Manganese

Manganese can have an oxidation state as high as +7 in the form of permanganate (KMnO4), which has some use in the environmental field as an oxidizing agent. However, within the biological realm, the natural states are +2 to +4, called, respectively, the "manganous" and "manganic" forms. Soluble Mn2+ and insoluble MnO2 represent the principal forms that are found.

The dominant reservoir for manganese is the lithosphere; it forms ~0.1% of Earth's crust. Waters, both fresh and marine, rarely contain manganese concentrations above a few tenths of a part per million, and often far less. Like iron, manganese can also be chemically oxidized by atmospheric oxygen, but in this case pH levels of 8 or higher (as opposed to pH 4 to 5 for iron) are necessary to promote this spontaneous reaction. (This fact is used in the Winkler test, the standard chemical method of measuring dissolved oxygen; a solution of Mn2+ is added to the sample, followed by alkali, leading to formation of a stoichiometric amount of MnO2 that is then analyzed iodometrically.)

13.5.1 Manganese Reduction

Manganese in the +4 state (i.e., MnO2) can be reductively converted to Mn(II) by microbial activity under anoxic conditions. At least in some cases it appears that it is being used as an alternative electron acceptor for anaerobic respiration.

13.5.2 Manganese Oxidation

There is a relatively small group of bacteria capable of directly using manganese as an oxidative energy source, including some strains of Arthrobacter, Bacillus manganicus, Corynebacterium, Flavobacterium, Gallionella, Hyphomicrobium, Pseudomonas, and Vibrio. Several strains of iron-oxidizing bacteria, such as Leptothrix and Sphaerotilus, are also believed to be capable of cooxidizing manganese, precipitating it within their sheaths and metallic coatings.

13.5.3 Manganese in Environmental Engineering and Science

The cycling of manganese is not often regarded as a major environmental engineering and science concern, particularly since it is usually present only in the low parts per billion range. However, as with iron, higher levels of manganese may cause aesthetic problems in waters, or even difficulties with groundwater pumping. These issues tend to develop in relation to site-specific conditions.

Like iron, manganese can impose objectionable tastes in waters at quite low concentrations, particularly with hot coffee and tea. Discoloration with manganese generates a pinkish color. To address these concerns, the secondary standard for drinking water has been set at 50 parts per billion.

Typically, the problem is seen in communities using a lake or reservoir as their potable water source. Such water bodies tend to stratify (Section 15.2.1), with an aerobic zone (epilimnion) above an anaerobic one (hypolimnion). Seasonally, as the depths of these layers vary, the point of withdrawal for these waters (Figure 13.29) may extend

Nominal Mn2+ at water intake point

Nominal Mn2+ at water intake point

Fixed elevation intake pipe

Seasonal upward shift in hypolimnion (typically ''peaks' during Fall)

MnO2 settling reduced

MnO2 settling reduced

MnO2

Hypolimnion

Fixed elevation intake pipe

Seasonal upward shift in hypolimnion (typically ''peaks' during Fall)

Elevated Mn2+ at water intake point

Epilimnion oxidation Mn2+ -> MnO2

Thermocline reduced MnO2 Mn2+

Epilimnion oxidation Mn2+ -> MnO2

Hypolimnion

Figure 13.29 Seasonal shifts in lake stratification leading to increased presence of soluble reduced manganese (Mn2+) in potable waters.

into the anaerobic zone. This can draw reduced manganese into the system, where it may later be oxidized.

Another potential problem occurs when groundwater is contaminated by landfill lea-chate or other degradable organic compounds, leading to anaerobic conditions. This can lead to mobilization of manganese and iron, potentially causing clogging of nearby wells (Figure 13.30).

Fe migration toward well

Figure 13.30 Mobilization of reduced manganese and iron by groundwater contamination.

Fe migration toward well

Figure 13.30 Mobilization of reduced manganese and iron by groundwater contamination.

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