Page 116 - Soil and water contamination, 2nd edition

Major dissolved phase constituents 103

Mn oxide is preferentially precipitated on existing Mn oxide surfaces, which results in
nodules being formed around a central nucleus. Coatings or small discrete particles of Mn
oxide are very common in soils and bed sediments . Subsurface water in contact with these
2+
Mn precipitates under moderately reduced conditions may exhibit enhanced dissolved Mn
-1
concentrations up to about 1 mg l , even at neutral pH . Accumulations of Mn oxides are
also found in acidic leached laterite soils in tropical regions and in the dark stains on rocks
in arid regions. The manganese nodules and coatings usually contain significant amounts
of coprecipitated iron and sometimes other metals as well, especially cobalt, lead , zinc ,
copper , nickel , and barium. Because of the different redox potential s at which Fe and Mn are
oxidised (compare Figures 5.4 and 5.5), the zones in which Fe and Mn precipitate may occur
spatially separated.
The Mn taken up by plants accumulates mostly in the leaves. As soon as the plant
parts die back or the leaves are shed, the Mn is released into solution in surface runoff or
soil moisture. Accordingly, the released Mn may cause temporary increases in the Mn
concentrations in river water during autumn. Similar to Fe, chelates may play an important
2+
role in the environmental transport of Mn in some situations. Note, however, that the Mn
2+
ion is considerably more stable than Fe under oxic conditions, and can also be transported
in larger concentrations without organic complexation .
A possible anthropogenic source of Mn in streamwater is the discharge of acidic mine
water. These mine water discharges may also contain considerable amounts of Fe and other
metals . Manganese usually persists in the water for greater distances downstream from the
discharge location than Fe. Downstream from the inflow, the acidity is buffered , so the pH
increases gradually. The Fe is removed from the water first, due to precipitation of ferric
hydroxide. Later (and thus after more distance has been travelled) Mn also disappears from
solution.

5.8 ALUMINIUM

Aluminium (Al), the third most abundant element after oxygen and silicon, is by far the
most common metal in the Earth’s crust. In nature it is almost always found in the 3+
oxidation state combined with other elements and there are many different compounds
which contain Al. Whilst Al is abundant in the environment, the naturally occurring
forms are usually stable under near-neutral pH , so Al rarely occurs in natural waters in
concentrations greater than a few tens or hundreds of milligrams per litre. Because of this, Al
is not usually considered to be a major dissolved phase constituent . However, under extreme
pH conditions, Al may become soluble and because of its toxicity it may be a relevant
parameter in environmental assessment studies. Therefore, the most important chemical
properties of Al are discussed below.
Aluminium does not appear to be an essential trace element and it has no known
biological role. Under acidic conditions, Al occurs in a soluble form which can be
absorbed by plants and animals. Some plants (ferns and tea, for example) naturally
accumulate high amounts of aluminium compounds in their leaves. However, Al in large
concentrations is toxic for most plants because it inhibits the root growth. Dissolved Al
in surface waters of low pH has a deleterious effect on fish and other aquatic organisms.
Humans have highly effective barriers to exclude aluminium. Only a very small fraction
of aluminium in the diet is taken up from the stomach and intestines, and in healthy
individuals most of this absorbed aluminium is excreted by the kidneys. When the natural
barriers that limit the absorption of aluminium are bypassed, or if the ability of the
kidneys to excrete aluminium is impaired, this metal accumulates in the body and may
cause adverse health effects.

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