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Palladium [Pd, 46]
32.1 INTRODUCTION
Palladium (Pd) is a rare, lustrous silvery-white metal of group 10 in the periodic table
of elements and is a member of the platinum group metals (PGMs). It is the least
dense of them and has the lowest melting point. Its average abundance in the Earth’s
crust is estimated within the range of 4–10 μg/kg, but in the upper layer, it is much
less, about 1 μg/kg. Relatively high Pd contents may be in ultramafic rocks, some
coals, black shales, and phosphorites. Its mean content of coal in the United States
is <1 μg/kg (Finkelman 1999). Basalts and shale schists contain Pd up to 0.6 μg/kg.
The highest Pd contents are associated with Mn ores and Mn concretions, where it
may be accumulated up to about 12 μg/kg.
Palladium has various oxidation states, from +1 to +4, and most often it is +2.
It exhibits relatively strong chalcophilic character and chemical reactivity, and thus, it
forms more mineral compounds than other PGMs. Its better-known minerals are as
follows: potarite, PdHg; arsenopalladinite, Pd 3 As; stibiopalladinite, Pd 3 Sb; cooperite,
(Pt,Pd)S; and braggite, (Pt,Pd,Ni)S. It may be associated with several sulfide minerals,
at the approximate concentration from 0.7 to 10 mg/kg, and may also be accumulated
in ilmenite, zircon, and chromite.
The global Pd production (rounded) in 2010 was estimated to be 197 t, of which
87 t was produced in Russia, 73 t in South Africa, and 11 t in the United States
(USGS 2011). Recycling is also a source of Pd, mostly from scrapped catalytic converters. Above half of Pd supply is used for the automotive catalytic converters, and
recently, it has been highly increased (Schäfer and Puchelt 1998). It is used to reduce
emissions of various carbon compounds and nitrous oxides. However, Pd emission
may also be of potential risk to human and environment. It is used in electronics,
jewelry production, as well as chemical and medical instruments. Its compounds
are used in photography and electroplatings, and as a catalyst, mainly in organic
synthesis. Radioactive 103 Pd is proving to be a successful form of treatment for some
types of cancer. Several Pd compounds have antiviral, antibacterial, and fungicide
properties. Due to the numerous applications and limited supply sources of Pd, it is
a metal of a great value.
32.2 SOILS
The mean Pd contents of soils, as reported by various authors, range from 1 to 40 μg/kg
and average 20 μg/kg. According to the data of the World Health Organization (WHO
2002a), its mean content is 40 μg/kg. Agricultural soils of Sweden contain Pd within
the range of <40–150 μg/kg, at the mean value of 40 μg/kg (Eriksson 2001a). Recently,
Pd levels in soils and dust along roads, especially high traffic, have been elevated,
within the range of 6.6–280 μg/kg (Ravindra et al. 2004). The highest Pd content,
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