Manganese in Water
Manganese metal is not found pure in nature, but its ores are very common and widely distributed. Although manganese is much less abundant than iron in the Earth’s crust, it is one of the most common elements and is widely distributed in rocks and soils
In igneous rock minerals, manganese is usually in the reduced form Mn+2. The ferromagnesian minerals such as biotite and hornblende commonly contain some manganese. The most common forms of manganese in rocks and soils, however, are oxides and hydroxides in which the oxidation state of the element is +2, +3 or +4.
These oxides tend to absorb other metallic ions very strongly and the common naturally occurring species of the manganese oxides usually have many impurities.
Manganese is an essential element in plant metabolism and it is to be expected that organic circulation of manganese can influence its occurrence in natural water. Manganese oxide deposits also occur in fresh water lakes apparently as a result of processes similar to those causing deposits of bog iron ore.
In reservoirs and lakes when thermal stratification occurs, the water near the bottom may be able to dissolve considerable concentrations of manganese from deposits left there under oxidizing conditions prevailing at an earlier time. Sometimes this is the reason why water withdrawn from many public supply reservoirs contains excessive concentration of manganese at times.
Aquatic plants have been noted to be accumulators of manganese. Manganese in plant parts that die back or are shed, as for example in leaves, become available for solution in runoff and soil moisture. The possible importance of suspended manganese oxide in streams as controls over the behavior of minor cations has been pointed out. However, the importance of this source of manganese in river water is not completely understood.
One of the notable properties of manganese oxide is a tendency to form coatings on other mineral surfaces. This is a further complicating factor in determining the amounts present in ordinary stream flow.
Although manganese forms complexes with many kinds of organic material, the divalent Mn+2 ion is relatively stable in uncomplexed form in the conditions normally to be expected in river water. Studies to established correlation between organic color and manganese concentrations in river water are ongoing.
Manganese is often present to the extent of more than 1 mg/L in streams that have received acid drainage from coal mines. Manganese usually persists in the river water for greater distances downstream from the pollution source than the iron contained in the same drainage inflows. As the acidity is gradually neutralized, ferric hydroxide precipitates first, however, manganese also disappears from solution after a longer time.
Some ground waters that contain objectionable concentrations of iron also have considerable amounts of manganese, but ground waters that contain more manganese than iron are rather unusual. Many of the ground waters reported to carry large manganese concentrations are from thermal springs. In many places, these springs seem to be closely associated with manganese oxide deposits.
The drinking water limits established for manganese have been established for manganese on the basis of aesthetic and economic considerations rather than physiological hazards. In concentrations not causing unpleasant tastes, manganese is regarded by most investigators to be of no toxilogical significance in drinking water.
However, manganese is essential for plant growth apparently as an enzyme activator. It is especially abundant in the reproductive parts of plants, seeds being the highest, while woody sections contain the least manganese.
The toxicity of manganese towards fish is dependent upon many factors. The toxic action is slow and manganese does not appear to precipitate on gill secretions as does iron. The following concentrations of manganese have been tolerated by fish under the stated conditions.
Concentration in Mg/L Time of Exposure Type of Fish
1 – River Crayfish
15 7 days Tench Carp and Trout
40 4 days Fingerling Catfish
50 3 days Stickle back
2700 50 hours Eels
It should be noted that the permanganates are much more toxic to fish than the manganous salts. Permanganates killed fish in 8 to 18 hours at concentrations of 2.2 to 4.1 mg/L of manganese. However, permangates are not stable for long.