28
Molybdenum [Mo, 42]

28.1 INTRODUCTION
Molybdenum (Mo), a metal of the group 6 in the periodic table of elements, occurs
in the Earth’s crust at the mean content of 1.5 mg/kg, within the range of 1–2 mg/
kg. In acidic magmatic rocks and granites, its content is up to 2 mg/kg, and in argillaceous sediments, up to 2.5 mg/kg. In coal, its contents vary between 3 and 6 mg/kg,
and in fly ash, it may be concentrated up to 15 mg/kg.
Molybdenum reveals lithophilic properties, and may occur at various degrees of
oxidation, from +2 to +6, but most often has the highest degree. Its behavior is
unusual, as in the most environmental compartments it is likely to form oxyanions. The most common Mo mineral is molybdenite, MoS 2 ; less frequent minerals
are molibdite, MoO 3 ; wulfenite, PbMoO 4 ; and powellite, CaMoO 4 . Several other
Mo minerals, mainly oxidates, are present in various sediments and are associated
mainly with Fe and Ti minerals. Especially Mn–Fe nodules reveal a great capability
to concentrate Mo, up to 2000 mg/kg (Fairbridge vide Kabata-Pendias 2011). Under
anoxic and S-rich environments, which dominate in sea-bottom sediments, Mo may
be accumulated in Mn–Fe nodules, up to about 500 mg/kg (Szefer 2002).
Molibdenite, a primary Mo ore deposit, is a major source of this metal. It is also
a by-product of some metal ores, mainly of Cu and W. World mine Mo production
(rounded) in 2010 was 234 kt, of which China produced 94; the United States produced 56; Chile produced 39; and Peru produced 12 (USGS 2011).
Molybdenum is an alloying element in steel and cast-iron production. It is also
used for refractory materials in high-temperature electric furnaces, and as catalyst
in chemistry. It is used in some red and orange pigments.
28.2 SOILS
The worldwide average soil content of Mo is established at 1.1 mg/kg, within the
range of <1–2  mg/kg. However, its higher concentrations, up to about 7  mg/kg,
are also noticed in soils (Table 28.1). Mean Mo content in organic surface soils of
Norway is 0.56 mg/kg, and is a bit lower than mean value (0.67 mg/kg) for all investigated soils (Nygard et al. 2012).
It behaves like both a chalcophille and a lithophille element, and in aerobic condition, mainly as molybdate oxyanion, MoO
2
4
. Molybdenum is likely to form various
complex compounds and minerals in soils, for example, wulfenite, PbMoO 4 ; ferrimolibdite, Fe 2 (MoO 4 ) 3
. 8H 2 O, as well as other semicrystalline Fe–Mo forms. Under
reducing conditions, Mo easily form soluble thiomolybdates (e.g., MoS
2
4
).
During weathering processes, Mo in minerals is oxidized to oxyanions, which
occurs mainly at lower pH values. These Mo anions are readily coprecipitated by
organic matter (OM), CaCO 3 , and by several cations. Strong adsorption of Mo reveal
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