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7 Use of Wetland Plants in Bioaccumulation of Heavy Metals 121

are considered to be the best candidates among all living organisms to remediate
most of the environmental contaminants, especially inorganic contaminants like
heavy metals into the natural biogeochemical cycle (Lovley 2003).

7.4 Phytoremediation: The Process Overview

Phytoremediation (Ancient Greek: phyto-“plant”, and Latin remedium-“restoring
balance”) is a low-cost, natural solar-powered, environment-friendly, less/no main-
tenance, aesthetically pleasing technology that can treat diverse environmental
pollutants including heavy metals. It is a better alternative to costly mechanized
methods like extraction, pump and treat systems, or soil washing.
Phytoextraction, phytostabilization, rhizofiltration, and phytovolatilization are
the basic mechanisms of phytoremediation technology by which plant uptake heavy
metals. Phytoextraction involves the uptake/absorption and translocation of heavy
metals by roots into the above ground parts (shoots) of the plants. Shoot part of the
plant may be harvested periodically and incinerated for energy and the ash may be
recycled for metals. In general, metal uptake and phytoextraction coefficients
decrease in the order Cr 6+ > Cd 2+ > Ni 2+ > Zn 2+ > Cu 2+ > Pb 2+ > Cr 3+ (USEPA
2000). Immobilization of contaminants using certain plant species in the soil and
groundwater is the basis of phytostabilization. The process involves either adsorp-
tion of contaminants onto roots or precipitation within the root zone avoiding their
exodus in soil or movement by erosion. Rhizofiltration is the technology for cleaning
up communal wastewater, where adsorption or precipitation onto plant roots or
absorption and sequestration of contaminants take place in the roots that are present
in the adjacent solution (Fig. 7.1). Contaminant uptake and transpiration by a plant is
known as phytovolatilization. It occurs as along with the growth of the plant as it
takes up water along with the pollutant (i.e., for Hg, Se, As) (Noctor et al. 1998;
Meagher 2000; Lasat 2000; Salido et al. 2003; Ghosh and Singh 2005; Tangahu et al.
2011; Using phytoremediation to Clean Up Sites http://www.epa.gov/superfund/
accomp/news/phyto.htm; accessed on 30-8-2012). Again, many plants have the
capacity to accumulate heavy metals at much higher concentration without affecting
their metabolic process. A plant of this category may be hyperaccumulator when it
can concentrate the pollutants in a least proportion which differs according to the
1
pollutant concerned (e.g., more than 1,000 mg kg of dry weight for chromium,
1
copper, cobalt, nickel, or lead or more than 10,000 mg kg for zinc or manganese)
due to adaptive evolution towards hypertolerance or phytotolerance. Metal
hyperaccumulation in plants may lead to several interactions like defense, mutual-
ism (mycorrhizae, pollen, and seed dispersal), interferences with neighboring
plant species, commensalism, and biofilm formation (Baker and Brooks 1989;
Barron 2003; Michel et al. 2007; Burken et al. 2011).