sewage sludge to arable lands contribute to the load of metals in the soil (Giller et al.
1989; McGrath et al. 1995; Anderson 1997). Industrial and mining enterprises
directly discharge liquid effluent and solid waste; these discharges are not managed
strictly, leading to a decline of soil quality and HM pollution. Smoke and exhaust
emissions from these factories also contain heavy metals, which eventually enter the
soil through natural sedimentation and rain. The industrial effluents, both solid and
liquid, are highly enriched with HMs when compared with other soil background
values.
2.1 Coal Mining and Combustion
As one of the main sources of electric power, coal mining and combustion have
promoted the economy to a great extent. At the same time, the procurement,
machining, and combustion processes for electric power production from both raw
coal and waste by-products release numerous contaminants into the ecosystem. After
coal is combusted for electricity, the ash, enriched with toxic metals, could flow into
the atmosphere and then fall to the soil when wet or dry deposition occurs. The
residues of combustion will also pollute the soil if there are no measures for
separating them. The power industry wastes are HM enriched because the raw
materials have already undergone a natural concentration process in their formation.
2.2 Metals Mining and Smelting
The environmental problems associated with the mining and smelting of precious
and semiprecious metals, especially soil HM pollution, have a long history. As a
result of the constant increase in the market value of precious metals such as gold and
silver and other semiprecious metals (e.g., nickel), new sites are explored; mining
and smelting results in the addition of HM loads in China and other developing
countries. In the exploitation of photogenic sulfide deposits, HMs are easily poured
into the mine soil through natural oxidation, the long-term leaching of the discarded
sulfide minerals. When mine drainage and rainfall occur, the stream is mixed with
metal ions, which when released by acid wastewater will flow into the soil. Moreover, the dust particles produced in mining, transport, and the casting process cannot
be ignored as a soil HM source. The physical and chemical treatments employed in
the ore-smelting process to extract a specific metal frequently release other metals
(especially cadmium, arsenic) and constituents into the water that is used during the
process or other solid waste which results from the process. During heating
(smelting), metals are volatilized and emitted into the air in gas form, then adhere
to atmospheric dust particles when the temperature decreases.
4 Recent Advances in Phytoremediation of Toxic Metals from Contaminated. . .
83
1989; McGrath et al. 1995; Anderson 1997). Industrial and mining enterprises
directly discharge liquid effluent and solid waste; these discharges are not managed
strictly, leading to a decline of soil quality and HM pollution. Smoke and exhaust
emissions from these factories also contain heavy metals, which eventually enter the
soil through natural sedimentation and rain. The industrial effluents, both solid and
liquid, are highly enriched with HMs when compared with other soil background
values.
2.1 Coal Mining and Combustion
As one of the main sources of electric power, coal mining and combustion have
promoted the economy to a great extent. At the same time, the procurement,
machining, and combustion processes for electric power production from both raw
coal and waste by-products release numerous contaminants into the ecosystem. After
coal is combusted for electricity, the ash, enriched with toxic metals, could flow into
the atmosphere and then fall to the soil when wet or dry deposition occurs. The
residues of combustion will also pollute the soil if there are no measures for
separating them. The power industry wastes are HM enriched because the raw
materials have already undergone a natural concentration process in their formation.
2.2 Metals Mining and Smelting
The environmental problems associated with the mining and smelting of precious
and semiprecious metals, especially soil HM pollution, have a long history. As a
result of the constant increase in the market value of precious metals such as gold and
silver and other semiprecious metals (e.g., nickel), new sites are explored; mining
and smelting results in the addition of HM loads in China and other developing
countries. In the exploitation of photogenic sulfide deposits, HMs are easily poured
into the mine soil through natural oxidation, the long-term leaching of the discarded
sulfide minerals. When mine drainage and rainfall occur, the stream is mixed with
metal ions, which when released by acid wastewater will flow into the soil. Moreover, the dust particles produced in mining, transport, and the casting process cannot
be ignored as a soil HM source. The physical and chemical treatments employed in
the ore-smelting process to extract a specific metal frequently release other metals
(especially cadmium, arsenic) and constituents into the water that is used during the
process or other solid waste which results from the process. During heating
(smelting), metals are volatilized and emitted into the air in gas form, then adhere
to atmospheric dust particles when the temperature decreases.
4 Recent Advances in Phytoremediation of Toxic Metals from Contaminated. . .
83