Iron is one of the most common elements found in nature, accounting for at least five percent of the earth's crust. It is understandable, therefore, that just about all water supplies, surface or ground, contain some measurable amount of iron. In nature, iron usually occurs as an insoluble oxide, ferric oxide (Fe 2 O 3 ). Under favorable conditions on the earth's surface, the iron is converted to a soluble form and dissolves in water will which it comes in contact. For this reason, iron can be found in almost every natural
water source, but particularly in well waters. Well waters are usually high in carbon dioxide (CO 2 ) and low in dissolved oxygen (0 2 ), which contributes to the conversion of insoluble iron oxide to the soluble form of ferrous bicarbonate [Fe(HCO 3 ) 2 ]. Ferrous iron is colorless in solution, and the sample is clear when drawn.
When clear water containing ferrous bicarbonate is exposed to the atmosphere for a period of time, it will adsorb oxygen from the air and react to form insoluble iron, most often as ferric oxide (commonly referred to as rust). Because iron oxidizes readily and precipitates as an insoluble substance, it will cause red-brown staining of laundry and porcelain fixtures. In addition, iron will impart a metallic taste to drinking water and beverages.
Water that contains iron at 0.1 ppm-mg/L or lower may be considered acceptable for domestic uses. However, if the total Fe level is 0.3 ppm-mg/L or higher, staining can result on kitchen and bathroom fixtures, dishes, cookware, laundry, and masonry surfaces. Several industrial applications call for an almost total absence of iron content in water used for process work.
In addition to natural sources of iron in water, this metallic substance can result from corrosion of exposed steel or iron. Corrosion can also dissolve other heavy metals. The more corrosive the water, the more iron and other heavy metals will dissolve from metal surfaces with which the water supply comes in contact. Where oxygen is present in a low pH value water, corrosion of iron, steel, cadmium, copper, lead, and zinc will be accelerated. Also, higher water temperatures, such as in hot water heaters and boiler heating water, add another dimension for corrosion. As a result of corrosive conditions, iron can be present most often as rust particles, and to some extent as dissolved iron. The iron content of a water sample should always be analyzed and reported as the total iron ____ the combination of all forms present.
All forms of iron create an unacceptable characteristic in water for drinking, dishwashing, bathing, and laundering- Another serious problem of water corrosion occurs where lead-soldered piping and brass faucets are installed. Evidence of lead in drinking water has been reported at kitchen faucets. 2 Where corrosive conditions of this nature do occur, a safe practice is to let the cold water run for a few minutes each morning before drawing water for consumption/ cooking. This will reduce the contact time during which lead may dissolve.
Another step, of course, is to treat the water at the kitchen tap for removal of heavy metals with a cartridge-type filter or suitable whole-house treatment.
As mentioned in Chapter Five, a cation exchange water softener regenerated at 10 pounds (4.5 kg) or more of NaCI will reduce and remove dissolved iron, plus soften the water. Likewise, other dissolved cations, including cadmium, copper, lead, manganese, and zinc, can be reduced to acceptable drinking water standards by cation water softening.
Even at very low levels, iron can produce a favorable climate for the growth of what is called iron bacteria, These microorganisms, such as Crenothrix, Leptothrix, and Gallionella utilize energy obtained from the oxidation of ferrous to ferric iron to "fix" dissolved carbon dioxide into organic molecules necessary for their existence. These organisms need only a continuous supply of ferrous iron and air or oxygen to metabolize ferric iron into their cell structures, and to deposit gelatinous ferric hydroxide iron compounds. The growth of these organisms will result in the formation of a jelly-like mass, cause pipe encrustation, and can produce foul-tasting drinking water. If the interior of a water closet has a gelatinous sludge and the surface reflects an iridescent (rainbow) slick, it is usually a telltale sign of the presence of iron bacteria. Because of its organic nature, iron bacteria by whatever name, is one of the most difficult forms of iron to remove and control.
While colloidal iron can be observed visually in a water sample, as can ferric iron and, to some degree, organic iron, it does differ from the other two. Colloidal iron stays in suspension, giving a red-pink, turbid cast to the water sample. It is very highly dispersed and has a very low specific gravity almost equal to that of water. The specks of iron appear to be floating, and sometimes are attached to silica. The colloidal particles can have a slight negative charge. It may take a water sample containing colloidal iron 48 hours for the iron to drop out or begin to settle at the bottom of the container. In municipal/industrial water treatment plants, colloidal iron is removed by adding a coagulant such as ----- allowing it to coagulate, form a floc with the colloids, and partially settle out; then passing the water through a granular medium filter system.