Iron (Fe) and water
Iron and water: reaction mechanisms, environmental impact and health effects
|Seawater contains approximately 1-3 ppb of iron. The amount varies strongly, and is different in the Atlantic and the Pacific Ocean. Rivers contain approximately 0.5-1 ppm of iron, and groundwater contains 100 ppm. Drinking water may not contain more than 200 ppb of iron. |
Most algae contain between 20 and 200 ppm of iron, and some brown algae may accumulate up to 4000 ppm. The bio concentration factor of algae in seawater is approximately 104 - 105. Sea fish contain approximately 10-90 ppm and oyster tissue contains approximately 195 ppm of iron (all are dry mass).
Dissolved iron is mainly present as Fe(OH)2+ (aq) under acidic and neutral, oxygen-rich conditions. Under oxygen-poor conditions it mainly occurs as binary iron. Iron is part of many organic and inorganic chelation complexes that are generally water soluble.
Iron does not clearly alter in pure water or in dry air, but when both water and oxygen are present (moist air), iron corrodes. Its silvery colour changes to a reddish-brown, because hydrated oxides are formed. Dissolved electrolytes accelerate the reaction mechanism, which is as follows:
4 Fe + 3 O2 + 6 H2O -> 4 Fe3+ + 12 OH- -> 4 Fe(OH)3 or 4 FeO(OH) + 4 H2O
Elementary iron dissolves in water under normal conditions. Many iron compounds share this characteristic. Naturally occurring iron oxide, iron hydroxide, iron carbide and iron penta carbonyl are water insoluble. The water solubility of some iron compounds increases at lower pH values.
The main naturally occurring iron minerals are magnetite, hematite, goethite and siderite. Weathering processes release the element into waters. Both mineral water and drinking water contain iron carbonate. In deep sea areas the water often contains iron fragments the size of a fist, manganese and small amounts of lime, silicon dioxide and organic compounds.
Iron is a dietary requirement for most organisms, and plays an important role in natural processes in binary and tertiary form. Oxidized tertiary iron cannot be applied by organisms freely, except at very low pH values. Still, iron usually occurs in this generally water insoluble form.
The total amount of iron in the human body is approximately 4 g, of which 70% is present in red blood colouring agents. Iron is a dietary requirement for humans, just as it is for many other organisms. Men require approximately 7 mg iron on a daily basis, whereas women require 11 mg. The difference is determined by menstrual cycles. When people feed normally these amounts can be obtained rapidly. The body absorbs approximately 25% of all iron present in food. When someone is iron deficit feed iron intake may be increased by means of vitamin C tablets, because this vitamin reduces tertiary iron to binary iron. Phosphates and phytates decrease the amount of binary iron.
Iron removal from water is mostly carried out in drinking water preparation, because mineral water contains high amounts of iron ions. These influence water colour, odour and turbidity.