Iron is one of the most troublesome minerals encountered in domestic and commercial water supplies. The various forms of iron can leave stains and deposits that range in color from yellow to brown, but commonly has a "rusty" color. As organic residue it may be gray to black, slimy to rock solid. Iron can also impart undesirable odors and tastes to water.
Iron occurs in many forms in natural water supplies. The most common forms are described below.
1. DISSOLVED IRON: Ferrous bicarbonate [Fe(HCO3)2] is found only in oxygen free water. Dissolved iron is measured in parts per million (ppm). One ppm is equivalent to approximately 1/4 ounce of iron in 1,900 gallons of water. The recommended limit of iron in drinking water is 0.3 ppm and will begin staining at 0.5 ppm. The water containing it is clear and colorless when drawn. Upon contact with the air, oxygen is absorbed and reacts with the dissolved iron to form insoluble ferric hydroxide (commonly known as rust). This clouds the water and colors it in shades of yellow to red-brown.
This reaction produces carbon dioxide as follows
2Fe(HCO3)2 + 1/2O2 + H2O = Fe(OH)3 + 4CO2
The dissolution of the above carbon dioxide in water also forms carbonic acid. The presence of carbonic acid lowers the pH, and in low alkalinity water (2-3 gpg total solids), this can cause some corrosion problems in the system. Filtration through a neutralization media is then required to remediate this. The neutralization media commonly used is composed of calcium carbonate such as limestone, or marble.
Small quantities (less then 0.5 ppm) of ferrous iron, in the absence of dissolved oxygen, may be removed by ion exchange water conditioning. If this method is used, a chemical resin cleaner is required to be added to the brine. This will help remove the exchanged iron and prevent any oxidized iron from fouling the resin bed.
Ferrous iron, in higher concentrations, is most commonly removed by a two step oxidation then filtration process. Chlorine, oxygen, and potassium permanganate are the most common oxidizers used. The filter media is most commonly manganese greensand.
2. INSOLUBLE IRON: Insoluble ferric hydroxide in water produces a red water condition. It is found in some natural waters as a suspension of fine, denser-than-water particles. More often ferric hydroxide, in water, is formed when exposed to oxidizing conditions, like percolating from a spring, or being exposed to air at a water tap.
Another source of insoluble iron is from the action of bacteria. Bacteria can produce ferric hydroxide or iron carbonate as a waste product. Chlorine fed into the well or before the pressure tank will control this type of bacterial nuisance.
Manganese and hydrogen sulfide compounds are also found in ground water. Due to their chemical similarities to the above discussed iron compounds, they can be treated and removed from the water using the same methods described above.
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