Page 208 - Soil Degradation, Conservation and Remediation

6.2 Sources of Soil Pollutants 197

Cadmium may be adsorbed by clay minerals, carbonates, or hydrous oxides of
iron and manganese or may be precipitated as cadmium carbonate, hydroxide, and
phosphate. Adsorption mechanisms may be the primary source of cadmium removal
from soils. In soils and sediments polluted with metal wastes, the greatest
percentage of total cadmium was associated with the exchangeable fraction.
Cadmium concentrations have been shown to be limited by cadmium carbonate
in neutral and alkaline soils.
Cadmium in the human body is known to affect several enzymes. It is believed that
the renal damage that results in proteinuria is the result of Cd adversely affecting enzymes
responsible for reabsorption of proteins in kidney tubules. Cadmium also reduces the
activity of delta-aminolevulinic acid synthetase, arylsulfatase, alcohol dehydrogenase,
and lipoamide dehydrogenase, whereas it enhances the activity of delta-aminolevu-
linic acid dehydratase, pyruvate dehydrogenase, and pyruvate decarboxylase (Manahan
2003 ). The most spectacular and publicized occurrence of cadmium poisoning resulted
from dietary intake of cadmium by people in the Jinzū River Valley, near Fuchu, Japan.

Lead

Lead (Pb) is a bright and silvery metal which is soft, ductile, malleable, and resistant
to corrosion. Lead belongs to group IV and period 6 of the periodic table. It has
−3
atomic number 82, atomic mass 207.2, density 11.4 g cm , melting point 327.4 °C,
and boiling point 1,725 °C. Lead is generally found as a mineral combined with
other elements, such as sulfur (i.e., PbS, PbSO 4 ) or oxygen (PbCO 3 ). It ranges from
−1
10 to 30 mg kg in the earth’s crust (USDHHS 1999 ). Typical mean Pb concentration
−1
−1
for surface soils worldwide averages 32 mg kg and ranges from 10 to 67 mg kg
(Kabata-Pendias and Pendias 2001 ). Some metals are alloyed with lead; they are
antimony (in storage batteries), calcium (Ca) and tin (Sn) (in maintenance- free
storage batteries), silver (Ag) (for solder and anodes), strontium (Sr) and Sn (as
anodes in electrowinning processes), tellurium (Te) (pipe and sheet in chemical
installations and nuclear shielding), Sn (solders), and antimony (Sb) and Sn (sleeve
bearings, printing, and high-detail castings) (Manahan 2003 ). The general forms of
lead that are released to the environment are ionic lead, Pb(II), lead oxides and
hydroxides, and lead–metal oxyanion complexes. The most stable forms of lead are
Pb(II) and lead–hydroxy complexes. The predominant insoluble Pb compounds are
lead phosphates, lead carbonates, and lead hydroxides. Lead sulfide (PbS) which is

the most stable solid form in soil forms under reduced conditions. Under anaerobic
conditions a volatile organolead (tetramethyl lead) can be formed due to microbial
2−
2+
alkylation. Lead (II) compounds are predominantly ionic (e.g., Pb SO 4 ), whereas
Pb(IV) compounds tend to be covalent (e.g., tetraethyl lead, Pb(C 2 H 5 ) 4 ). Some Pb
(IV) compounds, such as PbO 2 , are strong oxidants. In addition to the inorganic
compounds of lead, there are a number of organolead compounds such as tetraethyl
lead which was earlier widely used as a gasoline additive.
More than 1,000 organolead compounds have been synthesized for commercial
and toxicological reasons. They are largely limited to the alkyl (methyl and ethyl) lead
compounds and their salts. Inhalation and ingestion are the two routes of exposure,