Iron and Manganese as a Water Contaminant
WETS has designed numerous systems for removal of iron and manganese. Iron and manganese are common elements found in the earth’s crust. Water carries these minerals as it passes through soil and rock. Since well water is typically not exposed to oxygen, the dissolved iron and manganese, on exposure to air, oxidize to form insoluble reddish-brown particles. The presence of only a very small amount of iron and manganese can cause difficulty when the water is used for process or industrial use. Many waters that contain iron also contain manganese; however, iron is more common.
Iron in natural water supplies may be present in one or more of the following forms:
- Ferrous Bicarbonate Fe(HCO3)2
- Ferrous Carbonate FeCO3
- Ferric Hydroxide Fe(OH)3
- Ferrous Hydroxide Fe(OH)2
- Ferrous Sulfate FeSO4
- Organic Iron
The most common form in which iron is present is ferrous bicarbonate. Ferrous bicarbonate is a soluble colorless salt. Its solubility will increase with an increase in the free carbon dioxide content of the water. In cold water saturated with carbon dioxide, the solubility of ferrous bicarbonate exceeds 150 ppm.
Iron is almost invariably present as soluble, colorless ferrous bicarbonate. Such waters are usually perfectly clear and colorless when first drawn, but on exposure to the atmosphere, they slowly cloud and finally deposit a yellowish to brownish precipitate of ferric hydroxide.
Removal of soluble ferrous bicarbonate from water is achieved by oxidizing and precipitating the iron as ferric hydroxide.
2 Fe (HCO3)2 + ½ O2 +H2O 2Fe(OH)3 + 4 CO2
. Ferrous Ferric
. Bicarbonate Hydroxide
Aerators have the advantage of reducing the free carbon dioxide content of the water, which simultaneously raises its pH.
Instead of using dissolved oxygen for oxidizing the ferrous iron content of a water, chlorination will additionally help to oxidize ferrous bicarbonate to ferric hydroxide.
2 Fe (HCO3)2 + Cl2 + Ca (HCO3)2 2Fe(OH)3 + 4 CO2 + Ca (HCO3)2
. Ferrous Mg Ferric Mg
. Bicarbonate Na2 Hydroxide Na2
While 1 ppm of oxygen will oxidize 7 ppm of ferrous iron, it is evident from the above reaction that 1 ppm of chlorine will oxidize only about 1.6 ppm of ferrous iron. Therefore, an aerator is the best method to oxidize ferrous bicarbonate.