Nitrate fertilisers, such as ammonium nitrate and potassium nitrate, are very important in agriculture because they add the much needed nitrogen to the soil. Plants require nitrogen for growth and for protein synthesis. In order to be absorbed by the plants, the fertiliser has be soluble in water. However, the solubility of the fertilisers creates a lot of environmental problems.
The fact that nitrate fertilisers are soluble, means that they can be washed, or leached, out of the soil by rain into groundwater and eventually end up in rivers and lakes. In the rivers and lakes, the nitrates promote the growth of the water plants and algae leading to eutrophication.
Eutrophication is the process of excess growth of aquatic plants leading to the destruction of life in the water.
The chain reaction of disaster continue as follows:
- Algae is a simple water plant that grows on the surface of the water. Nitrates in the water causes a bloom of algae all over the surface of the water, blocking out sunlight for other water plants.
- When the water plants and algae die, bacteria and other decomposers in the water feed on them and multiply rapidly due to the abundance of food.
- As the bacteria multiply, they respire, using up the dissolved oxygen in the water.
- Since fish extract dissolved oxygen from water, lack of oxygen in the water causes the fish to die.
- The balance of the ecosystem is affected.
The problem doesn’t end there. Nitrates eventually get into drinking water supplies and they are difficult to remove during water treatment. The nitrates in the drinking water affect the ability of babies under 6 months to carry oxygen in the bloodstream, leading to "blue baby" syndrome, as well as stomach cancer.
Nitrogen oxides in the atmosphere
Nitrogen is the most abundant gas in the atmosphere, due to the fact it is very unreactive. Its inertness is due to the large amounts of energy needed to break the N2 triple bond. However,
- during a thunderstorm, lightning can provide the needed activation energy and trigger the reaction between atmospheric nitrogen and atmospheric oxygen to produce nitrogen(II) oxide (NO) and nitrogen(IV) oxide (NO2).
- inside the cylinders of a car engine, a mixture of air (nitrogen and oxygen) and fuel is compressed and ignited by a spark. The spark and the high pressure creates the necessary conditions required for nitrogen to form its oxides. These oxides are released into the atmosphere in the car’s exhaust fumes.
Oxides of nitrogen cause acid rain and photochemical smog. These oxides contribute to respiratory problems. They also create a chain reaction of disaster which continues as follows:
- nitrogen oxides oxidises another pollutant, sulphur dioxide gas, SO2, in the upper atmosphere to form sulphur trioxide, SO3.
- SO2(g) + NO2(g) → SO3(g) + NO(g)
- the sulphur trioxide gas, SO3, then dissolve in rain water forming sulphuric acid. The sulphuric acid then falls as acid rain.
- SO3(g) + H2O(l) → H2SO4(aq)
- then NO then reacts with O2 forming more NO2 that can go on to oxidise more SO2.
- 2NO(g) + O2(g) → NO2(g)
However, modern car exhaust systems are fitted with catalytic
converters to help reduce the emission of nitrogen oxides, by converting the nitrogen oxides to harmless nitrogen gas.
- 2CO(g) + 2NO(g) → 2CO2(g) + N2(g)