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4 Remediation Mechanisms of Tropical Plants for Lead-Contaminated Environment 69

mining activities. Three grasses have been made commercially available after a
field study conducted in Liverpool, England (Recent developments for in situ
treatment of metals contaminated soil 1997):
• Agrostistenuis, cv Parys for copper waste
• Agrostistenuis, cv Coginan for acid lead and zinc wastes
• Festucarubra, cv Merlin for calcareous lead and zinc wastes

4.4.3 Rhizofiltration

Rhizofiltration is primarily used to remediate extracted groundwater, surface water,
and wastewater with low contaminant concentrations. It is defined as the use of
plants, both terrestrial and aquatic, to absorb, concentrate, and precipitate
contaminants from polluted aqueous sources in their roots. Rhizofiltration can be
used for Pb, Cd, Cu, Ni, Zn, and Cr, which are primarily retained within the roots
(United States Environmental Protection Agency 2000).
Sunflower, Indian mustard, tobacco, rye, spinach, and corn have been studied for
their ability to remove lead from water, with sunflower having the greatest ability.
In one study, after only 1 h of treatment, sunflowers reduced lead concentrations
significantly (Raskin and Ensley 2000). Indian mustard has a bioaccumulation
coefficient of 563 for lead and has also proven to be effective in removing a wide
1
concentration range of lead (4–500 mg L ) (Raskin and Ensley 2000). The
advantages associated with rhizofiltration are the ability to use both terrestrial and
aquatic plants for either in situ or ex situ applications. Another advantage is that
contaminants do not have to be translocated to the shoots. Thus, species other than
hyperaccumulators may be used. Terrestrial plants are preferred because they have
a fibrous and much longer root system, increasing the amount of root area (Raskin
and Ensley 2000). Disadvantages and limitations include the constant need to adjust
pH; plants may first need to be grown in a greenhouse or nursery; there is periodic
harvesting and plant disposal; tank design must be well engineered; and a good
understanding of the chemical speciation/interactions is needed. The cost of reme-
diation by rhizofiltration has been estimated to be $2–$6 per 1,000 gallons of water
(United States Environmental Protection Agency 2000a, b).

4.5 The Mechanism of Tropical Plants in Remediation of Lead

4.5.1 Mechanisms of Lead Tolerance

Plants respond to noxious effects of lead in various ways, such as selective metal
uptake, metal binding to the root surface, binding to the cell wall, and induction of
antioxidants. There are several types of antioxidants to which plants may respond: