54
Trace Elements in Abiotic and Biotic Environments
TABLE 9.1
Cadmium Contents in Soils, Water, and Air
Environmental Compartment
Range/Mean
Soil (mg/kg)
Light sandy
0.01–0.2
Medium loamy
0.08–0.3
Heavy loamy
0.2–0.5
Calcerous (calcisols)
0.4–0.8
Organic
0.2–2.5
Water (μg/L)
Rain
<0.01–0.05
River
0.02–0.05
Sea, ocean
0.07–0.11
Air (ng/m 3 )
Urban areas
0.2–150
Rural areas
0.1–4.0
Greenland
0.1–6.0
Antarctica
0.015
Source: Data are given for uncontaminated environments,
from Kabata-Pendias, A. and Mukherjee, A.B., Trace
Elements from Soil to Human, Springer, Berlin,
Germany, 2007; Reimann, C. and de Caritat, P.,
Chemical Elements in the Environment, Springer,
Berlin, Germany, 1998.
During weathering, Cd goes into soil solutions, where its concentrations vary
highly from 0.2 to 300  μg/L. It may occur as both cationic and anionic species,
as well as in organic chelates (Kabata-Pendias and Sadurski 2004). According to
Sposito and Page (1984), Cd speciation in soil solutions varies depending on soil
type, in oxic and acid soils: Cd 2+ , CdSO
q
4 and CdCl + ; and in alkaline soil: CdHCO 3
+ .
However, Cd in soil solution is present as free cation Cd 2+ between 55% and 90% and
is readily available to plants (Taylor and Percival 2001). Especially, interactions with
dissolved organic carbon affect the Cd speciation in soil solution (Ge et al. 2005).
Under anaerobic conditions, Cd in soil solution is governed by sulfide precipitation.
The highest proportion of Cd in soil occurs in residual fraction (~40%) and in fraction
adsorbed by hydrous oxides (~20%). After sludge application, more Cd became fixed
by metal hydrous oxides (Kabata-Pendias 2011). Wang et al. (2012) reported that Cd in
contaminated irrigated desert soil occurs mainly in the Fe–Mn oxide-bound fractions.
Cd fixation by metal hydrous oxides controls its behavior in soils at low pH (<5.5),
whereas at higher pH, mainly clays control the Cd fixation. In alkaline soil, at pH > 7.5,
two compounds, CdCO 3 and Cd 3 (PO 4 ) 2 , are mainly responsible for Cd behavior. There
are several evidences that Eh–pH controls the formation of these compounds, and thus