9
Cadmium [Cd, 48]
9.1 INTRODUCTION
Cadmium (Cd) is a metal of the group 12 in the periodic table of elements, and its
mean content in the Earth’s upper crust is within the range of 0.1–0.2 mg/kg. In igneous and sedimentary rocks, its contents are fairly similar, 0.2–0.3 mg/kg. The lowest
Cd contents are in sandstones. Coals may contain variable amounts of Cd, broadly
ranging from 0.5 to 170 mg/kg. Especially Zn-bearing coals, of Carboniferous age,
are resources of Cd.
Cadmium is considered as one of the most ecotoxic metals that exhibit adverse
effects on all biological processes. It reveals very harmful impact on the environment and the quality of food.
Cadmium occurs at +2 oxidation state, and easily forms complex cations and
anions, especially with Cl, S, and OH. It exhibits chalcophillic properties and
behaves very similar to Zn, but has a stronger affinity for S, and is more mobile in
acidic environments. During weathering processes, Cd forms simple, mobile compounds, such as CdO, Cd(OH) 2 , CdCl 2 , and CdF 2 . They are also isotypic with corresponding compounds of cations such as Co 2+ , Ni 2+ , Fe 2+ , Mg 2+ , and Ca 2+ . Common
minerals of Cd are greenockite, CdS; octavite, CdSe; and monteponite, CdO. They
all are associates with Zn and Pb ore deposits. The host minerals of Cd are sphalerite
(αZnS), wurtzite (βZnS), biotite, smithsonite, and amphibolites. Sphalerite is the
most common source of Cd.
World production (rounded) of Cd in 2010 was 22,000 t of which China produced
5600t; Republic of Korea produced 3200t; and the United States produced 650t
(USGS 2011). Secondary Cd production takes place mainly at NiCd battery recycling facilities. Industrial uses of Cd are varied, especially for NiCd battery production (about 70% of the total use). Cadmium is also added to nonferrous alloys.
The other traditional uses of Cd, such as coatings, pigments (yellow), and stabilizers
for plastics, have gradually decreased, owing to environmental and health concerns.
Concern over Cd toxicity has spurred various recent legislative efforts, especially in
the EU countries, to restrict the use of Cd in the most applications.
9.2 SOILS
Average Cd contents in soil worldwide is estimated at 0.41 mg/kg, within the range
of mean values for various countries from 0.06 to 1.1 mg/kg. Parent materials are
the main factor determining the Cd contents in soils. Its highest contents are in
calcerous and organic soils, <0.8 and <2.5 mg/kg, respectively (Table 9.1). In some
soils derived from bed rocks enriched in Cd, its concentration may be up to 8.9 mg/kg
(Čurlik and Šefčik 1999). Mean Cd content in organic surface soils of Norway is
0.57 mg/kg, ranging from 0.05 to 6.7 mg/kg (Nygard et al. 2012).
53
Cadmium [Cd, 48]
9.1 INTRODUCTION
Cadmium (Cd) is a metal of the group 12 in the periodic table of elements, and its
mean content in the Earth’s upper crust is within the range of 0.1–0.2 mg/kg. In igneous and sedimentary rocks, its contents are fairly similar, 0.2–0.3 mg/kg. The lowest
Cd contents are in sandstones. Coals may contain variable amounts of Cd, broadly
ranging from 0.5 to 170 mg/kg. Especially Zn-bearing coals, of Carboniferous age,
are resources of Cd.
Cadmium is considered as one of the most ecotoxic metals that exhibit adverse
effects on all biological processes. It reveals very harmful impact on the environment and the quality of food.
Cadmium occurs at +2 oxidation state, and easily forms complex cations and
anions, especially with Cl, S, and OH. It exhibits chalcophillic properties and
behaves very similar to Zn, but has a stronger affinity for S, and is more mobile in
acidic environments. During weathering processes, Cd forms simple, mobile compounds, such as CdO, Cd(OH) 2 , CdCl 2 , and CdF 2 . They are also isotypic with corresponding compounds of cations such as Co 2+ , Ni 2+ , Fe 2+ , Mg 2+ , and Ca 2+ . Common
minerals of Cd are greenockite, CdS; octavite, CdSe; and monteponite, CdO. They
all are associates with Zn and Pb ore deposits. The host minerals of Cd are sphalerite
(αZnS), wurtzite (βZnS), biotite, smithsonite, and amphibolites. Sphalerite is the
most common source of Cd.
World production (rounded) of Cd in 2010 was 22,000 t of which China produced
5600t; Republic of Korea produced 3200t; and the United States produced 650t
(USGS 2011). Secondary Cd production takes place mainly at NiCd battery recycling facilities. Industrial uses of Cd are varied, especially for NiCd battery production (about 70% of the total use). Cadmium is also added to nonferrous alloys.
The other traditional uses of Cd, such as coatings, pigments (yellow), and stabilizers
for plastics, have gradually decreased, owing to environmental and health concerns.
Concern over Cd toxicity has spurred various recent legislative efforts, especially in
the EU countries, to restrict the use of Cd in the most applications.
9.2 SOILS
Average Cd contents in soil worldwide is estimated at 0.41 mg/kg, within the range
of mean values for various countries from 0.06 to 1.1 mg/kg. Parent materials are
the main factor determining the Cd contents in soils. Its highest contents are in
calcerous and organic soils, <0.8 and <2.5 mg/kg, respectively (Table 9.1). In some
soils derived from bed rocks enriched in Cd, its concentration may be up to 8.9 mg/kg
(Čurlik and Šefčik 1999). Mean Cd content in organic surface soils of Norway is
0.57 mg/kg, ranging from 0.05 to 6.7 mg/kg (Nygard et al. 2012).
53