38
Rhodium [Rh, 45]
38.1 INTRODUCTION
Rhodium (Rh) is a silvery-white, hard metal of group 9 in the periodic table of elements, is the least frequent element of the platinum group metals (PGMs), and is
chemically inert. It reveals both siderophilic and chalcophilic properties. Its average
content in the Earth’s crust is estimated to be 0.06 μg/kg, and its contents in rocks
vary within the range of 0.01–20 μg/kg. It may be accumulated in coal, up to about
100 μg/kg. It tends to occur along with deposits of PGMs and is primarily obtained as
a by-product of mining and refining Pt. It is also mined from the Cu–Ni deposits,
mainly in Monchegorsk area, the Kola Peninsula (Russia). The main minerals of
these ores are pyrrhotite, magnetite, pentlandite, chalcopyrite, Ti magnetite, and
pyrite, in which Rh contents vary from 4 to 2780 μg/kg (Gregurek et al. 1999).
Rhodium oxidation states vary from +2 to +6, and the most common is +3. It may
be associated with some Pt minerals. Its typical arsenosulfide minerals are hollingworthite, (Rh,Pd,Pt)AsS, and irarsite, (Ir,Ru,Rh,Pt)AsS. Several other minerals,
such as ilmenite, zircon, and chromite may accumulate Rh. Higher Rh amounts are
accumulated in siderites and pyrrhotites, up to >100 μg/kg, and therefore, its elevated amounts around some Fe processing industries may be expected.
Rhodium is one of the most rarest and valuable precious metals. Its worldwide
production is about 25 t/yr, mainly in South Africa, Russia, and Canada. About 85%
of Rh is used for autocatalysts. It is also used frequently, as an alloying agent with
other metals, mainly Pt and Pd. These alloys are used to make such things as catalytic converters, electrodes for aircraft spark plugs, laboratory crucibles, electrical
contacts, chirurgical tools, and jewelry.
38.2 ENVIRONMENT
Rhodium contents of soils in Sweden are given as follows (in μg/kg): background
soil, 0.07–0.13; urban soil, 0.14–20.5; arable soil, mean <40, and sewage sludge, 0.26
(Eriksson 2001a). Soils along motorways in Australia contain Rh, at the mean value,
13 μg/kg. The highest Rh content, above 91 μg/kg, is in roadside soil samples from
Perth, Australia (Whiteley and Murray 2003).
Its content in dust samples collected (in 1999) along highways in Germany ranges
from 30 to 42 μg/kg (Djingova vide Kabata-Pendias 2011). Its sources are mainly
catalytic converters of cars. There is an estimation that all Rh in sewage sludge are
from automobile emission (Schäfer and Puchelt 1998).
The median Rh concentration in worldwide ocean waters, as well as in the North
Pacific, is estimated to be 0.08  ng/L (Nozaki 2005). Its similar concentration, at
about 0.1 ng/L, is reported by. These authors emphasized that unlike the other PGMs,
Rh is enriched in pelagic sediments (0.05–3 μg/kg), hydrothermal S deposits, and
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