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4 Remediation Mechanisms of Tropical Plants for Lead-Contaminated Environment 61

4.2 Phytoremediation

Traditionally techniques of soil remediation are costly and may cause secondary
pollution. Phytoremediation is an evolving field of science and technology to clean
up polluted soil, water, or air. It may be defined as the use of green plants to remove,
destroy, or sequester hazardous substances from the environment. Plants that
uptake heavy metals from the soil offer an alternative and less expensive method
to strip heavy metals directly from the soil. Plants have constitutive and adaptive
mechanisms for accumulating or tolerating high contaminant concentrations in
their rhizospheres (Yang et al. 2005). Phytoremediation takes advantage of the
fact that a living plant acts as a solar-driven pump, which can extract and concen-
trate certain heavy metals from the environment (Raskin et al. 1997).
Phytoremediation can provide a cost-effective, long-lasting aesthetic solution for
remediation of contaminated sites (Ma et al. 2001). It maintains the biological
properties and physical structure of the soil (Yang et al. 2005). One of the strategies
of phytoremediation of metal-contaminated soil is phytoextraction, i.e., uptake and
accumulation of metals into plant shoots, which can then be harvested and removed
from the site. Another application of phytoremediation is phytostabilization where
plants are used to minimize metal mobility in contaminated soils. Plant metal
uptake is influenced by soil factors including pH, organic matter, and cation
exchange capacity as well as plant species, cultivar, and age. The mobility and
availability of heavy metals in soil are generally low, especially when soil is high in
pH, clay, and organic matter (Jung and Thornton 1996; Rosselli et al. 2003). It is
important to use the native plants for phytoremediation because these plants are
often better in terms of survival, growth, and reproduction under environmental
stress than plants introduced from other environments. There has been a continuing
interest in searching for native plants that are tolerant to heavy metals; however,
studies have evaluated the phytoremediation potential of native plants under field
conditions (Shu et al. 2002; McGrath and Zhoa 2003; Abioye et al. 2012). Heavy
metals can cause severe phytotoxicity and may act as powerful force for the
evolution of tolerant plant populations. Therefore, it is possible to identify metal-
tolerant plant species from natural vegetation in the field sites that are contaminated
with various heavy metals. Hyperaccumulators which are often found growing in
polluted areas can naturally accumulate higher quantities of heavy metal in their
shoots than roots. In view of this fact, metal removal from soil can be greatly
enhanced by the judicious selection of plant species; the knowledge about the
ability of various plant species or tissues to absorb and transport metals will provide
an insight into choosing appropriate plants for phytoremediation (Deng et al. 2004;
Zhou and Song 2004). Identification of hyperaccumulators is an imperious and
important task as the key to successful implementation of phytoremediation (Zhou
2002; Zhou and Song 2004). The hyperaccumulators characterized at first were
members of the Brassicaceae and Fabaceae families (Salt et al. 1998). Presently at
least 45 families are known to contain metal accumulating species. To date, more
than 400 plant species of metal hyperaccumulator plants have been reported in the