12
Chromium [Cr, 24]
12.1 INTRODUCTION
Chromium (Cr), a metal of the group 6 in the periodic table of elements, has
strong lithophillic tendencies. Its content on the Earth’s crust is within the range
of 102–185  mg/kg. The highest Cr contents in ultramafic rocks may be up to
3800 mg/kg, which decreased in acidic ones (e.g., granites) to 10–50 mg/kg. Its
distribution in sedimentary rocks is as follows (in mg/kg): argillaceous (shales),
80–120; sandstones, 20–40; and limestones, 5–16. Coal contains Cr within the
range 10–40 mg/kg, whereas fly ash may have up to about 150 mg/kg.
Chromium occurs at several oxidation states, of which the most common are +3
and +6. It is precipitated from magma mainly in the Cr spinel mineral groups of
the following formula: (Mg,Fe)O (Cr,Al,Fe) 2 O 4 . Its relatively common minerals are
chromite (FeCr 2 O 4 ) and crocoite (PbCrO 4 ). These minerals are likely to be associated with pyroxenes, amphibolites, and micas. The geochemical association of Cr
with Fe and Mn resulted in its increased contents in Fe concretions of soils, and in
ferromanganese nodules in sea bottom sediments.
World mine Cr production (rounded) in 2010 was 22 Mt, of which (in Mt) South
Africa, 8.5; India, 3.8; and the United States, 3.8 (USGS 2011). Chromium is mined
mainly as chromite ore, which is converted into ferrochromium, broadly used in
metallurgical industries, and most of that is consumed to make stainless and heatresisting steel. It is also used for chromium chemicals, applied in paints, varnishes,
glazes, and inks. A great proportion of Cr compounds is utilized for leather tanning.
It is also used in paper manufactories and various refractory products.
The main sources of Cr pollution are from dyestuffs and leather tanning. Also
chromite ore processing residue is a significant source of Cr, mainly in surface water,
which resulted in bottom sediments contamination.
12.2 SOILS
The world soils average Cr content has been estimated at 60 mg/kg. However, soils
derived from parent rocks with high Cr levels may contain much higher amounts. For
example, soils in some Carpathian rocks have about 6000 mg Cr/kg (Čurlik and Šefčik
1999). Soils developed from serpentines contain especially high Cr levels, sometimes
above 100,000 mg/kg (Kabata-Pendias 2011). The most common average Cr contents
are within the range 20–80 mg/kg, the lowest in organic soils and the highest in heavy
loamy soils (Table 12.1). Mean Cr content in organic surface soils of Norway is 7.8 mg/kg,
within the range <0.05–191 mg/kg (Nygard et al. 2012). Chromium contents of soils in
Spitsbergen range from 0.1 to 0.8 mg/kg (Gulińska et al. 2003).
The oxidation states of Cr are the most important factors controlling Cr behavior.
Cr 3+ is slightly mobile in very acidic soils, and at about pH 5.5, it forms very stable
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