Iodine [I, 53]
147
Atmosphere is considered the main source of I, which is emitted mainly from sea
and ocean surface. In some soils of islands, as well as in seacoast areas, higher
I accumulations, up to around 150 mg/kg, are reported. In areas where I contents
in soils is <1 mg/kg (e.g., Sri Lanka) symptoms of I deficiency disorders (IDD) in
the populations are observed (Chandrajith et al. vide Kabata-Pendias 2011). Usually,
light soils in humid climate regions are poor, whereas clayed soils are enriched in I.
Iodine distribution in soils is associated with soluble organic matter (SOM),
hydroxides of Fe and Al, and clay minerals, especially chloride–illite group. Some
microorganisms may play a significant role in I cycling in soils, due to their great
capability of I absorption. Microorganism biomass may contain up to above 3% of
I present in surface soil layer. Some fungi (e.g., Penicillium chrysogenum) may accumulate even much higher amounts of I.
Iodine behavior in soils varies depending on its species. In general, large proportions of I in soils occurs in organically bound forms. The absorption rate differs for
I species and is higher for I
than for IO 3
. In the aquatic phase of soil are present
mainly anions I , I , IO , and H IO
3
3
4
6
, of which the first two are the most common.
The anionic form (I – ) is very mobile in soils and is easily leached out from soils,
especially under anaerobic conditions. However, this I form is easily available to
plants. There is an estimation that only a small fraction (<1%–25%) of the total I
in soils is easily mobile, and thus phytoavailable. Iodine occurs in soils to a large
extent in fixed forms, being adsorbed by humic and fresh OM, as well as on clay
and crystal lattice of minerals. Usually, organoiodine species are slightly mobile in
soils. Exchange of volatile I compounds between soil surface and the atmosphere
is relatively a common process. Iodine levels in soils are highly dependent on the
atmosphere precipitation. Therefore, the distance from seas and oceans has an influence on I status in soils.
Soil acidity favors I sorption by soil components. Alkali soils of arid and semiarid
regions have elevated amounts of I, due to both salinity and a low I mobilization
under alkaline conditions. Contents of I correlate positively with clay fraction, but
only of noncalcareous soils. Liming soils is an important factor reducing the mobility of all I species, and thus reducing its bioavailability.
Soils in the surroundings of some industries (e.g., fossil-fuel combustion plants and
kelp-burning facilities), as well as in close distance to high traffic roads, have usually
elevated I contents. Additional I sources in soils may be due to some sewage sludge.
Great attention has been recently focused on the I radioisotopes, 129 I and 131 I,
released into the environment during nuclear bomb testing and nuclear accidents,
especially after the 1986 Chernobyl reactor accident. These are, however, depleted
relatively faster from soils, due to their volatilization and slow vertical migration
(Kashparov et al. vide Kabata-Pendias and Mukherjee 2007).
21.3 WATERS
The median I values in the worldwide seawater are estimated within the range of
50–70 μg/L (Reimann and de Caritat 1998). Much lower I concentrations are seen
in river water, 2–15 μg/L (Table 21.1). The dominant I species in seawater are iodide
ion (I – ), and organically bound compounds. However, iodate (IO 3
) anion is also