The role of nitrogen in the environment - nitrogen cycle nitrogen fixation the Haber-Bosch process nitrogen pollution
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The role of nitrogen in the
environment
nitrogen cycle
nitrogen fixation
the Haber-Bosch process
nitrogen pollution
The nitrogen cycle
The nitrogen cycle The nitrogen cycle is the biogeochemical cycle that describes the transformations of nitrogen and nitrogen- containing compounds in nature. It is a gaseous cycle. The Earth's atmosphere is about 78% nitrogen, making it the largest pool of nitrogen. Nitrogen is essential for many biological processes; and is crucial for any life here on Earth. It is in all amino acids, is incorporated into proteins, and is present in the bases that make up nucleic acids, such as DNA and RNA. Processing, or fixation, is necessary to convert gaseous nitrogen into forms usable by living organisms. Some fixation occurs in lightning strikes, but most fixation is done by free- living or symbiotic bacteria.
Nitrogen fixation Nitrogen-fixing bacteria have the nitrogenase enzyme that combines gaseous nitrogen with hydrogen to produce ammonia, which is then further converted by the bacteria to make its own organic compounds. Some nitrogen fixing bacteria, such as Rhizobium, live in the root nodules of legumes (such as peas or beans). Here they form a mutualistic relationship with the plant, producing ammonia in exchange for carbohydrates. Nutrient-poor soils can be planted with legumes to enrich them with nitrogen. A few other plants can form such symbioses. There are also bacteria species such as Azotobacter that are capable of nitrogen fixation in the soil.
Amplification of nitrogen fixation Nitrogen fixation has been thoroughly studied in recent years, based on the hope that genetic engineering can provide techniques that improve the nitrogen supply of plants. The production of synthetic nitrogen fertiliser is expensive and extraordinarily costly in terms of energy. Bacteria, too, are not able to produce ammonia at low energy costs. The triple bond of nitrogen belongs to the strongest covalent bonds occurring in biologically important molecules. The conversion of 1 mole nitrogen to 2 mole ammonia requires 25 mole ATP, i.e. the fixation of 1 gm. nitrogen costs 10 gm. glucose - under favourable conditions. Azotobacter’s reaction is especially pricey: it needs 100 gm. glucose for the fixation of 1 gm. nitrogen.
Nitrogen fixation The genetic basis of nitrogen fixation is largely known. The preferred test object was and still is Klebsiella pneumoniae, an enterobacterium related to Eschericia coli. In nitrogen fixation, the nitrogenase complex is the key enzyme. The reduction of molecular N2 to NH3, is catalysed by the nitrogenase enzyme system (EC 1.18.6.1). The overall reaction is: N2 + 8 H+ + 8 e- 2 NH3 + H2
Nitrogenase: active site components
Molybdenum nitrogenase (Mo nitrogenase), which is found
in all nitrogen fixing organisms, consists of two components:
component I component II
[nitrogenase molybdenum-iron [nitrogenase iron (Fe) protein,
(MoFe) protein, or dinitrogenase] or dinitrogenase reductase]
This is the site of N2 reduction. Electron transfer protein.Nitrogen fixation: a laboratory model An electrochemical system will convert N2 to NH3 In the laboratory. Bonds between the nitrogen atoms Break in stages with bonds forming between the nitrogen And molybdenum at the same time.
Nitrogen fixation: energetics
Even if the biological process does not involve tearing apart
the nitrogen molecule, but goes along in stages from N2 to
diazene (N2H2), then to hydrazine (N2H4), and finally to NH3,
there is still an energy problem. This pathway results in the
overall release of energy only when the last stage - the
production of ammonia - is reached. To carry the system over
the energy barrier, a lot of energy must be added to the system.
This is why symbiotic relationships between bacteria and plants
are common in nitrogen fixation.
185 kj/mol N2H2 95 kj/mol
N2H4
185 kj/mol
N2
NH3The fate of nitrogen in the soil Other plants get nitrogen from the soil by absorption at their roots in the form of either nitrate ions or ammonium ions. All nitrogen obtained by animals can be traced back to the eating of plants at some stage of the food chain. Due to their very high solubility, nitrates can enter groundwater. Elevated nitrate in groundwater is a concern for drinking water use because nitrate can interfere with blood-oxygen levels in infants and cause methemoglobinemia or blue-baby syndrome. Where groundwater recharges stream flow, nitrate-enriched groundwater can contribute to eutrophication, a process leading to high blue-green algal populations and the death of aquatic life due to excessive demand for oxygen. While not directly toxic to fish life like ammonia, nitrate can have indirect effects on fish if it contributes to this eutrophication.
Nitrogen as a waste product Nitrogen has contributed to severe eutrophication problems in some water bodies. As of 2006, the application of nitrogen fertilizer is being increasingly controlled in Britain and the United States. This is occurring along the same lines as control of phosphorus fertilizer, restriction of which is normally considered essential to the recovery of eutrophied waterbodies. Ammonia is highly toxic to fish life and the water discharge level of ammonia from wastewater treatment plants must often be closely monitored. To prevent loss of fish, nitrification prior to discharge is often desirable. Land application can be an attractive alternative to the mechanical aeration needed for nitrification.
The Haber-Bosch process
In the Haber Process, nitrogen (N2) and hydrogen (H2)
gases are reacted over an iron catalyst (Fe3+) in which
aluminium oxide (Al2O3) and potassium oxide (K2O) are
used as promoters. The reaction is carried out under
conditions of 250 atmospheres (atm), 450-500 °C; resulting
in a yield of 10-20%:
N2(g) + 3H2(g) → 2NH3(g) ∆Ho = -92.4 kJ/mol
(Where ∆Ho is the standard heat of reaction or standard
enthalpy change)
These conditions are chosen due to the high reaction rate
which they foster despite the poor relative amount of
ammonia produced.Synthesis gas preparation
One must obtain hydrogen from methane using hetero-
geneous catalysis for the Haber-Bosch process.
First, the methane is cleaned, mainly to remove sulphur
impurities that would poison the catalysts. This is done by
turning sulphur into hydrogen sulphide:
CH3SH + H2 → CH4 + H2S
and then reacting with zinc oxide to form zinc sulphide:
H2S + ZnO → ZnS + H2O
The clean methane is then reacted with steam over a
catalyst of nickel oxide. This is called steam reforming:
CH4 + H2O → CO + 3H2 (3 moles of hydrogen out)
CO + H2O → CO2 + H2 (1 extra mole of hydrogen out)
Note that 4 moles of hydrogen are produced per mole of
methaneHistory The process was first patented by Fritz Haber. In 1910 Carl Bosch successfully commercialized the process at BASF and secured further patents. Haber and Bosch were later awarded Nobel prizes, in 1918 and 1931 respectively, for their work in overcoming the chemical and engineering problems posed by the use of large-scale high-pressure technology. Ammonia was first manufactured using the Haber process on an industrial scale in Germany during World War I to meet the high demand for ammonium nitrate (for use in explosives) at a time when supply of Chile saltpeter from Chile could not be guaranteed because this industry was then almost 100% in British hands. It has been suggested that without this process, Germany would almost certainly have run out of explosives by 1916, thereby ending the war.
Reaction Rates and Equilibrium There are two opposing considerations in this synthesis: the position of the equilibrium and the rate of reaction. At room temperature, the reaction is slow and the obvious solution is to raise the temperature. This may increase the rate of the reaction but, since the reaction is exothermic, it also has the effect, according to Le Chatelier's Principle, of favouring the reverse reaction and thus reducing equilibrium constant, given by: As the temperature increases, the equilibrium is shifted and hence, the constant drops dramatically according to the van't Hoff equation. Lower temperatures cannot be used since the catalyst itself requires a temperature of at least 400 °C to be efficient.
Reaction Rates and Equilibrium Pressure is the obvious choice to favour the forward reaction because there are 4 moles of reactant for every 2 moles of product, and the pressure used (around 200 atm) alters the equilibrium concentrations to give a profitable yield. Economically, though, pressure is an expensive commodity. Pipes and reaction vessels need to be strengthened, valves more rigorous, and there are safety considerations of working at 200 atm. In addition, running pumps and compressors takes considerable energy. Thus the compromise used gives a single pass yield of around 15%. Another way to increase the yield of the reaction would be to remove the product (i.e. ammonia gas) from the system. In practice, gaseous ammonia is not removed from the reactor itself, since the temperature is too high; but it is removed from the equilibrium mixture of gases leaving the reaction vessel.
Reactive N vs Unreactive N2
• Unreactive N is N2 (78% of earth’s atmosphere)
• Reactive N (Nr) includes all biologically, chemically and
physically active N compounds in the atmosphere and
biosphere of the Earth
• N controls productivity of most natural ecosystems
• N2 is converted to Nr by biological nitrogen fixation (BNF)
• N2 is converted to Nr by humans fossil fuel combustion, the
Haber Bosch process, and cultivation-induced BNF.Reactive N vs Unreactive N2
• Unreactive N is N2 (78% of earth’s atmosphere)
• Reactive N (Nr) includes all biologically, chemically and
physically active N compounds in the atmosphere and
biosphere of the Earth
• N controls productivity of most natural ecosystems
• N2 is converted to Nr by biological nitrogen fixation (BNF)
• N2 is converted to Nr by humans fossil fuel combustion, the
Haber Bosch process, and cultivation-induced BNF.
• Bottom Lines
– Humans create more Nr than do natural terrestrial processes.
– Nr is accumulating in the environment.
– Nr accumulation contributes to most environment issues of
the day.
– Challenge is to reduce anthropogenic Nr creation.Reactive N vs Unreactive N2
• Unreactive N is N2 (78% of earth’s atmosphere)
• Reactive N (Nr) includes all biologically, chemically and
physically active N compounds in the atmosphere and
biosphere of the Earth
• N controls productivity of most natural ecosystems
• N2 is converted to Nr by biological nitrogen fixation (BNF)
• N2 is converted to Nr by humans fossil fuel combustion, the
Haber Bosch process, and cultivation-induced BNF.
• Bottom Lines
– Humans create more Nr than do natural terrestrial processes.
– Nr is accumulating in the environment.
– Nr accumulation contributes to most environment issues of
the day.
– Challenge is to reduce anthropogenic Nr creation.
• But, this is complicated by fact that Nr creation sustains
most of the world’s food needs.
– The real challenge is how can we provide food (and energy)
while also reducing Nr creation rates and arresting the
nitrogen cascade?Impact of Nitrogen Historical perspective – Human discovery; human ingenuity – N cycle in 1860 and 1995 Consequences of being ingenious – Nitrogen is nutritious – Nitrogen cascades How can one atom do all those things? – Impacts on atmosphere – Impacts on grasslands, forests and agroecosystems – Impacts on freshwater, coastal waters and oceans
Human population (millions)
The History of Nitrogen
7,000
6,000
5,000
4,000
3,000
2,000 BNF
N-Discovered N-Nutrient
1,000
0
1750 1800 1850 1900 1950 2000 2050
Humans , millio ns
Year
Galloway JN and Cowling EB. 2002; Galloway et al., 2002aNr Creation by Haber-Bosch
Human population (millions)
7,000 200
NOx emissions (Tg/year)
6,000
150
5,000 N2 + 3H2
--> 2NH3
4,000
100
3,000
2,000 BNF H-B 50
N-Discovered N-Nutrient
1,000 N2 + O2
--> 2NO
0 0
1750 1800 1850 1900 1950 2000 2050
Humans , millio ns Habe r Bo s c h
Le g ume s /Ric e , Tg N NOx e mis s io ns , Tg N
Galloway JN and Cowling EB. 2002; Galloway et al., 2002aThe Global Nitrogen Budget in 1860 and mid-1990s, TgN/yr
5
NOy N2 NHx
6
8 6 9
1860
120
6 7 15 11 8
0.3
27
5 6
NOy N2
mid-1990s
NHx
16 33 23 26 18
21 25 110 100 39
25
N2 + 3H2
48
2NH3
Galloway et al., 2002bAtmosphere
Terrestrial
Ecosystems
Human Activities
The Nitrogen
Cascade
Galloway et al., 2002a
Aquatic EcosystemsAtmosphere
NOx Ozone
Effects
Energy
Production
Terrestrial
Ecosystems
Human Activities
The Nitrogen
Cascade
Galloway et al., 2002a
Aquatic EcosystemsAtmosphere
Air Quality
NOx Ozone
Visibility
Effects
Effects
Energy
Production
Terrestrial
Ecosystems
Human Activities
The Nitrogen
Cascade
Galloway et al., 2002a
Aquatic EcosystemsAtmosphere
Air Quality
NOx Ozone
Visibility
Effects
Effects
Energy
Production
Terrestrial
Ecosystems
Forests &
Grassland
Soil
Human Activities
The Nitrogen
Cascade
Galloway et al., 2002a
Aquatic EcosystemsAtmosphere
Air Quality
NOx Ozone
Visibility
Effects
Effects
Energy
Production
Terrestrial
Ecosystems
Forests &
Grassland
Soil
Human Activities Groundwater
Effects
The Nitrogen Surface water
Effects
Cascade
Galloway et al., 2002a
Aquatic EcosystemsAtmosphere
Air Quality
NOx Ozone
Visibility
Effects
Effects
Energy
Production
Terrestrial
Ecosystems
Forests &
Grassland
Soil
Human Activities Groundwater
Effects
The Nitrogen Surface water
Effects
Coastal
Effects
Cascade
Galloway et al., 2002a
Aquatic EcosystemsAtmosphere
Air Quality
NOx Ozone
Visibility
Effects
Effects
Energy
Production
Terrestrial
Ecosystems
Forests &
Grassland
Soil
Human Activities Groundwater
Effects
The Nitrogen Surface water
Effects
Coastal
Effects
Ocean
Effects
Cascade
Galloway et al., 2002a
Aquatic EcosystemsAtmosphere
Air Quality
NOx Ozone
Visibility
Effects
Effects
Energy
Production
Terrestrial
Ecosystems
Food
NHx Agroecosystem Effects
Production Forests &
Crop Animal Grassland
People Soil Soil
(Food; Fiber) Norg
Human Activities Groundwater
Effects
The Nitrogen Surface water
Effects
Coastal
Effects
Ocean
Effects
Cascade
Galloway et al., 2002a
Aquatic EcosystemsAtmosphere
Air Quality
NOx Ozone
Visibility
Effects
Effects
Energy
Production
Terrestrial
NOx NH3
Ecosystems
Food
NHx Agroecosystem Effects
Production Forests &
Crop Animal Grassland
People Soil Soil
(Food; Fiber) Norg NO3
Human Activities Groundwater
Effects
The Nitrogen Surface water
Effects
Coastal
Effects
Ocean
Effects
Cascade
Galloway et al., 2002a
Aquatic EcosystemsAtmosphere
Air Quality
NOx Ozone
Visibility
Effects
Effects
Energy
Production
Terrestrial
NOx NH3
Ecosystems
Food
NHx Agroecosystem Effects
Production Forests &
Crop Animal Grassland
People Soil Soil
(Food; Fiber) Norg NO3
Human Activities Groundwater
Effects
The Nitrogen Surface water
Effects
Coastal
Effects
Ocean
Effects
Cascade
--Indicates denitrification potential Aquatic EcosystemsAtmosphere Stratospheric
Effects
Air Quality
NOx Ozone
Visibility GH
Effects Effects
Effects
Energy
Production N2O
Terrestrial
NOx NH3
Ecosystems
Food
NHx Agroecosystem Effects
Production Forests &
Crop Animal Grassland
People Soil Soil
(Food; Fiber) Norg NO3
N2O
Human Activities Groundwater
Effects
The Nitrogen Surface water
Effects
Coastal
Effects
Ocean
Effects
Cascade
--Indicates denitrification potential Aquatic EcosystemsNr and Agricultural Ecosystems
• Haber-Bosch has facilitated
agricultural intensification
• 40% of world’s population is
alive because of it
• An additional 3 billion people
by 2050 will be sustained by it
• Most N that enters
agroecosystems is released to
the environment.Nr and the Atmosphere
NOx emissions contribute to
OH, which defines the
oxidizing capacity of the
atmosphere
NOx emissions are responsible
for tens of thousands of excess-
deaths per year in the United
States
O3 and N2O contribute to
atmospheric warming
N2O emissions contribute to
stratospheric O3 depletionNr and Terrestrial Ecosystems
• N is the limiting nutrient in
most temperate and polar
ecosystems
• Nr deposition increases and
then decreases forest and
grassland productivity
• Nr additions probably decrease
biodiversity across the entire
range of depositionNr and Freshwater Ecosystems
• Surface water
acidification
– Tens of thousands of lakes
and streams
– Significant biodiversity
losses
– Negative feedbacks to
forested ecosystemsNr and Coastal Ecosystems
• Riverine and atmospheric
deposition are significant Nr
sources to coastal systems
• Nr inputs into coastal regions
result in eutrophication,
biodiversity losses, emissions
of N2O to the atmosphere.
• Most coastal regions are
impacted.There are significant effects
of Nr accumulation within each
reservoir
These effects are linked temporally
and biogeochemically in the
Nitrogen CascadeNr Riverine Fluxes
1860 (left) and 1990 (right)
TgN/yr
9.1
5 21.8
7.8
4.4
8.3
7.7 8.5
9.7
7.4
2 2.1
-> all regions increase riverine fluxes
-> Asia becomes dominant
Galloway et al, 2002b; Boyer et al., in preparationNitrogen Deposition
Past and Present
mg N/m2/yr
5000
2000
1000
750
500
250
100
50
25
5
1860 1993
Galloway and Cowling, 2002; Galloway et al., 2002bYou can also read
