Page 114 - Soil and water contamination, 2nd edition

Major dissolved phase constituents 101

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than 10 μg l (Hem, 1989). Under reducing conditions, the most common form of dissolved
3+
2+
iron is ferrous Fe , which is much more soluble than the ferric Fe . The concentrations can
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range up to 50 mg l , but are controlled by the solubility of pyrite , marcasite, and siderite , as
noted above.
Because the solubility of Fe depends on both the pH and the redox potential , the stability
of the solid and dissolved forms of Fe can conveniently be depicted in a pH–Eh diagram .
The construction, use, and limitations of these types of diagrams were outlined in Section
2.10. Figure 5.4 shows the fields of stability of Fe species in a system containing a constant
total amount of dissolved sulphur and carbon dioxide (inorganic carbon) species both of
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2- -1
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1 mmol l (equivalent to 96 mg SO l and 61 mg HCO l ). The solid species indicated
4 3
by the shaded areas would be thermodynamically stable in their designated domains. The
Fe activity does not affect the boundaries between the dissolved species. The boundaries
between the solid and dissolved species have been drawn for a dissolved Fe activity of
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56 μg l , but if more Fe is present, the domains of the solid species will increase.
Figure 5.4 confirms that there is one general Ph–Eh domain in which Fe is relatively
soluble: under moderate reducing conditions, especially at low pH . Comparison with Figure
2.3 shows that these conditions may prevail in groundwater, acidic podsolic soils and peat
soils, and in the hypolimnion of lakes . The high-solubility domain at high pH is outside
the range that is common in natural waters. Under the conditions specified for Figure 5.4,
siderite (FeCO ) saturation is not reached. To attain siderite saturation and an accompanying
3
stable solid-phase domain of siderite in the pH–Eh diagram requires a higher activity of

inorganic carbon species.
Although Fe is very insoluble under oxidising conditions in soil, the presence of organic
ligands can keep considerable amounts of Fe(III) in a mobile form (see also Section 4.3.2).
This is a major pathway of the plant uptake of Fe and some plants even produce ligands that
chelate soil Fe. Therefore, most plants are not deficient in Fe. The formation of chelates is
also an important process of Fe leaching in podsolic soils.

5.7 MANGANESE
Like Fe, manganese (Mn) is a transition metal and its chemistry is similar to that of Fe.
However, Mn can occur in three possible oxidation state s: 2+, 3+, and 4+, or Mn(II),
Mn(III), and Mn(IV). In organisms, Mn is involved in many enzyme systems and helps
with the protein, carbohydrate and fat metabolisms, with building of bone and connective
tissues, and also with the clotting of the blood. In broad-leaved plants, Mn deficiency cause
interveinal chlorosis of the younger leaves. Manganese may become noticeable in drinking
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water at concentrations greater than 0.05 mg l by imparting an undesirable colour, odour,
or taste to the water. However, the effects on human health are not a concern until the
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concentration of Mn is approximately 0.5 mg l . Long-term exposure to high concentrations
of Mn may be toxic to the nervous system. Like Fe, Mn is usually removed by aeration
during the preparation of drinking water.
Because of its three possible oxidation state s, manganese can form a wide range of
mixed-valence oxides. Divalent Mn is present as a minor constituent in many igneous rock
minerals, such as olivines, pyroxenes, and amphiboles, and in sedimentary limestone and
dolomite . Divalent Mn is released into solution during weathering and is more stable than
2+
Fe . Figure 5.6 shows the pH–Eh diagram of Mn species in a system containing a constant
-1
-1
total amount of 1 mmol l dissolved sulphur and 1 mmol l inorganic carbon species. From
2+
Figure 5.5 it can be seen that the concentration of dissolved Mn is largely controlled by the
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pH. Therefore, acidic groundwater may contain more than 1 mg l of Mn. Comparison with
2+
2+
Figure 5.4 demonstrates that Mn has a greater stability domain than Fe . This implies that

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