Page 17 - Plant-Based Remediation Processes

4 S. Chatterjee et al.

3+
effectively reduce the highly toxic and soluble Cr 6+ compounds to insoluble Cr ,
which does not pose an environmental risk (James 1996). Chemical species of Pb in
soil are usually somewhat bioavailable, whereas, chloropyromorphite, a Pb phos-
phate mineral is both extremely insoluble and non-bioavailable (Ma et al. 1995).
The roots of Agrostis capillaris growing in highly contaminated Pb/Zn mine wastes
are known to form pyromorphite from soil lead and phosphate by an unknown
mechanism, thus minimizing the escape of lead movement (Cotter-Howells and
Capom 1996). Advantage of using grass species for phytostabilization is that they
bioaccumulate less metals in their shoots in comparison to dicot species, in this way
minimizing exposure of wildlife to toxic elements (Pilon-Smits 2005).

1.3 Phytoextraction

Phytoextraction involves the cultivation of higher plants that concentrate and
translocate soil contaminants in their above ground tissues that can be harvested
at the end of the growth period (Salt et al. 1998). It is the most effective among
several phytoremediation methods, although technical difficulties are there for its
applications (Kramer 2005). Selection of suitable plant species is crucial for
effective phytoextraction and biomass derived from shoot of a phytoremediator
crop plant should be capable of depositing metal(oid) species at concentration
50–500 times higher than those in the contaminated soil substrate (Kramer 2005).
The best-known natural hyperaccumulators plants are alpine pennycress (Thlaspi
2+
caerulescens L.) capable of hyperaccumulating Zn , and occasionally Cd 2+ and
Ni 2+ (Milner and Kochian 2008), the serpentine endemic shrub Alyssum sp., Indian
mustard Brassica juncea (Brassicacea) and Astragalus racemosus (Leguminosae).
The Asian stonecrop Sedum alfredii (Crassulaceae) has gained increased attention
due to higher accumulation rate of Zn, Cd, and Pb (Lu et al. 2008; Deng et al. 2008).
Plants ideal for phytoextraction besides having an inherent capacity to tolerate
and hyperaccumulate metals should possess multiple traits like (1) high and fast
growing biomass; (2) extensively branched root systems; (3) ability to grow outside
their area of collection; (4) relatively easy to cultivate; and (5) possible repulsive
to herbivores to avoid the escape of accumulated metals to the food chain (Seth
2012). Unfortunately, most of the naturally hyperaccumulating plants have slow
growth, poor biomass, and often strong association with a specific habitat, therefore
limiting the phytoextraction potential (Chaney et al. 2005). However, non-
hyperaccumulator plants having higher growth rate and biomass could be modified
or engineered to achieve the above-mentioned attributes. To increase the potential
of phytoextraction, factors limiting trace element accumulation in plants have to be
resolved, which may include mobilization of poorly available contaminant in the
soil, root uptake, sequestration by metal-complex formation and deposition in
vacuoles for detoxification within roots, translocation to symplast, efficient xylem
loading, distribution and storage inside the aboveground organ and tissues, and
eventually expulsion of accumulated metal to less metabolically active cells, e.g.,
trichomes (Clemens et al. 2002). Two approaches are currently being explored to