Chemistry of Iron removal from drinking water.
Iron Removal by physical-chemical ways
Iron is one of the most abundant metals of the Earth's crust. It occurs naturally in water in soluble form as the ferrous iron (bivalent iron in dissolved form Fe2+ or Fe(OH)+) or complexed form like the ferric iron (trivalent iron: Fe3+ or precipitated as Fe(OH)3). The occurrence of iron in water can also have an industrial origin ; mining, iron and steel industry, metals corrosion, etc.
In general, iron does not present a danger to human health or the environment, but it brings unpleasantness of an aesthetic and organoleptic nature. Indeed, iron gives a rust color to the water, which can stain linen, sanitary facilities or even food industry products. Iron also gives a metallic taste to water, making it unpleasant for consumption. It can also be at the origin of corrosion in drains sewers, due to the development of microorganisms, the ferrobacteries.
In aerated water, the redox potential of the water is such as it allows an oxidation of the ferrous iron in ferric iron which precipitates then in iron hydroxide, Fe(OH)3, thus allowing a natural removal of dissolved iron.
4 Fe2+ 3 O2 --> 2 Fe2O3
Fe2O3 + 3 H2O --> 2Fe(OH)3
The form of iron in water depends on the water pH and redox potential, as shown in the Pourbaix diagram of Iron below. Usually groundwater has a low oxygen content, thus a low redox potential and low pH (5.5- 6.5)
However ground waters are naturally anaerobic: so iron remains in solution and therefore it is important to remove it for a water use.
The elimination of the ferrous iron, by physical-chemical way, is obtained by raising the water redox potential by oxidation thanks to oxygen of the air and this by simple ventilation. In the case of acid water, the treatment could be supplemented by a correction of the pH. Thus, the ferrous iron is oxidized in ferric iron, which precipitates in iron hydroxide, Fe(OH)3. The precipitate is then separated from water by filtration on sand or decantation. The stage of precipitation by chemical oxidation can also be carried out with the stronger oxidants such as the chlorine dioxide (ClO2), ozone (O3) or the potassium permanganate (KMnO4).
This elimination can be carried out by cascade or spraying open-air systems (for an acceptable maximum content of Fe2+ of 7mg.L-1) known as gravitating systems. Those systems require a significant place on the ground, but, in addition to an easy and a cheap exploitation cost, they also make possible aggressive CO2 and hydrogen sulfide (H2S) removal. There are also pressure systems, which in addition to their compactness, make possible to treat water whose Fe2+ concentrations between 7 and 10mg.L-1.
Iron is often found in water in complexed forms. In order to be eliminated, iron complexed requests a coagulation stage, which comes in between oxidation and filtration.
Remark : Thanks to microorganisms, it is possible to remove iron from water by biological way. Indeed, there are many bacteria, whose metabolism and thus their survival, are related to the oxidation of iron. However this biological removal requires conditions specific for the pH, the temperature, the redox potential, etc