Page 77 - Plant-Based Remediation Processes

4 Remediation Mechanisms of Tropical Plants for Lead-Contaminated Environment 65

absorption by calcium is well known (Garland and Wilkins 1981; Kim et al. 2002)
and is associated with competition between these two cations for calcium channels
2+
(Huang and Cunningham 1996). Several authors have demonstrated that Ca -
permeable channels are the main pathway by which lead enters roots (Wang et al.
2007; Pourrut et al. 2008). The use of transgenic plants has shown that lead can
penetrate into roots through alternative nonselective pathways, such as cyclic
nucleotide-gated ion channels (Arazi et al. 1999; Kohler et al. 1999), or via low-
affinity cation transporters (Wojas et al. 2007).
Reduced uptake and translocation of lead to aerial parts of vegetables is consid-
ered to be beneficial in preventing lead from entering the food chain. However,
reduced uptake and translocation of lead to aerial plant parts, when plants are used
to remediate polluted soils, is a major problem. Indeed, soil remediation requires
plants (hyperaccumulators) that can take high lead levels up and translocate it to
aerial plant parts with no or minimal toxicity. The amount of lead that moves from
soil to penetrate into plants can be measured by the transfer factor; transfer factor is
defined as the ratio that exists between the concentration of lead in the plant vs. the
concentration of lead in the soil (Arshad et al. 2008; Bi et al. 2010; Liu et al. 2010).
Transfer factor will be different for different plant species and will change as soil’s
physical and chemical properties are altered (Arshad et al. 2008; Bi et al. 2010; Liu
et al. 2010). Generally, plants having a transfer factor greater than 1 are categorized
as hyperaccumulators, whereas those with transfer factor less than 1 are termed as
non-accumulators of lead (Arshad et al. 2008).

4.3 Tropical Plants Useful in Remediation of Lead

4.3.1 Phytoextraction of Lead by Tropical Plants

Certain tropical plants have been identified which have the potential to uptake lead.
Many of these plants belong to the following families: Brassicaceae,
Euphorbiaceae, Asteraceae, Lamiaceae, and Scrophulariaceae. Brassica juncea,
commonly called Indian mustard, has been found to have a good ability to transport
lead from the roots to the shoots, which is an important characteristic for the
phytoextraction of lead (United States Environmental Protection Agency
2000a, b). The phytoextraction coefficient for Indian mustard (Brassica juncea)is
1
1.7 and it has been found that a lead concentration of 500 mg L is not phytotoxic
to this Brassica species (United States Environmental Protection Agency 2000a, b).
A phytoextraction coefficient is the ratio of the metal concentration found within
the surface biomass of the plant over the metal concentration found in the soil. Thus
the greater the coefficient, the greater the uptake of contaminant (Fig. 4.2) (United
States Environmental Protection Agency 2000). Some calculations indicate that
Brassica juncea is capable of removing 1,550 kg of lead per acre Thlaspi
rotundifolium sp. Cepaeifolium, a non-crop Brassica, commonly known as