Waterguide - Iron
What is iron?
Like oxygen, silicon and aluminium, iron (symbol Fe, atomic number 26, atomic mass 55.85 g/mol) is one of the most abundant elements in the Earth’s crust (5 %). Iron is known for its brown colour which it takes on when it comes into contact with oxygen. It is a base metal, which is why it has a strong tendency to corrode. In water, iron is often present in combination with manganese.
In what form can iron be found in the Earth's crust?
It is impossible to find iron as an element in nature, as it immediately binds with oxygen. This is why iron is usually found as iron oxide (FeO) or as iron sulphate (FeS) as a reaction with sulphur. There are different iron oxidation levels depending on the reaction. The most common are: Iron1+, iron2+ and iron3+.
Iron can be found as an element in meteorites from space.
How does iron enter water and in what concentration can it be found there?
Iron is present in water as hydrated iron oxide. The soluble form of iron in water always has the oxidation level Fe2+.
Iron gets into water in the following ways:
- Animal metabolic products
- Through humans
- Reduction of trivalent iron compounds
- Oxidation of iron sulphates (e.g.: pyrite FeS2)
Animal excretions contain iron, which makes its way onto land as trace elements through sea birds.
Humans generate approximately 2 billion tons of dust every year. This dust is swept into the seas/oceans by the wind and is carried away from there.
The reduction process is carried out by lowering the pH value to < 5 and oxidation is performed using nitrate or oxygen.
In surface water, you differentiate between the total iron content and iron2+. Iron2+ constitutes 67 % of the total iron content in surface water.
Why does iron have to be removed from water?
The iron has to be removed in order to avoid the undesirable reaction between iron and oxygen in the water. Iron oxide would corrode the pipes and cause the water to turn a yellowy brown colour.
Even if iron is important for the human body, too much iron intake can cause damage.