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non-protein thiol (NP-SH), cysteine, glutathione, ascorbic acid, proline, and anti-
oxidant enzymes, such as superoxide dismutase (SOD), ascorbate peroxidase
(APX), guaiacol peroxidase (GPX), catalase (CAT), and glutathione reductase
(GR). However, the response varies with plant species, metal concentration, and
exposure conditions.

4.5.1.1 Chelates Assisted Mechanism in Phytoextraction of Lead

One major factor limiting the potential for lead phytoextraction is low metal
bioavailability for plant uptake (Raskin and Ensley 2000). To overcome this
limitation, synthetic chemical chelators may need to be added to the contaminated
soil to increase the amount of lead that is bioavailable for the plants. The use of
synthetic chelates in the phytoremediation process is not only to increase heavy
metal uptake by plants through increasing the bioavailability of the metal, but also
to increase micronutrient availability, which decreases the possibility of plant
nutrient deficiencies (Blaylock and Huang 1999). The goal of commercial
phytoextraction is to remove or reduce the level of toxic metals within the
contaminated soils to meet regulatory standards within 1–3 years (Raskin and
Ensley 2000). The regulatory standard for lead-contaminated soil set by the EPA
is 500 ppm. Plants that accumulate more than 1 % of the target contaminant in the
harvestable portion and produce more than 20 metric tons of shoot biomass per
hectare per year are required to achieve this goal (Raskin and Ensley 2000).
Researchers have found that through the application of soil amendments and
chemical chelates this goal can be achieved. Based on scientific studies, it has
been shown that only 0.1 % of the total amount of lead in contaminated soils is in
solution and bioavailable to plants for remediation. With the addition of synthetic
chelators, the total amount of lead in solution can be increased up to 100 times
(Raskin and Ensley 2000). Increasing the mobility and bioavailability of lead in the
soil through certain chelators, organic acids, or chemical compounds allows for the
hyperaccumulation of metals in some plants. For lead, a number of different
chelators have been tested: EDTA (ethylene-dinitrilo-tetra acetic acid), CDTA
(trans-1,2-cyclohexylene-dinitrilo-tetra acetic acid), DTPA (diethylenetrinitrilo-
penta acetic acid), EGTA (ethylebis[oxyethylenetrinitrilo]-tetra acetic acid),
HEDTA (hydroxyethyl-ethylene-dinitrilo-tri acetic acid), citric acid, and malic
acid. Addition of the chelates resulted in enhanced shoot lead concentrations.
EDTA proved to be the best and least expensive, costing around $1.95 per pound.
In soils with a pH of 5 and amended with EDTA, plants accumulated nearly
1
2,000 mg kg more lead in their shoots when compared to other treatments in
soil limed to a pH of 7.5. EDTA, DTPA, and CDTA all achieved shoot lead
1
concentrations of more than 10,000 mg kg .
1
In order for substantial lead accumulation (>5,000 mg kg ) to occur in the
shoots, the concentration of synthetic chelates (EDTA, DTPA, and CDTA) must
1
exceed 1 mol kg . It was also noted that plants grown in soils amended with
chelators varied in their lead concentration uptake. For example, the lead