Iron exists in three basic forms as elemental metallic iron, in ferrous (Fe++) and ferric (Fe +++) states. Ferrous iron usually occurs in water drawn from wells. It is present due to the solubility of ferrous bicarbonate as a result of the action of carbon dioxide on iron deposits in the ground.
Iron remains in this soluble ferrous state as long as the water remains underground, where molecular oxygen is scarce. Carbon dioxide is commonly found together with high iron concentrations, but this is not necessarily the case. When this iron-bearing water is first brought to the surface, it is usually clear and colorless with a distinct iron taste. After aeration or exposure to the air, the water develops a milk-like haze, which soon turns reddish-brown in color.
Chemically what happens is this: upon exposure to the air, molecular oxygen begins to enter the water as carbon dioxide escapes. The oxygen then oxidizes the ferrous ions (Fe++) changing them to ferric ions (Fe+++) At this point, the ferric ions combine with free hydroxyl ions (OH-) to form the insoluble gelatinous compound ferric hydroxide [Fe (OH)3] As the individual molecules join together, characteristic rust color (often called "red water" or "rusty water") appears. And finally, a gelatinous precipitate of ferric hydroxide settles to the bottom of the container. In this way, the soluble ferrous ions convert into the insoluble ferric hydroxide state. Iron flavor noted in water containing ferrous ions markedly decreases as the ferrous iron passes into the ferric form.
Actually iron in natural water supplies may be present in a number of forms including:
- soluble ferrous ions;
- ferric ions, soluble in very acid water;
- ferric hydroxide, insoluble in neutral or alkaline water;
- ferric oxide, which show up as particles of rust from pipes; and
- in combination with organic compounds or iron bacteria.
Iron, as you can see, is present in water either in a ferrous or a ferric state.
Iron is generally found in the ferrous state (colorless and soluble) in groundwater supplies. As iron oxidizes upon exposure to the air, it usually settles out. For this reason, it is rarely found in surface water supplies.
When iron is found in surface supplies, the water may well be extremely acid, or the iron may be combined in various complex molecules, which resist oxidation. In some surface waters, iron may be present in an organic (chelated*) form. Such water usually contains a great deal of colored colloidal turbidity, which does not settle and is difficult to remove by filtration.
Unfortunately, organic iron can be quite troublesome, although significant progress in the treatment of this type has been made.
Iron Bacteria
Iron bacteria frequently thrive in iron-bearing water. As they develop, these bacteria form reddish-brown growths that may clog pipes and reduce flow rates. A decaying mass of these iron bacteria can cause bad tastes and odors in a water supply, and often cause extremely discolored water when the slimy growths break free in slugs at high flow rates. These iron bacteria can grow either in darkness or in light but are most frequently noticed in toilet flush tanks. They require water containing an adequate supply of ferrous ions and free oxygen. While they have been grown in cultures containing no iron, they thrive best in iron-bearing waters. The most common names for the various types of iron and manganese bacteria include: crenothrix**, gallionella, and leptothrix.
As this brief discussion indicates, iron can be in the water in a number of forms, the cause of which can be quite varied. The chemistry of iron removal is not difficult once the cause has been clearly determined. Corrective measures present difficulties in some instances only because it is not always easy to determine the cause of the problem and because the operation of certain types of water conditioning equipment may not be well understood.
To determine the proper corrective steps requires a bit of sleuthing. The iron, as we have seen, maybe in water either in a ferrous or ferric state. Further, it may be the result of corrosion. Again the problem may in a large measure be due to the presence of iron bacteria. Because the problem of iron-bearing water is complex, it is difficult to establish rules for treatment. What must be done depends on the cause and the type of equipment available.
Chelate: To combine into a complex molecule having great stability due to the molecular arrangement.
**Crenothrix: This term is sometimes incorrectly used in referring to all iron or manganese bacteria. Some 18 or more varieties of iron and manganese bacteria have been classified and studied over the years. A recent study, however, indicates that some of the varieties of bacteria are simply different forms of the same bacteria. This study shows that the different forms develop differently under differing environmental conditions. The most widely accepted classifications include a) Gallionella b)Crenothrix c)Leptothrix. One authority in the subject of iron bacteria states that despite a long search for crenothrix, no evidence of a separate bacteria of this type has been discovered. This same authority feels that all iron bacteria are in fact forms of gallionella or sphareotilus which includes what are popularly called crenothrix and leptothrix.