Page 209 - Soil Degradation, Conservation and Remediation
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198 6 Soil Pollution
and the effects from both are the same. Pb accumulates in the body organs (i.e., brain),
which may lead to poisoning (plumbism) or even death. The gastrointestinal tract,
kidneys, and central nervous system are also affected by the presence of lead. Children
exposed to lead are at risk for impaired development, lower IQ, shortened attention
span, hyperactivity, and mental deterioration. Adults usually experience decreased
reaction time, loss of memory, nausea, insomnia, anorexia, and weakness of the joints
when exposed to lead. Lead can cause serious injury to the brain, nervous system, red
blood cells, and kidneys (Baldwin and Marshall 1999 ). Lead performs no known
essential function in the human body, it can merely do harm after uptake from
food, air, or water. The most serious source of exposure to soil lead is through direct
ingestion (eating) of contaminated soil or dust. In general, plants do not absorb or
accumulate lead. However, in soils testing high in lead, it is possible for some lead to
be taken up. Studies have shown that lead does not readily accumulate in the fruiting
parts of vegetable and fruit crops (e.g., corn, beans, squash, tomatoes, strawberries,
and apples). Higher concentrations are more likely to be found in leafy vegetables
(e.g., lettuce) and on the surface of root crops (e.g., carrots). Since plants do not take
up large quantities of soil lead, the lead levels in soil considered safe for plants will be
much higher than soil lead levels where eating of soil is a concern (pica). Generally, it
has been considered safe to use garden produce grown in soils with total lead levels
less than 300 ppm. The risk of lead poisoning through the food chain increases as the
soil lead level rises above this concentration. Even at soil levels above 300 ppm, most
of the risk is from lead-contaminated soil or dust deposits on the plants rather than
from uptake of lead by the plant (Rosen 2002 ).
The proportion of total soil Pb that exists in the soil solution is very small. Alloway
−1
et al. ( 1985 ) found Pb concentrations ranging from 61 to 12,537 μg L in soil solutions
enriched from a weathering Pb ore and soil contaminated by Pb/Zn mining and
2+
sewage sludge. Lead may exist in the soil solution as the free metal ion, Pb , and
soluble organic and inorganic complexes. The speciation in the soil solution depends
to a large extent on pH, dissolved organic carbon, and the presence of ligands. Lead
2+
activity decreases with increasing soil pH. It is widely acknowledged that Pb is the
most significant Pb species in solution at pH values less than either 7 (Wang and
Benoit 1997 ) or 8 (Lindsay 1979 ). The dominant form of Pb in solution at pH values
greater than 7 is PbCO 3 (Wang and Benoit 1997 ). Other minor Pb species include
+
+
+
PbOH , Pb(OH) 2 , Pb(OH) 3 , PbCl 3 , PbCl , PbNO 3 , and Pb(CO 3 ) 2 (Wang and Benoit
2+
1997 ). Lead exists as Pb in solution at pH 4 (Markus and McBratney 2001 ).
Mercury
Mercury belongs to the same group of the periodic table with Zn and Cd. It is the
only liquid metal at standard temperature and pressure. It has atomic number 80,
−3
atomic weight 200.6, density 13.6 g cm , melting point 13.6 °C, and boiling point
357 °C. It is generally obtained as a by-product of ore processing (Smith et al.
2+
1995 ). After its release to the environment, Hg usually exists in mercuric (Hg ),
0
2+
mercurous (Hg 2 ), elemental (Hg ), or alkylated form (methyl/ethyl mercury). The
redox potential and pH of the system determine the stable forms of Hg that will be