4.1.6 Phytodegradation
The complex organic molecules of the contaminants are either broken down and
released as simple molecules, or the disrupted molecules will be incorporated into
the tissues of the plant for growth and other metabolic activities. As the transformation of pollutants occurs, this may also referred to as a phyto-transformation process.
Polycyclic aromatic hydrocarbons, polychlorinated biphenyls (PCBs), and other
inorganic pollutants such as oxynitride and oxysulfide can be absorbed and degraded
by plants (root, stem, leaf).
5 Phytoremediating Plants
Plants that absorb heavy metals 100 times more than ordinary species, also known as
hyper-accumulators, are selected for phytoremediation. The seed selection of hyperaccumulators is the basis for phytoremediation. The earliest report about hyperaccumulators can be traced back to the year 1583. Cesalpino, an Italian botanist,
found a special plant growing on a “black rock.” It was afterward confirmed as a kind
of hyper-accumulator of Ni. Generally, the definition of hyper-accumulator refers to
the standard confirmed by Baker and Brooks (1989). Hyper-accumulators are
reported for the metal-accumulating capacity in their leaves: the dry leaf tissue of
such plants may contain more than 100 μg/g Cd and Se, and comparatively more for
other metals such as Cr, Pb, Ni, Cu, Mn, and Zn at about 1000 μg/g or more. In many
hyper-accumulators the concentration of heavy metal was found to be greater in
shoots than in the roots. The threshold of accumulation by these hyper-accumulators
will be more than thrice than that of most species growing on normal soils and at
least one order of magnitude higher compared to those growing on metal-enriched
soils.
The more than 500 species of metal hyper-accumulators already found are widely
distributed in about 50 families of vascular plants. Brassicaceae, which contain many
food crop species, make up the most, approximately 25%. Most belong to Ni hyperaccumulators, approximately 400 species. For Cu, cobalt (Co), Zn, Se, Pb, and Mn,
the totals are, respectively, 37, 29, 21, 20, 17, and 13 species. The number of Cd and
As hyper-accumulators is relatively less. Baker (1987) found that the shoots of
Thlaspi caerulescens can accumulate 7000 μg g
À1 Pb under hydroponics condition.
With the discovery of Pteris vittata, a well-known As hyper-accumulator,
phytoremediation of As-polluted soils has made considerable progress.
5.1 How Do Plants Adsorb Toxic Metals?
As in nutrient and water absorption from the environment by the roots of the plants,
the heavy metals also absorbed and are translocated from the roots to the
90
M. K. Awasthi et al.
The complex organic molecules of the contaminants are either broken down and
released as simple molecules, or the disrupted molecules will be incorporated into
the tissues of the plant for growth and other metabolic activities. As the transformation of pollutants occurs, this may also referred to as a phyto-transformation process.
Polycyclic aromatic hydrocarbons, polychlorinated biphenyls (PCBs), and other
inorganic pollutants such as oxynitride and oxysulfide can be absorbed and degraded
by plants (root, stem, leaf).
5 Phytoremediating Plants
Plants that absorb heavy metals 100 times more than ordinary species, also known as
hyper-accumulators, are selected for phytoremediation. The seed selection of hyperaccumulators is the basis for phytoremediation. The earliest report about hyperaccumulators can be traced back to the year 1583. Cesalpino, an Italian botanist,
found a special plant growing on a “black rock.” It was afterward confirmed as a kind
of hyper-accumulator of Ni. Generally, the definition of hyper-accumulator refers to
the standard confirmed by Baker and Brooks (1989). Hyper-accumulators are
reported for the metal-accumulating capacity in their leaves: the dry leaf tissue of
such plants may contain more than 100 μg/g Cd and Se, and comparatively more for
other metals such as Cr, Pb, Ni, Cu, Mn, and Zn at about 1000 μg/g or more. In many
hyper-accumulators the concentration of heavy metal was found to be greater in
shoots than in the roots. The threshold of accumulation by these hyper-accumulators
will be more than thrice than that of most species growing on normal soils and at
least one order of magnitude higher compared to those growing on metal-enriched
soils.
The more than 500 species of metal hyper-accumulators already found are widely
distributed in about 50 families of vascular plants. Brassicaceae, which contain many
food crop species, make up the most, approximately 25%. Most belong to Ni hyperaccumulators, approximately 400 species. For Cu, cobalt (Co), Zn, Se, Pb, and Mn,
the totals are, respectively, 37, 29, 21, 20, 17, and 13 species. The number of Cd and
As hyper-accumulators is relatively less. Baker (1987) found that the shoots of
Thlaspi caerulescens can accumulate 7000 μg g
À1 Pb under hydroponics condition.
With the discovery of Pteris vittata, a well-known As hyper-accumulator,
phytoremediation of As-polluted soils has made considerable progress.
5.1 How Do Plants Adsorb Toxic Metals?
As in nutrient and water absorption from the environment by the roots of the plants,
the heavy metals also absorbed and are translocated from the roots to the
90
M. K. Awasthi et al.