Phytoextraction of Trace Metals: Principles and Applications - Environmental Sustainability: Role of Green Technologies - page 209

more than one TE might require sequential
phytoextraction with different species, a process
that will lengthen the time needed to clean up
the site. On average, the time required for phyto-
extraction to clean even a moderately contami-
nated soil is in the order of decades.
Induced phytoextraction to increase metal
availability has been studied in the past. It con-
sists in addition of solubilising metal chelators
(i.e. EDTA, ethylenediaminetetraacetate) to the
soil to increase the mobility and allowing the
metals to be taken up more easily by the plant.
However, induced phytoextraction has no appli-
cation in situ because it poses an environmental
risk through metal leaching to groundwater and it
is cost prohibitive (Chaney et al. 2007 ).
A more promising approach to increase the
biomass and obtain hyperaccumulation is being
studied where all genes needed for hyperaccumu-
lation and hypertolerance are cloned and
expressed in a high biomass plant (Cherian and
Oliveira 2005 ; Clemens et al. 2002 ; Dhankher
et al. 2012 ; Rugh et al. 1998 ). However, the
development of a high biomass bioengineered
metal hypertolerant and hyperaccumulator plants
is still in its infancy. Hyperaccumulator traits are
expressed in several genes, and the engineering
of transgenic plants might become impractical
beyond a certain number of genes (Krämer 2010 ).
Phytoextraction is a low-cost environmentally
friendly technology, but the fact that it is 'time
consuming' (Conesa et al. 2012 ) adds additional
unpredictable costs and makes it less appealing
relative to engineering methods or nonaction.
One of the major drawbacks of phytoextraction
that hinders the commercial development of the
technology is the cost effi ciency. Growing plants
for cleanup of a contaminated soil is costly, and
the cost of production (fertilisers, irrigation
water, harvest machines, etc.) adds up to the cost
of biomass disposal. Farmers and landowners
may desire soil cleanup, but the decision to take
action is dependent upon the economics of their
farm operations.
The importance of agricultural management
practices for producing high yield for phytoextrac-
tion crops has often been overlooked. Chaney et al.

( 2007 ) describe the agronomy of phytoextraction
and point out the importance of practices such as
fertilisation, pH optimising, weed control, and
scheduling of harvest as critical fi eld management
practices for the success of phytoextraction.
However, fi eld management operation and agricul-
tural practices need to be included in the cost analy-
sis of phytoextraction and in models to estimate
value and cost of phytoextraction products
(Robinson et al. 2003b ). When the phytoextraction
is combined with a profi t-making operation, then
the time constraint may become less important.
In addition, when the phytoextraction product
(biomass) has commercial value (timber, bioen-
ergy, fertiliser, food supplement, etc.), it can be
sold to offset the cost of production and farm prac-
tices operation.
There are only few successful demonstrations
of the possibility of using phytoextraction in fi eld
sites and production of secondary products that
offset the costs of production. Perhaps the most
economically viable example is the use of
Se-enriched crops (i.e. Opuntia fi cus - indica
(cactus pear) and Brassica oleracea L. (broccoli))
(Bañuelos et al. 2012 ) for human consumption and
production of forages ( Brassica napus L., canola)
with enough Se to replace Se supplements nor-
mally added to livestock feed (Bañuelos 2006 ).
The oil and seed meal extracted from Brassica
seeds grown in Se-rich soils of the San Joaquin
Valley in Central California have been used as
source of biofuels, green fertilisers, and bioherbicide
(Bañuelos 2009 ; Bañuelos and Hanson 2010 ).
Two other important ways to combine phyto-
extraction with production of secondary prod-
ucts, such as biomass for bioenergy and recovery
of metals from the plant as bio-ore, are described
in the following sections.

3 Phytoextraction

and Bioenergy Production

Biofuels are renewable fuels derived from
biological feedstock and are largely carbon neu-
tral because the CO 2 released during biofuels
combustion is offset by carbon fi xation during