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4 Remediation Mechanisms of Tropical Plants for Lead-Contaminated Environment 67

(Huang et al. 1997). One way to increase Pb solubility is to lower the pH.
Decreasing the soil pH has also been shown to increase the amount of metal that
a plant can accumulate (Huang et al. 1997). Alternatively, chelating agents such as
EDTA have the potential to increase the bioavailability of metal in the soil solution
and may increase the amount of metal accumulated in plant tissue (Huang et al.
1997).

4.4 Methods of Phytoremediation

Phytoremediation is actually a generic term for several ways in which plants can be
used to clean up contaminated soils and water. Plants may break down or degrade
organic pollutants, or remove and stabilize metal contaminants. This may be done
through one of or a combination of the methods described.

4.4.1 Phytoextraction (Phytoaccumulation)

Phytoextraction is primarily used for the treatment of contaminated soils (United
States Environmental Protection Agency 2000a, b). To remove contamination from
the soil (Fig. 4.2), this approach uses plants to absorb, concentrate, and precipitate
toxic metals from contaminated soils into the aboveground biomass (shoots, leaves,
etc.) (Emerging Technologies for the Phytoremediation of Metals in Soil 1997).
Discovery of metal hyperaccumulator species demonstrates that plants have the
potential to remove metals from contaminated soils. A hyperaccumulator is a plant
species capable of accumulating 100 times more metal than a common non-
accumulating plant. Thus, a hyperaccumulator will concentrate more than
1,000 Fg g 1 (0.1 %) of Co, Cu, Cr, Pb, or 1 % of Zn and Ni in their leaf dry
matter. Most hyperaccumulator species accumulate Ni while others have been
shown to accumulate Cd, Co, Cu, and Zn. Currently there are no known Pb
hyperaccumulators. Certain plants can extract lead from contaminated soils, but
only when certain soil amendments have been added (United States Environmental
Protection Agency 2000a, b). There are several advantages of phytoextraction. The
cost of phytoextraction is fairly inexpensive when compared to conventional
methods. For example, phytoremediation of an acre site contaminated with lead
was estimated to require 30 years and cost $200,000 compared to $12 million for
excavation and disposal, $6,300,000 for soil washing, and 600,000 for a soil cap
(United States Environmental Protection Agency 2000a, b). Another benefit is that
the contaminant is permanently removed from the soil (Emerging Technologies for
the Phytoremediation of Metals in Soil 1997). In addition, the amount of waste
material that must be disposed of is substantially decreased up to 95 % (United
States Environmental Protection Agency 2000), and in some cases, the contaminant
can be recycled from the contaminated plant biomass. The use of hyperaccumulator