Eco-Friendly Technologies for Heavy Metal Remediation: Pragmatic Approaches - Environmental Sustainability: Role of Green Technologies - page 197

(soil, sediment, sludge) or water through an
engineered containment system. In general, the
rate and extent of biodegradation are greater in a
bioreactor system than in in situ or in solid-phase
systems because the contained environment is
more manageable and hence more controllable
and predictable.

absorption of contaminants into the root, the
chemical migrates through the root xylem and
via the sap and eventually reaches the leaves
(Suresh and Ravishankar 2004 ). Thus, phytore-
mediation shows potential for accumulating,
immobilizing, and transforming a low level of
persistent contaminants. Based on the fate of
contaminants, phytoremediation techniques can
be categorized broadly into fi ve types such as
phytoextraction, phytostabilization, phytodegra-
dation, phytovolatilization, and rhizofi ltration
(Table 1 ).

3 Plant-Assisted

Bioremediation

Phytoremediation for environmental cleanup is a
promising tool, which is cost-effective, and has
aesthetic compensation and long-term applicabil-
ity. Phytoremediation is defi ned as the use of
plants for removal of environmental pollutants or
detoxifi cation to make them harmless. Macek
et al. ( 2000 ) have reported that the advantages of
phytoremediation, which is due to its eco-friendly
approach, public acceptance, and capability of
degradation of diverse range of contaminants.
Plants function in phytoremediation in two ways,
the major one being facilitation of favorable con-
ditions for microbial degradation, specifi cally by
plant root-colonizing microbes, and the second
aspect is plant root itself, providing a simple and
inexpensive means of accessing contaminants
existing in subsurface soils and water. Upon

3.1 Phytoextraction or

Phytoaccumulation

This is a process used by the plants to accumu-
late contaminants into the harvestable parts of
roots and aboveground shoots (Kumar et al.
1995 ; Chaney et al. 1997 ). This technique saves
tremendous remediation cost by accumulating
low levels of contaminants from a widespread
area. Unlike the degradation mechanisms, this
process produces a mass of plants and contami-
nants (usually metals) that can be then har-
vested and incinerated and the ash related to a
confi ned area or the heavy metals are extracted
from it.

Table 1 Categories of phytoremediation processes and mechanisms of contaminant removal
Sl. no. Process Plant mechanism Contaminant Substrate

1 Phytoextraction Hyperaccumulation, uptake and

concentration of metals via soils,
direct uptake into the plant tissue
with subsequent removal
of the plants

Inorganics Soils

2 Phytostabilization Complexation; root exudates

cause metal to precipitate soils,
groundwater, and mine tailing
and become less available

Inorganics Soils, groundwater,

mine tailing

3 Phytodegradation Degradation in plants, enhances

microbial degradation
in rhizosphere

Organics Soils, groundwater

within rhizosphere

4 Phytovolatilization Volatilization by leaves; plants

evapotranspirate selenium,
mercury, and volatile
hydrocarbons

Organics/inorganics Soils and

groundwater

5 Rhizofi ltration Rhizosphere accumulation, Organics/inorganics Surface water and

water pumped