Agua

IRON REMOVAL BY MICHAEL C. KELLER SYBRON CHEMICALS INC.

Iron is commonly found in rocks and soil throughout the United States. Under the proper conditions, iron will leach into the water supply from the rock and soil formations. If iron is present in the water where concentrations exceed 0.3 ppm, staining can occur. This staining can affect plumbing fixtures, dish ware, clothes, and willproduce a yellow to reddish appearance in water. These levels may also impart taste and/or odor. The EPA has established a secondary drinking water regulation of 0.3 ppm of water. This is not federally enforceable, since it is not considered a health risk. It may, however, be enforced at the state or county level. The following types of iron can be found in potable water supplies: 1. Sequestering iron2. Iron bound in organics - “Heme Iron” 3. Iron Bacteria 4. Ferric Hydroxide or Red Water Iron 5. Ferrous Bicarbonate or Clear Water Iron As one can see, iron can be found in many different forms. This creates a problem for the removal of iron from water, since treatment methods differ for each type of iron. Municipalities will feed sequestering agents, like sodium tripolyphosphate,hexametaphosphate, or sodium silicate, into the water to tie up metals like iron and keep them in solution. These sequestering agents will inhibit the reaction that would normally occur in untreated water. In this case, iron is prevented from precipitated and causing a characteristic red stain. The problem with these types of agents is that they can be broken down. As water treated with a sequestering agentpasses through a hot water heater, the polyphosphate or silicate bond can be broken. When this occurs staining can become a problem. Sequestering agents will also inhibit the exchange of iron onto softening resins. To test for polyphosphate in water, one must test for total phosphate and orthophosphate. The difference between these two values will be the polyphosphate. A minimum of 4 ppmpolyphosphate is fed into the water to treat 2 ppm of iron or less. For every 1 ppm of iron above 2 ppm, an additional 2 ppm of polyphosphate must be added. Unfortunately, many times the polyphosphate will be underfed, so only a portion of the iron will be sequestered; the balance will be one of the other types of iron. Laboratory testing has indicated that a strong base anion in the chloride form will removethe polyphosphate from the water up to approximately 90%. The removal of sodium silicate from the water is a little more difficult and requires the use of a strong base anion in the OH form (regenerating with NaOH {caustic}). No testing has been performed to verify this, but silicates

can only be removed by an OH form strong base anion. Further field testing is required to verify both ofthese types of treatments and their limitations. Iron can also be associated with organics such as tannins, Figure 7. This is also known as organically bound iron or “Heme Iron.” Tannins are very large molecular weight organics that are a breakdown product of dead vegetation. Most common in surface water and shallow wells, tannins can cause water to have a yellow to brown color. These molecules havethe ability to hold on to iron by a mechanism called chelation (claw). This type of iron will not be removed by softening resin. To treat for this type of iron, one must treat for the organic. A strong base macroporous anion has been shown to pick up theses organics. The tannins are picked up by both a ion exchange process and an absorption process. The regeneration must be performed frequentlywith salt to reduce the potential for organic fouling. unfortunately, each geographic region has its characteristic type of tannin, and removal by ion exchange may not be effective. Oxidation is another process by which tannins can be removed. Sodium Hypochlorite, for example, will oxidize both the tannin as well as the iron. Following the oxidant addition there must be sufficient retention time...