31
Osmium [Os, 76]

31.1 INTRODUCTION
Osmium (Os) is a hard, brittle, transition metal of group 8 in the periodic table of
elements and is a member of the platinum group metals (PGMs). It is considered the
least abundance stable element in the Earth’s crust, within the range of 0.5–1.0 μg/kg.
The highest concentrations of Os are in some igneous rocks and volcanic deposits.
Like other PGMs, Os occurs mainly in native alloys of variable composition, and
also in Pt ores.
Osmium forms compounds within the broad range of oxidation state, from −1
to +8. The most common states, however, are above +2. There are no Os minerals, but this metal is likely to be associated (at about 3%) with laurite, (Ru,Os)
S 2 . Some other minerals of PGMs exhibit an affinity for elevated amounts of Os,
up to 30%, for example, iridosmine and osmiridium—native alloys of variable
composition.
Osmium is obtained commercially as a by-product from Ni and Cu mining and
processing. There are not available data on the Os production. However, there is an
estimation that in 2012, the U.S. production of Os was 75 kg.
Osmium is mainly used for the production of hard-wearing alloys and for various
electronic devices (e.g., fountain pen tips, electrical contacts). It is very resistance to
all acids and alkalis. During metal processing, various Os compounds are formed,
of which OsF 6 , OsCl 4 , and volatile OsO 4 are toxic to humans. There are also some
suggestions indicating anticancer effects of Os compounds.
31.2 ENVIRONMENT
The mean Os content of soils is estimated to be 0.05 μg/kg, fairly similar to its content on the Earth’s crust. Although Os is known to be released from metal-processing
plants, mainly as volatile tetraoxide (OsO 4 ), there are no data on Os contaminations.
Osmium emitted from automobile catalytic converters is accumulated along motorways, where its contents vary between 0.03 and 2.36 μg/kg (Fritsche and Meisel vide
Kabata-Pendias 2011).
Its mean concentration in ocean waters is established at 2 pg/L (Nozaki 2005).
Its concentrations in river waters range from 3.3 to 42.1 pg/L and average 9 pg/L
(Gaillardet et al. 2003).
Mobile Os species are easily available to plants and are relatively toxic. Although
Os is considered as biologically unreactive metal, its increased levels in food and
fodder plants might be of a great health risk.
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