Biotechnological Aspects of Soil Decontamination - Biotechnologies and Biomimetics for Civil Engineering - page 381

tolerance and accumulate toxic metals within roots and above-ground tissues, such
as shoot, flower, stem, and leaves, etc. (Barcelo and Poschenrieder 2003 ). Such
extraordinary ability of plants to accumulate heavy metals is described as hyper-
accumulators. Hyperaccumulators are able to accumulate Zn concentration higher
than 1 % and Cu, Pb, and Ni higher than 0.1 % of the tissue weight. The current
criterion used to define a hyperaccumulator is a plant that can accumulate metal to
a concentration that is 100 times greater than ''normal'' plants growing in the same
environment (Brooks 1977 ; Reeves and Brooks 1983 ). This particular capacity to
accumulate and tolerate large metal concentrations has opened up the possibility to
use for remediation of polluted soils and waters. The use of hyperaccumulator
plants to remove, destroy, or sequester hazardous toxic heavy metals is termed
phytoremediation (Schnoor 1997 ).
Hyperaccumulator plants such as Thlaspi, Utrica, Chenopodium, Polygonum
sachalase, and Alyssim have the capability to accumulate cadmium, lead, nickel,
and zinc (Baker et al. 1991 ). In addition to hyperaccumulator, plants such as trees
(Poplar) and grasses (Vetiver) are now being actively evaluated though, their metal
bioconcentrating capability is well below that of hyperaccumulator plants (Burken
and Schnoor 1998 ; Sebastiani et al. 2004 ). Decontamination of soil from radio-
active contaminants by phytoremediation has been reported (Van denhove 2013 ).
Two novel approaches, the use of agrobacterium transformed plant roots and
mycelia cultures of fungi have been reported as research tool in the study of
remediation of contaminated soil by Wenzel et al. ( 1999 ).
Phytoremediation is often also referred as botanical bioremediation or green
remediation (Chaney et al. 1997 ). It involves the repeated cropping of plants on
heavy metal contaminated soils until the soils metal concentrations have reached
acceptable levels. After each cropping, the plant biomass is removed from the area
and may be washed to reduce its volume where upon it can be stored in an
appropriate area that does not pose a risk to the environment (Raskin et al. 1994 ;
Chaney et al. 1997 ). Currently phytoremediation is used for treating many classes
of contaminants including elemental (heavy metals and radionuclides) as well
as organic pollutants (PCBs, PAHs, nitroaromatics) (Cunningham et al. 1996 ;
Dushenkov 2003 ). This is most applicable to shallow soils with low levels of
contamination. The main disadvantage is that longer times are required compared
to other methods. Phytoremediation can be classified according to the mechanism
and nature of contaminant (Table 17.1 ).
17.4.6.1 Phytoextraction
It is the biological approach to remove the contamination primarily from soil and
isolate it, without destroying the soil structure and fertility. It is also called phy-
toaccumulation. It is the process that involves the uptake and translocation of
heavy metals by roots into the above ground portions of ''hyperaccumulator''
plants (Brown et al. 1994 ) (Table 17.2 ).