• Iron is a chemical element with symbol Fe (from Latin: ferrum) and atomic number 26.
It
is a metal in the first transition series. It is by mass the most common element on Earth,
forming much of Earth's outer and inner core. It is the fourth most common element in
the Earth's crust. Its abundance in rocky planets like Earth is due to its abundant
production by fusion in high-mass stars, where the production of nickel-56 (which
decays to the most common isotope of iron) is the last nuclear fusion reaction that
is exothermic.
• Huge amounts of iron are used to make steel, an alloy of iron and carbon. Steel typically
contains between 0.3% and 1.5% carbon, depending on the desired characteristics. The
addition of other elements can give steel other useful properties. Small amounts
of chromium improves durability and prevents rust (stainless steel); nickel increases
durability and resistance to heat and acids; manganese increases strength and
resistance to wear; molybdenum increases strength and resistance to heat; tungsten
retains hardness at high temperatures; and vanadium increases strength and
springiness. Steel is used to make paper clips, skyscrapers and everything in between.
• In addition to helping build the world around us, iron helps keep plants and animals
alive. Iron plays a role in the creation of chlorophyll in plants and is an essential part of
hemoglobin, the substance that carries oxygen within red blood cells. Iron sulfate
(FeSO4) is used to treat the blood disease anemia.
• Manganese is a chemical element with symbol Mn and atomic number 25. It is not
found as a free element in nature; it is often found in combination with iron, and
in many minerals. Manganese is a metal with important industrial metal alloy uses,
particularly in stainless steels. Today, most manganese is still obtained from
pyrolusite, although it is usually burned in a furnace with powdered aluminum or
is treated with sulfuric acid (H2SO4) to form manganese sulfate (MnSO4), which is
then electrolyzed.
• Nearly 90% of all of the manganese produced each year is used in the production
of steel. Manganese is added to molten steel to remove oxygen and sulfur and is
alloyed with steel to make it easier to form and work with and to increase steel's
strength and resistance to impact. Railroad tracks, for example, are made with
steel that contains as much as 1.2% manganese. Manganese is also used to give
glass an amethyst color and is responsible for the color of amethyst gemstones.
• Manganese dioxide (MnO2), the most common compound of manganese, makes
up about 0.14% of the Earth's crust. It is used in dry cell batteries to prevent the
formation of hydrogen, to remove the green color in glass that is caused by the
presence of iron contaminants, and as a drying agent in black paints.
• Iron and Manganese are concentrated in water by contact with rocks
and minerals, and occasionally fabricated materials like iron and
steel pipes. Groundwater supplies may require treatment for high
levels of iron and manganese; generally few surface water supplies
have high enough levels of either to cause problems. Occasionally
discharge of acid industrial wastes or mine drainage may increase
iron or manganese to problem levels in surface water.
• Both iron and manganese are readily apparent in drinking water
supplies. Both impart a strong metallic taste to the water and both
cause staining. Water coming from wells and springs with high iron
and/or manganese may appear colorless initially but orange-brown
(iron) or black (manganese) stains or particles quickly appear as the
water is exposed to oxygen.
• Neither iron nor manganese in water present a health hazard.
However, their presence in water may cause taste, staining and
accumulation problems.
• Because iron and manganese are chemically similar, they cause
similar problems. Iron will cause reddish-brown staining of
laundry, porcelain, dishes, utensils and even glassware.
Manganese acts in a similar way but causes a brownish-black
stain. Soaps and detergents do not remove these stains, and the
use of chlorine bleach and alkaline builders (such as sodium
carbonate) can actually intensify the stains.
• Iron and manganese deposits will build up in pipelines, pressure
tanks, water heaters and water softeners. This reduces the
available quantity and pressure of the water supply. Iron and
manganese accumulations become an economic problem when
water supply or softening equipment must be replaced. Also,
pumping water through constricted pipes or heating water with
heating rods coated with iron or manganese minerals increase
energy costs.
• Iron and manganese exist in many different chemical forms. The
presence of a given form of iron or manganese in geologic materials or
water depends on many different environmental factors. We can often
anticipate iron and manganese problems in water by observing a few
general principles that affect water chemistry.
• An important principle to remember about chemical reactions is that, if
allowed enough time, they will reach an equilibrium with the
surrounding environment. When the conditions of that environment are
changed, such as pumping water from an underground aquifer, the
chemical equilibrium is upset. This will lead to either solution or
precipitation of certain elements such as iron and manganese.
• A general rule of thumb is that oxygenated water will have only low
levels of iron and manganese. The reason is that both iron and
manganese react with oxygen to form compounds that do not stay
dissolved in water. Surface water and shallow groundwater usually have
enough dissolved oxygen to maintain iron and manganese in an
undissolved state. In surface water, iron and manganese are most likely
to be trapped within suspended organic matter particles.
The need to test for iron and manganese in water is not as
critical as it is for other types of contaminants that can cause
health problems. Iron and manganese are not a problem in
household water until they become detectable by the senses.
Consequently, elaborate laboratory analyses are not required to
determine if iron or manganese are a problem. Laboratory
analyses for iron and manganese are needed to quantify the
problem.
Exposure of the sample to air will cause precipitation of iron and
manganese. To get an estimate of the amount of iron and
manganese originally dissolved in the well water, precipitation
must be prevented or the precipitated material must be
redissolved. Before sampling for iron and manganese, a certified
laboratory should be consulted. They will recommend a sampling
procedure that will provide an accurate estimate of dissolved
iron and manganese in the source water.
• Polyphosphates react with dissolved iron and manganese by
trapping them in a complex molecule that is soluble in water (Figure
2). As a result the iron and manganese are not available to react with
oxygen and precipitate. Polyphosphates can be fed into the water
system with controlled injection equipment. Polyphosphates are not
stable at high temperatures. If water is treated prior to heating in a
water heater, the polyphosphates will release iron and manganese in
the heater as they break down. The released iron and manganese
will then react with oxygen and precipitate.
• Polyphosphate treatment is a relatively cheap way to treat water for
low levels of iron and manganese. Depending on the type of
polyphosphate used, water with 1 to 3 ppm of iron can be
adequately treated.
Soluble iron and manganese (iron and manganese
dissolved in water) can be exchanged for sodium on
an exchange resin or zeolite. This process of iron and
manganese removal is the very same ion exchange
process that removes hardness or calcium and
magnesium. Iron and manganese are removed during
normal operation of the water softener. They are later
removed from the exchange medium along with
calcium and magnesium during regeneration and
backwashing. Some water softeners are capable of
adequately treating water having iron up to 10 ppm.
However, others are limited to treating water with
iron no greater than 1 ppm. If iron and manganese
removal is desired in addition to hardness, the
manufacturer's recommendations should be checked.
• One of the disadvantages of depending on ion
exchange for iron and manganese removal is
precipitation by oxygen. Some of the precipitate
becomes tightly bound to the exchange resin and
over time reduces the exchange capacity by
plugging pores and blocking exchange sites. If
iron bacteria are present, the problem is even
worse. Also, if suspended particles of insoluble
forms of iron or manganese are present in the
water prior to softening, they will be filtered out
on the resin and cause plugging. Suspended iron
and manganese should be filtered out before
water enters the softener.
• A clogged water softener can be cleaned by acid
regeneration if the unit is made to withstand acid
corrosion. The manufacturer should be consulted
before this is attempted. Problem iron bacteria
can be eliminated by chlorinating and filtering
the water at some point before it reaches the
softener. As long as levels of iron and
manganese in the water do not exceed the
manufacturer's recommendations, iron and
manganese clogging should not be a significant
problem. When iron and manganese levels are
higher than recommended by the manufacturer,
iron and manganese removal will be necessary
prior to softening.
• One of the first types of filters to be used to treat
water was the "greensand" filter. The active
material in "greensand" is glauconite. Glauconite
is a green clay mineral that contains iron and has
ion exchange properties. Glauconite often occurs
mixed with other material as small pellets, thus
the name "greensand." The glauconite is mined,
washed, screened and treated with various
chemicals to produce a durable greenish-black
product that has properties that allow it to
adsorb (collect in a condensed form on a surface)
soluble iron and manganese.
• As water is passed through the filter, soluble iron and
manganese are pulled from solution and later react to
form insoluble iron and manganese. Insoluble iron and
manganese will build up in the greensand filter and must
be removed by backwashing. Backwashing should be done
regularly, twice a week, or as recommended by the
manufacturer.
• Eventually the greensand must also be regenerated by
washing with a permanganate solution. Regeneration will
leave the greensand grains coated once again with a
manganese material that adsorbs soluble iron and
manganese. Frequency of regeneration will depend on the
level of iron, manganese and oxygen in the water and the
size of the filter. The manufacturer's recommendations
should be followed.
Most greensand filters are rated as effective for treating
water with iron concentrations up to 10 ppm. Because
some greensand filters are not rated this high, the
manufacturer's recommendations should always be
checked. The acidity or pH of the water will influence the
ability of the filter to remove both iron and manganese. If
the pH of the water is lower than 6.8, the greensand will
probably not adequately filter out the iron and manganese.
The pH can be raised above 7.0 by running the water
through a calcite filter.
Regular backwashing is essential for effective filter
performance and requires flow rates that are often three to
four times the normal household usage rate. A backwash
rate of about eight ppm/square foot of filter bed is
recommended. If the household system cannot support
the needed flow rate for adequate backwashing, poor filter
performance and failure are likely.
• Chemical oxidation followed by filtration is
the accepted method of iron and manganese
removal when concentrations are greater than
10 ppm. A number of strong oxidants have
been used in this procedure; however,
chlorine is generally used in household
systems.
• A chlorine solution is injected with a chemical feed
pump ahead of a sand filter. Soluble iron and
manganese begin to precipitate almost immediately
after contact with the chlorine solution. However,
approximately 20 minutes of contact time is needed
for the precipitate to form particles that can be
filtered. Often the standard 42 gallon pressure tank
used on many household systems will provide the
needed contact time if water is forced through the
tank. A simple T-connection from the pipeline to the
pressure tank will not work, since much of the water
bypasses the tank. Additional contact time can be
provided by connecting another tank in series or
using a plastic pipe coil.
• This type of system will remove both soluble and suspended particles of
insoluble iron and manganese from the source water. Backwashing the
sand filter to remove precipitated iron and manganese is an important
part of continued filtration. As with the greensand filter, the system flow
rate should be checked to make sure it can provide the needed rates for
backwashing. An additional advantage of using the chlorination system
is its bactericidal effect. Iron and manganese bacteria along with other
bacteria, are destroyed. Potential clogging problems in the sand filter
are eliminated. Chlorination does produce trihalomethanes (THM) when
organic matter is present in the water. THMs are considered to be
carcinogenic (maximum contaminant level permissible in public water
systems is 0.1 parts per million) and if necessary can be filtered out with
an activated charcoal filter.
• The optimum rate of oxidation of iron and manganese by chlorination is
at a pH of about 8.0 and 8.5, respectively. Soda ash injected with the
chlorine will increase the pH to optimum levels. Adjusting the pH to
alkaline levels also reduces the corrosiveness of the water in pipes and
plumbing.
• The Water Quality Association (WQA) has set
voluntary performance standards for
oxidative filtration methods. They specify that
an oxidizing filter shall reduce:
10.0 ppm plus or minus 1.0 ppm soluble iron to not more
than 0.2 ppm
2.0 ppm plus or minus 0.2 ppm soluble manganese to not
more than 0.5 ppm
WQA also recognizes that the following water
treatment methods can be used to meet the
EPA's Secondary Drinking Water Standards for
both soluble iron and manganese:
◦ 1) oxidizing filters;
◦ 2) cation exchange; and
◦ 3) chlorination - precipitation/filtration.
Polyphosphate treatment does not meet the
drinking water standards, because it ties up
iron and manganese but does not remove it.
Reverse osmosis, distillation, and pressure
aeration/filtration are also recognized by
WQA as water treatment methods that can be
used to meet the iron and manganese
drinking water standards.
• The consumer should be cautioned to note that
different water treatment systems vary
considerably in their ability to treat a specific
contaminant. Water treatment equipment should
be selected only after careful consideration of the
water problem and type of equipment to be used
for its removal. As a part of the installation
procedure, the performance of the equipment
should be tested by water analysis. Periodic water
analysis is recommended to check for continued
equipment performance.
• Water Quality Association. Recommended industry
standards for household and commercial water
filters--a voluntary industry standard, S-200. 1988.
National Headquarters and Laboratory, Lisle, Illinois.
• Water Quality Association. Treatment Techniques For
Meeting the National Secondary Drinking Water
Regulations With the Application of Point-of-use
Systems. 1989. Recognized R28 National
Headquarters and Laboratory, Lisle, Illinois, 1989.
• Water quality and treatment - a handbook of public
water supplies, 3rd Edition. Prepared by The
American Water Works Association, Inc. McGraw-Hill
Book Co., New York.
