22
Iridium [Ir, 77]
22.1 INTRODUCTION
Iridium (Ir), a noble metal of the group 9 in the periodic table of elements, is one of
the rarest elements in the Earth’s crust, present in the range of 0.02–0.05 μg/kg in
the upper and bulk continental zones, respectively. Due to the tendency of Ir to bind
with Fe, its content in the Earth’s crust is lower than in deeper layers. It is concentrated mainly in the Fe–Ni core and in some meteorites. Its geochemical behavior is
close to that of Co and Ni. Like other platinum group metals, it is very hard, brittle,
silver-white, and the second densest metal (22.4 g/cm 3 ). Iridium contents of some
coals may be up to 200 μg/kg.
Iridium was, presumably, emitted in higher amounts with volcanic dusts (Kracatau,
in 1883), and later on, was deposited in sedimentary rocks. Increased Ir contents in
sedimentary rocks, at the border layer between Cretaceous and Paleogene deposits, is apparently related to an effect of the meteorite collision, 65 million years ago
(Dai et al. vide Kabata-Pendias 2011).
Main oxidation state of iridium is +4, but it may vary from +2 to +6, and has 34
radioisotopes. It occurs as natural alloys with Rh and Os, mainly in arsenosulfide
minerals. Its typical minerals are irarsite ([Ir,Ru,Rh,Pt]AsS) and hollingworthite
([Rh,Pd,Pt,Ir]AsS). It may also be associated with other minerals, including olivine
and ilmenite.
The annual production of Ir is estimated at about 3 t, and it is mined from Ni–Cu
ores deposits. Various Ir salts and organometallic compounds are used in industrial
catalysis. It is also used in alloys, for the corrosion and acid resistance at high temperature, and for electrodes in the chloralkali processes. Ir radioisotopes are used in
some radioisotope thermoelectric generators. Additionally, 192 Ir is used as a source
of gamma radiation for the treatment of cancer, especially when a sealed radioactive
source is placed inside or next to the area requiring treatment.
22.2 ENVIRONMENT
Average content of Ir in Swedish arable soils is given as <4 μg/kg (Eriksson 2001a).
Soil humus layer from the Kola Peninsula contains Ir up to 5  μg/kg, and in soils
along motorways in Austria, its average content is 1.1 μg/kg (Fritsche and Meisel vide
Kabata-Pendias 2011). Swedish arable soils contain Ir at the average value of <40 μg/kg
(Eriksson 2001a). Dust collected along highways in Germany contains Ir in the range
of 1.2–3.5 μg/kg, and plants contain from <0.02 to 0.4 μg/kg (Djingova et al. 2003).
Cereal grains from Sweden contain Ir at the average value of <0.1 μg/kg (Eriksson
2001a). Median Ir concentration in ocean water is 0.13 pg/L, whereas in the Baltic Sea,
it ranges from 0.13 to 1.17 pg/L (Anbar et al. vide Kabata-Pendias and Mukherjee 2007).
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