The Troublesome Trio & Water Purity: Part 1
Part 1: Iron
Three undesirable water characteristics-iron, manganese, and hydrogen sulfide–can he called the “Bermuda Triangle” of the water processing business. Singularly or in combination, this trio can present a very complex challenge to the water treatment specialist. The same processing method used for removal of one species can often be applied to all three, but because of the many variations in the nature of these substances, determining corrective treatment calls for a complete and reliable water analysis; thorough equipment know-how; and proper application of the “art” of water conditioning.
Both physical and water chemistry factors determine success or failure in removing these substances to improve the aesthetic quality of water. Important physical factors include (1) the pump and plumbing system, (2) available flow rates, and (3) water temperature. Water chemistry factors include (1) pH values, (2) concentration of the species, and (3) oxygen levels. Geographically, iron, manganese, and hydrogen sulfide often are objectionable characteristics in the same water supply. Along the Eastern Seaboard, the upper Great Lakes region, and the Great Plains states are areas where unacceptable levels of these three species are common in well waters and municipal water supplies. Below, and in the next installament, we’ll look at the background and chemical behavior of the troublesome trio in water sources.
Sometimes referred to as the “fly in the ointment”, iron can be the most bothersome water treatment problem to deal with. Perhaps that is why so many different labels have been given to the various categories of iron in water chemistry. The table here lists four basic forms of iron in water along with other alternate names in each class.
When clear water containing ferrous bicarbonate is exposed to the atmosphere for a period of time, it will adsorb oxygen 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 consdiered acceptable for most uses, however, if the FE level is 0.3 ppm-mg/L or higher, staining can result and thus most industrial and/or commercial applications will require a complete absence of iron 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 dissole 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 dissolvved 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, washing, laundering, and other industrial/commercial applications.
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 (Fe₂O₃). Under favorable conditions on the earth’s surface, the iron is converted to a soluble form and dissolves in water with 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 (CO₂) and low in dissolved oxygen (O₂), which contributes to the conversion of insoluble iron oxide to the soluble form of ferrous bicarbonate [Fe(CO₃)₂]. Ferrous iron is colorless in solution, and the sample is clear when drawn.
|Forms of Iron (Fe) in Water|
|1. Ferrous, Fe²⁺
|2. Ferric, Fe³⁺
Oxidized Iron (Iron Oxide)
Red-water Iron – Rust
|3. Organic Iron
|4. Colloidal Iron
Even at very low levels, iron can produce a favorable climate for the growth of what is called iron bacteria. These microorganisms, such as Crenthrix, 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 drinking water is the goal of your industrial/commercial needs). If the interior of a water closet has a gelationous sludge and the surface reflects an iridescent (rainbow) slick, it is usually a telltale sign of the presence of iron bacteria. Some iron bacteria can even cause sulfur as a byproduct. 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, ir does differ from the other two. Colloidal iron stays in suspension, giving a red-pink, tubid 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 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 alum; allowing it to coagulate, form a floc with the colloids, and partially settle out; then passing the water through a granular medium filter system.