nitric acid with water
• N₂O nitrous oxide laughing gas, from heating ammonium nitrate
• NaNO₃ sodium nitrate Chile saltpeter, the natural nitrate fertiliser
• N₂H₄ hydrazine endothermic hydride that burns cleanly to nitrogen and water

Occurrence, production and use

Crustal abundance

1.9×101 milligrams per kilogram

Oceanic abundance

5×10-1 milligrams per liter

Occurrence and sources

Nitrogen gas (N2) makes up 78.1% of the Earth’s air, by volume. The atmosphere of Mars, by comparison, is only 2.6% nitrogen. From an exhaustible source in our atmosphere, nitrogen gas can be obtained by liquefaction and fractional distillation. Nitrogen is found in all living systems as part of the makeup of biological compounds.

• dissolved in seawater about 0.5 mg/L; crustal estimate 19 mg/kg (Jefferson Lab figures via PubChem)
• N2 in air 78 percent of the atmosphere by volume; obtained by liquefaction and fractional distillation; the resource is unlimited
• sodium and potassium nitrate (saltpetre) Atacama Desert, Chile, and other dry regions; formed by decomposition of organic matter; minimal contribution to supply
• organic nitrogen all living things; hence coal and other fossil fuels

Extraction, production

Air separation by distillation of liquid air

Physical separation, no chemical reaction.

Haber process: ammonia synthesis from nitrogen and hydrogen

$\ce{N2 + 3 H2 <=> 2 NH3}$

Delta H0 = -46 kJ/mol, on an iron catalyst at 100 to 250 bar and 350 to 550 degrees C; only 20 to 30 percent of the synthesis gas converts per pass, so the unreacted gas is recycled after the ammonia is condensed out (AAF BREF, PDF p71). World capacity was 109 million tonnes of ammonia in 2003, 46 percent of it in Asia and 9 percent in Western Europe; the EU made about 11 million tonnes a year from around 50 plants (2001) (PDF p63). USGS puts 2024 world production at about 150 million tonnes of contained nitrogen.

Uses

The largest use of nitrogen is for the production of ammonia (NH3). Large amounts of nitrogen are combined with hydrogen to produce ammonia in a method known as the Haber process. Large amounts of ammonia are then used to create fertilizers, explosives and, through a process known as the Ostwald process, nitric acid (HNO3).

Nitrogen gas is largely inert and is used as a protective shield in the semiconductor industry and during certain types of welding and soldering operations. Oil companies use high pressure nitrogen to help force crude oil to the surface. Liquid nitrogen is an inexpensive cryogenic liquid used for refrigeration, preservation of biological samples and for low temperature scientific experimentation. Jefferson Lab's Frostbite Theater features videos of many basic liquid nitrogen experiments.

• Fertilisers: anhydrous ammonia for direct application; urea: $\ce{2 NH3 + CO2 <=> NH2COONH4}$ (ammonium carbamate), then $\ce{NH2COONH4 <=> CO(NH2)2 + H2O}$, both in the liquid phase in one reactor (AAF BREF, PDF p352); nitric acid by ammonia oxidation on a platinum catalyst, $\ce{4 NH3 + 5 O2 <=> 4 NO + 6 H2O}$, with side reactions to N2 and to the greenhouse gas N2O (PDF p124); ammonium nitrate by neutralisation, $\ce{NH3 + HNO3 -> NH4NO3}$, highly exothermic, the heat used to raise steam (PDF p393); ammonium phosphates and ammonium sulfate about 80 percent of ammonia is used as the nitrogen source in fertilisers (AAF BREF, PDF p63); 88 percent of US ammonia production in 2024 (USGS)
• Industrial chemicals: nylon and other polyamides; explosives and blasting agents, for which no practical substitute exists; sodium cyanide, hydrazine, amines, amides and nitriles the roughly 20 percent of ammonia not used in fertiliser (AAF BREF, PDF p63)
• Pharmaceuticals and dyes: amines, amides and nitriles from ammonia serve as intermediates in dye and pharmaceutical manufacture
• Food and beverage: nitrogen atmosphere to preserve foods; liquid nitrogen for rapid freezing that keeps moisture, colour, flavour and texture
• Electronics and steel: inert atmosphere in transistor and diode production; annealing of stainless steel and other mill products

Safety, toxicity

GHS classification, signal word Warning

• H280 Contains gas under pressure; may explode if heated Gases under pressure

Discovery and name

Discovered by

Daniel Rutherford

Discovered

1772

First isolated

not in sources

Named by

Jean-Antoine Chaptal

Origin of the name

from French nitre (“saltpetre”) + -gène ("forming")