A. Induced phytoextraction is also known as chelate-assisted phytoextraction.
Chelating agent can make the pollutant bioavailable. The chelating agents like
ethylene di-amine tetraacetic acid (EDTA) enhance the biosorption and the
accumulation ability of non-hyperaccumulating plant. Plants gain affinity toward
the metal by the chelate agent, and thus metal accumulation efficiency of the
plants gets enhanced. An outline of chelate-assisted phytoextraction of contaminated site is as follows:
• Evaluation of the site to be treated and determination of chelate and crop
combination.
• Preparation of the site by planting the plants.
• After the optimal biomass is reached, the soil is subjected to the metal chelate.
• After the metal accumulation phase starts, the crop can be harvested.
Depending on the plant and its seasons, the plant can be planted in the
contaminated or treatment site [34]. Kos and Leštan [35] had investigated the
effect of citric acid, EDTA, diethylene-tri-amine-penta-acetate (DTPA), and [SS]-stereoisomer of ethylenediamine disuccinate (EDDS) on phytoextraction of
copper from vineyard soil using Brassica rapa var. pekinensis. It was found that
EDDS showed better result of removal of Cu than the other chelates. Another
study showed that EDDS was effective than EDTA in terms of stimulation in
translocating metals to the shoots [36]. Metal-chelate complex is transported
through the xylem of the plant in the shoots. Water is found to evaporate;
however, metal-chelate complex remains. Water evaporates, leaving behind
the metal-chelate complex. Thus, the plant acts like a wick that transports the
metal ions from the soil into the leaves [34].
B. Continuous phytoextraction
Continuous phytoextraction is the utilization of a metal hyperaccumulating plant
which can accumulate, translocate, and also tolerate the high metal concentration
over the complete operation cycle. Phytoextraction of organic pollutant depends
on the absorption, translocation, and metabolism of organic pollutants in plants
[37]. Some organic compounds penetrate through the cell membrane easily and
enter into plant cells. Medicago sativa and Tagetes patula are suitable for the
phytoremediation of soils contaminated with phthalic esters and polycyclic
aromatic hydrocarbons (PAHs) [38–40]. Transformation and sequestration of
organic pollutants in plants are aided by several detoxification enzymes such as
cytochrome P450 enzymes (CYP) which catalyze the emulsification of hydrophobic pollutants [41, 42] and glutathione S-transferases (GSTs) which catalyze
conjugation of sulfhydryl (-SH) group of glutathione (GSH) and the organic
pollutant [43].
Epelde et al. [44] reported that a hyperaccumulator, Thlaspi caerulescens, is
the most widely studied plant for phytoextraction of metals due to its ability to
phytoextract Zn and Cd to a great extent from polluted soil. One of the drawbacks of employing metal hyperaccumulators for metal remediation is that they
yield low biomass, the growth rates are slow, and there is scarcity of such
hyperaccumulators for growth in metallic pollutant environment rich in lead,
uranium, arsenic, etc. [34].
50
S. Sophia and V. Shetty Kodialbail
Chelating agent can make the pollutant bioavailable. The chelating agents like
ethylene di-amine tetraacetic acid (EDTA) enhance the biosorption and the
accumulation ability of non-hyperaccumulating plant. Plants gain affinity toward
the metal by the chelate agent, and thus metal accumulation efficiency of the
plants gets enhanced. An outline of chelate-assisted phytoextraction of contaminated site is as follows:
• Evaluation of the site to be treated and determination of chelate and crop
combination.
• Preparation of the site by planting the plants.
• After the optimal biomass is reached, the soil is subjected to the metal chelate.
• After the metal accumulation phase starts, the crop can be harvested.
Depending on the plant and its seasons, the plant can be planted in the
contaminated or treatment site [34]. Kos and Leštan [35] had investigated the
effect of citric acid, EDTA, diethylene-tri-amine-penta-acetate (DTPA), and [SS]-stereoisomer of ethylenediamine disuccinate (EDDS) on phytoextraction of
copper from vineyard soil using Brassica rapa var. pekinensis. It was found that
EDDS showed better result of removal of Cu than the other chelates. Another
study showed that EDDS was effective than EDTA in terms of stimulation in
translocating metals to the shoots [36]. Metal-chelate complex is transported
through the xylem of the plant in the shoots. Water is found to evaporate;
however, metal-chelate complex remains. Water evaporates, leaving behind
the metal-chelate complex. Thus, the plant acts like a wick that transports the
metal ions from the soil into the leaves [34].
B. Continuous phytoextraction
Continuous phytoextraction is the utilization of a metal hyperaccumulating plant
which can accumulate, translocate, and also tolerate the high metal concentration
over the complete operation cycle. Phytoextraction of organic pollutant depends
on the absorption, translocation, and metabolism of organic pollutants in plants
[37]. Some organic compounds penetrate through the cell membrane easily and
enter into plant cells. Medicago sativa and Tagetes patula are suitable for the
phytoremediation of soils contaminated with phthalic esters and polycyclic
aromatic hydrocarbons (PAHs) [38–40]. Transformation and sequestration of
organic pollutants in plants are aided by several detoxification enzymes such as
cytochrome P450 enzymes (CYP) which catalyze the emulsification of hydrophobic pollutants [41, 42] and glutathione S-transferases (GSTs) which catalyze
conjugation of sulfhydryl (-SH) group of glutathione (GSH) and the organic
pollutant [43].
Epelde et al. [44] reported that a hyperaccumulator, Thlaspi caerulescens, is
the most widely studied plant for phytoextraction of metals due to its ability to
phytoextract Zn and Cd to a great extent from polluted soil. One of the drawbacks of employing metal hyperaccumulators for metal remediation is that they
yield low biomass, the growth rates are slow, and there is scarcity of such
hyperaccumulators for growth in metallic pollutant environment rich in lead,
uranium, arsenic, etc. [34].
50
S. Sophia and V. Shetty Kodialbail