158
Trace Elements in Abiotic and Biotic Environments
acidic soils, it may be above 2000 μg/L. In soil solution Fe is present in cationic
2
3
species as follows: Fe , Fe , FeCl , Fe(OH) , FeH PO
; and in anionic species,
2
2
4
2
it is as follows: Fe(OH) , Fe(SO ) , Fe(OH)
3
4 2
4
(Kabata-Pendias and Sadurski 2004).
Although Fe in soil, under most conditions, is slightly mobile, its organic complexes and chelates are relatively mobile species. These compounds are largely
responsible for Fe migration between soil horizons, and for its leaching from soil
profiles. Nevertheless, in most soils, there is an observed higher Fe input than output,
in surface soil layers.
Iron solubility is low in soils at the alkaline pH range, and its mobility increases
with increasing soil acidity. Thus, Fe cations in acid anaerobic soils may be toxic
to plants, whereas in alkaline well-aerated soils, it may not be available in enough
quantity to plants. Processes of reduction and oxidation of Fe are strongly controlled
by microorganisms.
Iron deficiencies in soils, for certain crops are relatively widespread, but most
common are in aridic climate zones, and are related to calcareous, alkaline, and
other specific soils. However, the assessment of Fe availability to plants is very difficult, due to several factors involved in these processes.
There are several sources of Fe contamination, including industrial and municipal
wastes. Iron in solid municipal waste is mainly in forms of carbonates and sulfides
(1.4 and 1.9 mg/kg, respectively) and has not been changed after composting (Ciba
and Zołotajkin 2001).
23.3 WATERS
The global mobility index (C w /C c ratio) of Fe is very low (<0.01). Thus, its contents
in surface water are relatively low. Iron concentrations in ocean water are estimated
within the range of 25–743 μg/L (Reimann and de Caritat 1998). The most common Fe concentration is given as 10–100 μg/L, and the worldwide mean at 30 μg/L
(Table 23.1). In the Baltic Sea, its contents range from 0.3 to 35 μg/L (Szefer 2002).
Worldwide riverine flux of Fe in seawater is estimated at 2.47 Mt/yr (Gaillardet et al.
2003), and at 7 Mt/yr (Kitano 1992).
The worldwide average Fe concentration in river water, in dissolved load of
<0.2 μm, is estimated at 66 μg/L, within the range of 11–739 μg/L (Gaillardet
et al. 2003). The behavior and chemistry of Fe in water systems are very complex and controlled by several parameters, of which the redox potential is the
most significant. Iron in stream water is subjected to various photochemical and
microbial oxidation reactions. Also, concentration of dissolved organic matter
has an important impact on Fe species in water. The predominated form of Fe are
colloids, but several hydrous ions of Fe 3+ and Fe 2+ may also be common, mainly
2+
0
as species: Fe(OH) , Fe(OH) 3 and Fe(OH) 4
− . Very stable in water are two forms
Fe 3+ and Fe(OH) 2
. Most probably, however, the solubility of Fe 3+ in seawater is
controlled by organic complexation.
The colloidal Fe oxides play a dominate role in the sorption and coagulation of
other colloidal substances and ions. Dissolved Fe compounds readily precipitate
in the most aquatic environments, and form various multimetallic concretions in