“Iron Water” (including Manganese). Secondary MCL: 0.3 mg/L Fe + Mn
Iron and manganese may be in the water as it is pumped up from wells, or iron pipes and other equipment may get oxidized (corroded) to form rust. Iron is almost always in the ferrous form, the Fe+2 ion, when fresh from the well or corrosion, but it is easily oxidized further, to the ferric form, Fe+3. Ferrous iron is the on producing metallic taste. Ferric iron in water attracts hydroxide ions so strongly that it will steal them from water if the pH is above 5.0. The reaction produces iron floc, which is a gooey, rust-colored mass.
Fe+3 + 3 H2O <=> Fe(OH)3 (solid) + 3 H+
KEq = 10+33
The exceptionally large equilibrium constant indicates that the reaction will go essentially to completion, and hardly even a single atom of Fe+ in solution will escape precipitation. This is the usual cause of consumer complaints of yellow stains on porcelain and laundry. The usual remedy is either oxidation to Fe+3 followed by filtration to remove the iron floc, or removal by ion exchange water softening if the concentration is not too high. Oxidation is usually done by feeding liquid chlorine bleach with a chemical feed pump, followed by a contact tank and filtration system to filter out the floc. Chlorination is preferred because it also disinfects, but it can also be done with oxidizing media such as manganese greensand and granular brass. Actual rust is a combination of the oxides of both ferrous and ferric iron: FeO/Fe2O3, sometimes written Fe3O4.
Iron in water from wells with high organic content is sometimes combined with huge tannin and lignin molecules from rotting vegetation, called “color bodies,” to form an even more deeply-colored product called heme iron, which has an atom of ferrous iron at remove (Heme refers to the blood pigment, which has an atom of ferrous iron at the center of an organic complex. But the organic part of heme and hemoglobin is nothing like tannins and lignins.) Some tannin molecules are large enough to be called colloidal, and it is often possible to remove most heme iron by fine-filtration in the sub-micron range. But it the molecular weight is too small, it must be removed either by ultrafiltration or nano-filtration, or by chemical coagulation followed by standard filtration.
Manganese usually occurs in combination with iron in well waters, and it makes the stains on laundry and porcelain even darker. When Mn+2 is oxidized by dissolved oxygen or chlorine, the result is manganese dioxide, MnO2, which is dark brown in color. It is removed by the same methods used to remove iron.