[28]. Plants have the ability to lower the pH and oxygenate the sediment in the soil
which affects the availability of the metals [116].
In the soil, the solubility and mobility of heavy metals are reduced naturally
through sorption, precipitation, and complexation reactions [117, 118]. Addition of
organic compounds to soils increases its cation exchange capacity [119] and accelerates the attenuation process [120]. This increases binding of heavy metals to soil,
thus rendering them less transportable.
The effect of high soil moisture content on the growth of plant species and
hyperaccumulation of Ni and Zn in Alyssum murale, Berkheya coddii, and Thlaspi
caerulescens was studied by Angle et al. [121]. Their results showed that the growth
and biomass of hyperaccumulators are higher at high soil moisture content.
3 Genetic Engineering in the Plant Species
Genetic engineering of the plant species is focused on genes whose protein products
are involved in the uptake and accumulation of metals. Plant biotechnologists target
on chelation and metal transport, making the two key processes for the success of
phytoremediation [122]. Genetic manipulation may be applied to plants and the
associated microorganisms to uptake or to volatilize the contaminants. Zhao and
Mcgrath [123] reported Se uptake by a plant in contaminated soil and drained water
from irrigation. When selenium is present as selenate, it is highly bioavailable to
plant roots. Some of the reports showed that the transgenic Brassica juncea (Indian
mustard), whose genes have overexpressed, are involved in sulfur (S)/Se metabolism
and have increased in the accumulation and tolerance to Se. The transgenic plants
overexpressing adenosine triphosphate sulfurylase (ATPS) catalyze sulfate/selenate
activation before they are reduced to sulfite/selenite. Selenite reduction and assimilation can be enhanced by overexpression of APS and/or the APS reductase (APR)
[123]. Agata et al. [122] enhanced the phytoremediation of a mercury-polluted soil
by integration of merT gene into ppk-transgenic tobacco which resulted in accelerated and enhanced mercury uptake into tobacco. One of the effective methods to
ameliorate the process is by introducing a xenobiotic microbe to the rhizosphere to
contribute in the secretions of the exudate and promote in the decomposition of
contaminants. The aim of this method is that plants need not absorb the contaminants
but the secreted enzymes can decompose the contaminants [124].
4 Drawback and Limitations of Phytoremediation
Phytoremediation is a long process and it may take many years to clean up a site
from the contaminant; still the site may not be fully remediated. The use of invasive,
non-native species can affect biodiversity. The consumption of contaminated plants
by wildlife is also a remarkable concern. After the cleanup process, the harvested
Phytoremediation of Soil for Metal and Organic Pollutant Removal
59
which affects the availability of the metals [116].
In the soil, the solubility and mobility of heavy metals are reduced naturally
through sorption, precipitation, and complexation reactions [117, 118]. Addition of
organic compounds to soils increases its cation exchange capacity [119] and accelerates the attenuation process [120]. This increases binding of heavy metals to soil,
thus rendering them less transportable.
The effect of high soil moisture content on the growth of plant species and
hyperaccumulation of Ni and Zn in Alyssum murale, Berkheya coddii, and Thlaspi
caerulescens was studied by Angle et al. [121]. Their results showed that the growth
and biomass of hyperaccumulators are higher at high soil moisture content.
3 Genetic Engineering in the Plant Species
Genetic engineering of the plant species is focused on genes whose protein products
are involved in the uptake and accumulation of metals. Plant biotechnologists target
on chelation and metal transport, making the two key processes for the success of
phytoremediation [122]. Genetic manipulation may be applied to plants and the
associated microorganisms to uptake or to volatilize the contaminants. Zhao and
Mcgrath [123] reported Se uptake by a plant in contaminated soil and drained water
from irrigation. When selenium is present as selenate, it is highly bioavailable to
plant roots. Some of the reports showed that the transgenic Brassica juncea (Indian
mustard), whose genes have overexpressed, are involved in sulfur (S)/Se metabolism
and have increased in the accumulation and tolerance to Se. The transgenic plants
overexpressing adenosine triphosphate sulfurylase (ATPS) catalyze sulfate/selenate
activation before they are reduced to sulfite/selenite. Selenite reduction and assimilation can be enhanced by overexpression of APS and/or the APS reductase (APR)
[123]. Agata et al. [122] enhanced the phytoremediation of a mercury-polluted soil
by integration of merT gene into ppk-transgenic tobacco which resulted in accelerated and enhanced mercury uptake into tobacco. One of the effective methods to
ameliorate the process is by introducing a xenobiotic microbe to the rhizosphere to
contribute in the secretions of the exudate and promote in the decomposition of
contaminants. The aim of this method is that plants need not absorb the contaminants
but the secreted enzymes can decompose the contaminants [124].
4 Drawback and Limitations of Phytoremediation
Phytoremediation is a long process and it may take many years to clean up a site
from the contaminant; still the site may not be fully remediated. The use of invasive,
non-native species can affect biodiversity. The consumption of contaminated plants
by wildlife is also a remarkable concern. After the cleanup process, the harvested
Phytoremediation of Soil for Metal and Organic Pollutant Removal
59