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7 Use of Wetland Plants in Bioaccumulation of Heavy Metals 129

Phytoremediation process, thus, may be improved using plant-associated
microorganisms that alter the solubility, availability, and transport of trace elements
and nutrients by reducing soil pH, secretion of chelators and siderophores, or redox
changes. Selenium (Se) phytoremediation (accumulation and volatilization) by
Indian mustard (Brassica juncea) was most effective in the presence of plant
growth promoting rhizobacteria (de Souza et al. 1999). Available data suggests
that bacteria such as Azotobacter chroococcum (N 2 -fixer), Bacillus megaterium
(P-solubilizer), and Bacillus mucilaginosus (K-solubilizer) and Bacillus sp. RJ16
can decrease soil pH, probably by excreting low weight molecular acids, enhancing
the bioavailability of heavy metals like Cd and Zn for plants (Morant et al. 2003;
Wu et al. 2006; Sheng and Xia 2006). It has been seen that the presence of different
rhizobacteria associated with three plants, Alyssum murale, A. serpyllifolium subsp.
lusitanicum, Thlaspi caerulescens, increased the potentiality of heavy metal accu-
mulation to their bodies (Whiting et al. 2001; Cloutier-Hurteau et al. 2008; Becerra-
Castro et al. 2009). Rhizosphere actinobacteria Alnus glutinosa living in symbiosis
with N 2 -fixing Frankia were found to tolerate more than 2.0 mM Ni along with
the increase yield of the plant (Wheeler et al. 2001). Likewise, a bacterial mixture
of bacteria Microbacterium saperdae, Pseudomonas monteilii, and Enterobacter
cancerogenus helped in higher zinc extraction by plants like T. caerulescens
(Delorme et al. 2001).
For wastewater treatment in wetlands, establishing a dense stand of vegetation is
more important than selecting a particular species. Any species that will grow well
can be chosen. However, for storm water wetlands, native plant species work best.
Selecting native, local plant species for wetland restoration is required as the plants
are adapted to the local climate, soils, and surrounding plant and animal
communities, and are likely to do well (Fig. 7.2). As for example, Bulrushes
(Scirpus sp.) are widely used in treating sewage and wastewaters due to their ability
to withstand high levels of nutrients, establish easily and noninvasive nature. Like
that, arrowhead (Sagittaria sp.) and pickerelweed (Pontederia cordata) may be
used in agricultural wetlands. The efficiency of water hyacinth (Eichhornia
crassipes) for nutrient uptake and their rapid growth rate have put them to use for
many years in cleaning up municipal and industrial wastewater (Vesk et al. 1999;
Lombi et al. 2000; Prasad et al. 2001; Prasad 2007; Espinoza-Quinones et al. 2009).
Water hyacinth has been shown to accumulate trace elements and as the recycling
process is run by photosynthetic activity and biomass growth, the process is
sustainable and is also energy and cost efficient (Garbisu et al. 2002; Lu et al.
2004; Bertrand and Poirier 2005). Few aquatic plants, as mentioned in the Table 7.1,
have already been identified for their potential role in the remediation of metal-
contaminated areas (Prasad et al. 2001; Prasad 2007).