Page 206 - Soil Degradation, Conservation and Remediation

6.2 Sources of Soil Pollutants 195

Cadmium

Cadmium (Cd) is a lustrous, silver-white, ductile, very malleable, and soft metal.
Cadmium is a metallic element with atomic number 48, atomic weight 112.4, density
−3
8.65 g cm , melting point 320.9 °C, and boiling point 765 °C. Cadmium has a
relatively low crustal abundance, although it occurs ubiquitously in rocks and
soils (Alloway 1995 ). It is rarely found in its elemental form. The main minerals
containing cadmium are greenockite (CdS), octavite (CdSe), and monteponite
(CdO) (Kabata-Pendias and Mukherjee 2007 ). Cadmium is often found in associa-
tion with zinc ores and zinc-bearing lead ores. Its compounds almost exclusively
involve the +2 oxidation state. Cadmium forms simple salts with oxygen, sulfur, and
many common anions such as chloride, nitrate, and carbonate. Cadmium often
+
forms simple hydrated hydroxyl ions such as [Cd(OH)(H 2 O) x ] in aqueous solution.
It also has appreciable coordination chemistry with ligands including halides,
hydroxides, cyanides, and nitrate (Alloway 1995 ). Organocadmium compounds are
rather reactive and unstable.
Cadmium is regarded as one of the most toxic trace elements in the environment.
The increased emissions with its persistence and its relatively rapid uptake and
accumulation by food chain crops contribute to its potential environmental hazards.
According to Heinrichs et al. ( 1980 ), the average concentration of cadmium in the
−1
lithosphere is 0.098 mg kg . Most mafic and granitic rocks contain a very low

−1
concentration of cadmium (<0.1 mg kg ). It has relatively high concentrations
−1
(about 10 mg kg ) in sedimentary rocks. Some shales are unusually high in Cd
−1
(>100 mg kg ).
Amounts of indigenous cadmium in cultivated and noncultivated soils are
determined by the quantities of Cd in the parent materials together with amounts
added through atmospheric deposition, fertilizers, pesticides, and irrigation
water. Generally soils of similar age derived from igneous rocks contain the lowest
Cd, soils derived from metamorphic rocks have intermediate, and soils derived
from sedimentary rocks contain the largest quantities of Cd. Non-contaminated
soils contain very low concentration of Cd. In a survey, a total of 3,305 soil samples
were collected from crop-producing areas in 36 states of USA. Concentrations of
−1
Cd ranged from 0.005 to 2.40 mg kg , with mean and median values of 0.27 and
−1
0.20 mg kg , respectively (Holmgren et al. 1986 ). Surface soils around mining
areas contain high concentrations of Cd. For example, Munshower ( 1977 )
−1
reported 29 mg Cd kg in surface soil at 2 km northeast of a lead–zinc smelter
in Montana. The Cd concentration of surface soil decreased with distance and
reached near background levels 24 km from the source. Buchauer ( 1973 ) reported
−1
concentrations of Cd in the surface organic horizon as great as 750 mg kg . The
soils contaminated by mining operations will remain high in Cd more or less
permanently because Cd is quite immobile in soil. Cadmium concentration in
−1
soil solution is relatively low ranging from 0.2 to 6.0 μg L (Kabata-Pendias and
Pendias 2000 ).