Manganese is a mineral that naturally occurs in rocks and soil and may also be present due to underground pollution sources. Manganese is seldom found alone in a water supply. It is frequently found in iron-bearing waters but is rare than iron. Chemically it can be considered a close relative of iron since it occurs in much the same forms as iron. When manganese is present in water, it is as annoying as iron, perhaps even more so. In low concentrations, it produces extremely objectionable stains on everything with which it comes in contact. Deposits collect in pipelines, and tap water may contain black sediment and turbidity due to precipitated manganese. When fabrics are washed in manganese-bearing water, dark brown or black stains are formed due to the oxidation of the manganese.
The U.S. EPA Secondary Drinking Water Regulations recommend a limit of 0.05 mg/l manganese because of the staining which may be caused. For many industrial purposes, the manganese content should not exceed 0.01 to 0.02 mg/l. And in some cases, this is even considered excessive. In concentrations higher than 0.05 mg/l the manganese may become noticeable by impairing color, odor, or taste to the water. However, according to the EPA health effects are not a concern until concentrations are approximately 10 times higher. If it is unbearable, one could use water treatment technologies including cation exchange water softening, distillation, filtration, and reverse osmosis, which can be effective in removing manganese from water.
What health effects can manganese cause?
Manganese can be consumed from our diet and in our drinking water. Bathing and showering in manganese-containing water do not increase your exposure since manganese does not penetrate the skin and doesn’t get into the air. High exposure to manganese has been associated with toxicity to the nervous system, producing a syndrome that resembles Parkinsonism. Manganese is unlikely to produce other types of toxicity such as cancer or reproductive damage. Young children appear to absorb more manganese than older age groups but excrete less. This makes it particularly important for pregnant women and children to have clean drinking water.
Due to the fact that dissolved manganese oxidizes slower than iron, it is generally more difficult to be removed from the water. Pure elemental manganese metal is gray tinged with pink, brittle, and somewhat harder than iron which it resembles. Pure metal is not found in nature. However, this chemically active element is found in many compounds. Deposits occur in certain portions of the US as well as in other parts of the world.
Manganese is present most frequently as a manganous ion (Mn++) in water. Salts of manganese are generally more soluble in acid than in alkaline water. In this way, they are similar to iron. The manganous ion is usually introduced to water through the solubility of manganous bicarbonate.
Further, some surface waters and shallow wells contain organic or colloidal* manganese compounds. Manganese bacteria can also cause problems similar to those caused by iron bacteria-clogging, staining, etc.
Suspended insoluble manganic hydroxide, known as "black water," while not rare, is less common. This is probably due to the fact that a much higher pH is necessary to precipitate manganic hydroxide than is necessary to the production of ferric hydroxide.
Manganese bicarbonate in solution is colorless. The result is that unaerated deep well waters containing manganous ions are clear when freshly drawn. Exposure to the air soon converts the clear, soluble manganous ions into the black insoluble substance that is manganese dioxide. Then the trouble begins. The reactions occurring when manganous ions are converted to manganese dioxide are as follows:
| Reaction occurring in the oxidation of manganese | |||||||||
|---|---|---|---|---|---|---|---|---|---|
| 2Mn++ | + | O | + | 2Hz0 | –› | 2Mn2 | + | 4H+ | |
| Manganous ions plus oxygen plus water reacts to produce manganese dioxide plus hydrogen ions | |||||||||
Light concentrations of manganese can be removed with a water softener. Higher concentrations may be removed with oxidizing filters with considerable success. Very high manganese concentrations, or those complicated by organic matter, etc., call for chemical oxidation, as with iron, plus filtration.
Note that chlorine will not completely oxidize manganese unless the pH is above 9.5, whereas potassium permanganate is effective at pH values above 7.5. Thus, permanganate is the preferred oxidizing agent in most cases.
*Colloidal: Containing or pertaining to colloids which are insoluble particles. These particles are larger than molecules but small enough so that they remain suspended in a liquid without settling. A colloid does not affect the freezing point, boiling point or vapor tension of the liquid in which it is suspended.