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Phytoremediation
S.C.Santra
Department of Environmental Science
University of Kalyani
Kalyani, Nadia
Email: [email protected]
Phytoremediation:
Application of biological processes for decontaminating the contaminated or
polluted sites is a challenging task because heavy metals cannot be
degraded and hence permit in the soil. In order to clean up the
contaminated sites, heavy metals should be extracted and concentrated by
an appropriate technique for proper disposal in designated secure landfill
sites. The established conventional techniques (viz. thermal processes,
physical separation, electrochemical methods, washing, stabilization etc.)
for clean up of metal contaminated soil are generally too expensive and
often harmful to soil microbial diversity. Plant mediated
decontamination/detoxification processes are commonly referred to as
phytoremediation. It has been proposed as an alternative method to
remove pollutants from air, soil, and water or to render pollutants harmless
and does not affect soil biological activity, structure and fertility (Prasad,
2003, 2004). There are several types of phytoremediation technologies
currently adopted as successful clean up process, both in contaminated
soils and water. (Schulz, and Beck, 2002; Prasad et al 2010).
 Phytoextraction – Reduction of metal concentration in the
soil by cultivating plants with a high capacity for metal
accumulation in the shoots;
 Rhizofiltration – Adsorption or precipitation of metals onto
roots or absorption by the roots of metal tolerant aquatic
plants;
 Phytostabilization – Immobilization of metals in soils by
adsorption onto roots or precipitation in the rhizosphere;
 Hydraulic control – Absorption of large amounts of water
by fast growing plants and thus prevent expansion of
contaminants into adjacent uncontaminated areas;
 Rhizodegradation – Decomposition of organic pollutants or
biotransformation of metals by rhizospheric organisms.
 Phyto volatilization – Detoxify soil metal contaminant by
bio-methylation processes.
 Phyto degradation – Uptake of contaminants and the
subsequent transformation, mineralization or metabilisation
by the plant itself through various internal enzymatic
reactions and metabolic processes (Prasad et al 2010).
 Phytosequestration – Phytochemical complexation in the root zone, leading to the
precipitation or immobilization of target contaminants in the root, such complexes
and there by stored in the vacuolar space of root cells. Transport proteins are also
present that facilitate transfer of contaminants between cells (Prasad, 2011).
 Phytoattenuation – Production of plant biomass as sink of toxic metals at
contaminated sites, where conventional agriculture is affected by the presence of
elevated amounts of plant available trace elements, causing economic losses and
endanger or diminish food and feed quality and safety (Meers et al 2005, 2010).
 Biofortification of phytofortification – It is the process of increasing the
bioavailable concentrations of essential nutrients in edible portions of food crops
through agronomic intervention or genetic selection; essential nutrients like trace
elements, vitamins and metabolites which are accumulated or synthesized during
the growth and development of selected plants can be used as food or feed (healthy
diet) in many areas (Kralova & Masarovicova, 2006).
 Metal Nanoparticles – The natural production of metal nanoparticles viz. TiO2,
ZNo, Al2O3, AgNO3 etc. in plants of contaminated sites are often offered
resistance to plants against invasion of disease and pests or render better stress
adaptability to salt or water (li & Xing, 2007; Havenkamp & Karshall, 2009).
Cadmium phytoremediation:
The main anthropogenic pathway through which Cd enters the
water bodies is via wastes and waste waters from industrial
processes such as electroplating, plastic manufacturing,
metallurgical processes and industries of pigments and Cd/Ni
batteries Cadmium exists in wastewaters in many forms including
soluble, insoluble, inorganic, metal organic, reduced, oxidized, free
metal, precipitated adsorbed and complexed forms. Watanabe et al.
(2009) grew selected species of family Amaranthaceae viz.
Amaranthus tricolor, and observed the higher Cd-accumulating
properties in plants. Later similar hyper accumulation properties of
Cd were reported in Brassica juncea (Family Cruciferae),
Chrysanthemum indicum (Family Asteraceae) and some other
plants, using EDTA chelator, the hyper accumulation of Cd – can be
enhanced many fold n vitro & in vivo.
Arsenic phytoremediation:
In lower part of gangetic delta & Brahmaputra basin,
groundwater arsenic contamination is a natural geogenic
processes. However, in many parts of Asia, arsenical
products have been widely used in agriculture and industrial
practices viz. pesticides, fertilizers, wood preservatives,
smelter wastes and coal fly ash, which are of great
environmental concern.
The Chinese brake fern Pteris vittata is a major arsenic
hyperaccumulating plants which can accumulate 23 g kg-1
of arsenic in its fronds. Similarlarly, other species Pteris
longifolia, Pteris umbrosa and Pityrogramma calomelanos
are also known to be hyperaccumualtors. In presence of
available phosphate, the uptake of As- by the plants appears
to be higher. The root associated VAM fungi in ferns also
helps in hyperaccumulation of arsenic.
Mercury phytoremediation:
The mercury contamination in environment is
primarily anthropogenic, more precisely, industrial
sources viz. coal thermal plant, iron and steel
industry, chloralkali plants, battery industry and so
on being the mail sources. Bioremediation by
microbes in mercury contaminated site
detoxification is quite established. Bacteria and
several higher plants have properties to make
phytovolatilization of mercury at contaminated
sites.
Chromium phytoremediation:
Chromium is the chief heavy metal contaminant found
in the tannery effluent and chromite mine areas.
Phytoremediation appears to be one of the major thrust
areas in chromium contaminated land detoxification.
Selected chromium tolerant plants root zone in
association with VAM fungi, helps in
hyperaccumulation of chromium on site.
A number of tree species helps in phytoextraction of
chromium at contaminated sites.
Cyanide phytoremediation:
Cyanide primarily found in waste
water of steel plants, gold mine
areas. Selected water plants like
water hyacinth (Eichhornia
crassipes) and bacteria are often
used for phytoremediation/
bioremediation purpose.
Constructed Wetland for
Removal of Metal in Aquatic
Environment
Processes Occurring in a Wetland
BIOTIC MECHANISMS TREATING ORGANIC COMPOUNDS IN WETLAND
BIOTIC MECHANISMS TREATING INORGANIC COMPOUNDS IN WETLAND
Abiotic Mechanisms Treating Organic Compounds in Wetland
Abiotic Mechanisms Treating Inorganic Compounds in Wetland
Organic Compound Removal in Wetland
Mechanisms of Metal Removal in Wetland
Suspended Solids Pathways
BOD/ Carbon Pathways
Legend
DM Dissolved Matter
PM Particular Matter
#
Denotes dissolved
or in solution
Se
Bu
Ao
An
Fb
Fm
Di
Sedimentation
Re Resuspension of bed particulates
Biofilm Uptake
Rs Respiration by algal biofilm and
Aerobic decomposition
phytoplankton
Anaerobic decomposition
Bio film fall and deposition
Microphyte litter
Dilution of O at water/air interface
Plants recommended for phytoremediation purpose
Sl.
No.
Plant’s name
Common name
Phytoremediation function
Agropyron repens
Wheat grass
Stabilization of lead in soil
Agropyron smithii
Wheat grass
Metal extraction (phyto extraction)
Agrostis castellana
Beat grass
Metal extraction (phyto extraction)
Agrostis tenuis
Beat grass
Metal extraction (phyto extraction)
Alyssum bertoloni
--
Hyperaccumulation of metal (phyto
extraction)
Asabidopsis halleri
--
Metal tolerance (phytoextraction)
Azolla pinnata
Water fern
Bioabsorption of toxic metals
(Rhizofiltration)
Azolla filculoides
Water fern
Bioabsorption of toxic metals
(Rhizofiltration)
Bacopa monnieri
Water hyssop
Metals accumulation
Brassica oleracea
cauliflower
Metals accumulation
(Phytoextraction)
Brasica napus
Indian mustard
Metals accumulation
(Phytoextraction)
Brassica juncea
Rape
Metals accumulation
(Phytoextraction)
contd…
Sl.
No.
Plant’s name
Common name
Phytoremediation function
Brassica campestris
Cabbage
Metals accumulation
(Phytoextraction)
Carex praegractlis
Sedge
Phytoirrigation
Elchhornia crussipes
Water hyacinth
Metal accumulation (Biosorption)
Hordeum brachyantherum
--
Phytoiirrigation metal hyper
accumulation
Hydrocotyle umbellate
Pannywort
Biosorption of toxic metals
Hygrophila corymbosa
--
Cadmium accumulation
Macademia neurophylla
--
Metal hyperaccumulation
Pistia stratoites
Water lettuce
Metal hyperaccumulation
Pteris vittata / P. longifolia
Brake fern
As-hyperaccumulation
Salvinia molesta
Kariba weed
Metal accumulation
(Rhizofiltration)
Silene vulgaris
Bladder champion
Phytostabilization
Solidago hispida
Hairy golden rod
Phytostabilization
Streptanthus polygaloides
--
Nickelhyperaccumulation
Vallisnaria spiralis
Eel grass
Metal hyperaccumulation
Vetiveria zizanordes
Vetiver grass
Metal hyperaccumulation
Source: Glass 1988, McGutcheon and Schnoor 2003, Prasad 2004, 2007, 2012, Prasad and Strzalka, 2002.
Phytoremediation and Genetic Engineering:
Understanding the physiology and biochemistry of metal accumulation in plants is
important for several reasons. The main implications are that this knowledge allows the
identification of agronomic practices capable of optimizing the potential for
phytoremediation and permits the identification and isolation of gene responsible for the
expression of the hyperaccumulating phenotypes. Thus ideal plant for the
phytoremediation of any metal must have a substantial capacity for metal uptake,
bioaccumulation and stability as well as durability to reduce the length of treatment as for
as possible and practicable. It then follows that there is a promising alternative in the
development of transgenic plants with enhanced properties of metal uptake and
translocation, bioaccumulation potential and higher tolerance to toxicity. Such
heightened metal bioaccumulation and tolerance could be mainly achieved by normally
over expression the natural or modified genes encoding antioxidant enzymes. Several
researchers have reported to date rather encouraging results using plants genetically
engineered with increased cadmium tolerance and uptake for phytoremediation
purposes. However, a majority of these genetically manipulated plants for
phytoremediation have only been tested under strict laboratory conditions and a very
scanty few have been analysed for their phytoremediation potential at field scale. Metal
hyperaccumulating plants and microbes with unique abilities to tolerate, accumulate and
detoxify metals, including cadmium and other metal or metalloids hence constitute an
essential pool of material for genetic modification for targeted enhancements in
phytoremediation potential (Fulekar et al 2009).
Genetic engineering modifications of the physiological and molecular mechanism of plants,
cadmium uptake and tolerance have also been successfully achieved and these show promose
in opening new avenues for enhancing the overall efficiency of cadmium phytoremediation
(Eapen and D’souza, 2005). The possibilities for genetic engineering plants for
phytoremediation is shown in the Fig.
Fig. : Possibilities for genetic engineering in phytoremediation
A number of genetically modified (transgenic) plants
have been generated and tested in recent studies
and they have demonstrated the merits of genetic
engineering in enhancing the tolerance, uptake and
/or bioaccumulation of Cadmium. Wojas et al. (2009)
have demonstrated in a study first of its kind that the
hetergenous expression of Arabidopsis MRP7 in
tobacco (Nicotiana tabacum var. xanthi L.) could
modify cadmium accumulation, distribution and
tolerance.
UNUSUAL ACCUMULATION OF METALIC
ELEMENTS /METALLOID BY PLANTS
Al
Club moss; Hydrangea sp., tea plants
As
Brown algae, fern like Pteris vittata
B
Brown algae, Plumbaginaceae
Ba
Rhizopods, Brazil nut
Cu
Caryophyllaceae
F
Dichopetalum cymosum
Li
Thallictrum sp., Cissium Sp.
Mn
Ferns, Digitalis purpurea
Mo
Papilionaceae
Ni
Alyssium sp., Hybanthus floridundus
Se
Cruciferae, Astragalus racemosus
Sr
Brown algae
V
Amonita sp. Brown algae
Zn
Thlaspi calaminare
Sources: Brown H.J.M. (1966), H eevit E.J. and Smith, T.A. (1975)
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