5.1.4.2 Phytostabilization
The use of indigenous hyperaccumulator plants to reduce the migration of Cd
through the soil medium is known as phytostabilization (Robinson et al. 2006).
Hamzah et al. (2016) also reported the potential of using indigenous
hyperaccumulator plants to stabilize heavy metals, and such remediated contaminated soil could be used as agricultural soil again. Among the Cd
hyperaccumulators, Solanum nigrum, Populus sp., Salix calodendron, and Arabis
paniculata may be good candidates for field conditions due to their potentially
higher biomass (French et al. 2006; Ji et al. 2011). Catharanthus roseus (a valued
medicinal plant) was exposed to different concentrations of heavy metals like CdCl 2
and PbCl 2, and its bioaccumulation efficiency was observed. AAS analyses of leaves
of treated plants show 5–10% accumulation of cadmium, but there was no accumulation of lead at all (Pandey et al. 2007). Mojiri (2011) found the potential of corn
(Zea mays) for phytoremediation of soil contaminated with cadmium and lead.
Result indicated that corn is an effective accumulator plant for phytoremediation
of cadmium- and lead-polluted soils. Physalis minima a fast growing weed plant has
been studied for extraction of heavy metals like Cd and Cr. The bioconcentration
factor (BCF) and translocation factor (TF) showed that Physalis minima is a high
heavy metal accumulator (Subhashini and Swamy, 2013). Alfalfa had a better ability
of accumulating Cd, and results indicate that alfalfa had the higher biomass when
grown in soil polluted by different concentrations of heavy metals (Wang et al.
2015). Kathal et al. (2016) found that Cd concentration was found highest in plant as
compared to the seeds in Brassica juncea grown in Cd and control soil under
greenhouse condition.
5.1.4.3 Rhizofiltration
It is a technique used to remove heavy metal (Cd) from aquatic environment using
plants. Phytoremediation can be enhanced by using chelating agents such as citric
acid, EDTA, DTPA, and EGTA (Luo et al. 2006). However, excessive use of
chelating agents in the field results in secondary pollution of soils, and leaching of
chelating agents may risk groundwater contamination and increase the cost of
phytoremediation (Robinson et al. 2006).
5.1.4.4 Mechanism of Heavy Metal Detoxification and Uptake by Plants
Heavy metal chelation by high-affinity ligands in cytosol followed by compartmentalization of the metal–ligand complex is a very important mechanism of heavy
metal detoxification and tolerance by plants. Chelators may be natural, i.e., amino
acids, organic acids, metallothionein (MT), and phytochelatins (PC), or synthetic,
i.e., ethylene glycol tetra acetic acid (EGTA) and ethylene diamine tetra acetic acid
(ETDA). Addition of EDTA has been found to accelerate the uptake of Cd, Zn, Cu,
15 Cadmium as an Environmental Pollutant: Ecotoxicological Effects, Health. . .
375
The use of indigenous hyperaccumulator plants to reduce the migration of Cd
through the soil medium is known as phytostabilization (Robinson et al. 2006).
Hamzah et al. (2016) also reported the potential of using indigenous
hyperaccumulator plants to stabilize heavy metals, and such remediated contaminated soil could be used as agricultural soil again. Among the Cd
hyperaccumulators, Solanum nigrum, Populus sp., Salix calodendron, and Arabis
paniculata may be good candidates for field conditions due to their potentially
higher biomass (French et al. 2006; Ji et al. 2011). Catharanthus roseus (a valued
medicinal plant) was exposed to different concentrations of heavy metals like CdCl 2
and PbCl 2, and its bioaccumulation efficiency was observed. AAS analyses of leaves
of treated plants show 5–10% accumulation of cadmium, but there was no accumulation of lead at all (Pandey et al. 2007). Mojiri (2011) found the potential of corn
(Zea mays) for phytoremediation of soil contaminated with cadmium and lead.
Result indicated that corn is an effective accumulator plant for phytoremediation
of cadmium- and lead-polluted soils. Physalis minima a fast growing weed plant has
been studied for extraction of heavy metals like Cd and Cr. The bioconcentration
factor (BCF) and translocation factor (TF) showed that Physalis minima is a high
heavy metal accumulator (Subhashini and Swamy, 2013). Alfalfa had a better ability
of accumulating Cd, and results indicate that alfalfa had the higher biomass when
grown in soil polluted by different concentrations of heavy metals (Wang et al.
2015). Kathal et al. (2016) found that Cd concentration was found highest in plant as
compared to the seeds in Brassica juncea grown in Cd and control soil under
greenhouse condition.
5.1.4.3 Rhizofiltration
It is a technique used to remove heavy metal (Cd) from aquatic environment using
plants. Phytoremediation can be enhanced by using chelating agents such as citric
acid, EDTA, DTPA, and EGTA (Luo et al. 2006). However, excessive use of
chelating agents in the field results in secondary pollution of soils, and leaching of
chelating agents may risk groundwater contamination and increase the cost of
phytoremediation (Robinson et al. 2006).
5.1.4.4 Mechanism of Heavy Metal Detoxification and Uptake by Plants
Heavy metal chelation by high-affinity ligands in cytosol followed by compartmentalization of the metal–ligand complex is a very important mechanism of heavy
metal detoxification and tolerance by plants. Chelators may be natural, i.e., amino
acids, organic acids, metallothionein (MT), and phytochelatins (PC), or synthetic,
i.e., ethylene glycol tetra acetic acid (EGTA) and ethylene diamine tetra acetic acid
(ETDA). Addition of EDTA has been found to accelerate the uptake of Cd, Zn, Cu,
15 Cadmium as an Environmental Pollutant: Ecotoxicological Effects, Health. . .
375