Phytoremediation of Hazardous Lead from Environment
By: Sudhakar Srivastava, Seema Mishra and R.D.Tripathi
Lead (Pb) pollution of the environment is a major problem
today. It causes health hazards to livestock and human beings, children being
most sensitive. Recently it has been established as a potential carcinogen.
Lead enters in the environment through air, water and soil and finally enters
the food chain through contaminated water, edibles and other foodstuffs.
Besides, human beings can be directly exposed through occupational and
environmental exposures.
Most of the heavy metal contaminated sites are lead
affected. Lead affects many physiological parameters in plants and causes sharp
decrease in crop productivity. Currently lead contaminated sites are being
remediated by a variety of rather costly engineering technologies.
Phytoremediation, popularly known as green clean, is an emerging technology for
the clean up of contaminated sites by the use of plants, and is ecofriendly and
low cost technology compared to traditional engineering remediation methods.
There are many ways in which plants may be utilized for
remediation and these constitute different subcategories of phytoremediation
viz. phytoextraction, rhizofiltration, phytostablization and
phytovolatilization. Of these phytoextraction is best suited for remediation of
lead contaminated sites and is much studied area. It is defined as accumulation
of metal in above ground plant parts and those plants which accumulate more
than 0.1% of lead as dry weight of shoot are known as "hyperaccumulators".
Phytoextraction of Lead:
Lead persists in soil for very long time. Its remediaion is
problematic due to its low availability to plants as it forms complexes and
gets precipitated and also due to its low traslocation from root to shoot.
A few plants are known to hyperaccumulate lead such as
Thlaspi rotundifolium, T. alpestre, Alyssum wulfenianum, Polycarpaea synandra,
Armeria martima, few bryophytes and lichens etc. Various aquatic
plants like Hydrilla, Vallisneria, Marsilea, Cyperus, Polygonum etc.,
algae like Phaeodactylum, Stichococcus, Stigeoclonium etc.
have also been found to be potential accumulators of lead. However, many of
these plants named above are not good for phytoremediation due to their very
low biomass and slow growth rates, thus will take very long time for effective
remediation of a site. It is speculated that even the best accumulators will
take 13-14 years to clean up a site.
To achieve lead remediation in reasonable time there is need
of plants which has short lifetime, could accumulate greater than 1% of lead of
shoot dry weight and produce more than 20 tonnes of shoot biomass ha-1 year-1.
To achieve this goal availability of lead in soil needs to be increased. Chelate mobilization has been studied in much detail in this context.
Chelates bind lead removing it from complexes and thus
increase its availability to plants. They also enhance its translocation from
root to shoot and cause more accumulatiom. Many chelates e.g. ethylenediaminetetraaceticacid
(EDTA),N-hydroxyethylenediamine-triaceticacid (HEDTA), ethyleneglycol-bis (β-aminoethylether)
(EGTA) etc. has been tested. Of all these EDTA has been proved to best chelate.
It considerably increases availability of lead, enhances its mobility from root
to shoot to even more than 100 times and increases lead accumulation in shoot
to more than 400 fold. Research is going on to find suitable amount and number
of doses of chelate and timing of its application. It has been found that
multiple doses in small quantities are better than single time application of
high dose. Application after planting gives good results as compared to
application before plantation. Soil condition like fertilizer applied, soil
temperature, salinity etc. also affect effectiveness of remediation.
Researchers have studied accumulation of lead by high
biomass plants like Brassica juncea, B. rapa, Helianthus annuus, Vicia faba,
Pisum sativum, Phaseolus vulgaris etc. on application of lead. Brassica
juncea could accumulate more than 1.5% of lead of its shoot dry weight on
EDTA application. It has been designated as best lead phytoremediator plant as
it is non-edible plant and poses no health hazards for public. It is also a
very high biomass plant and has short life cycle. However there are concerns
about side effects associated with chelate application. Pb-EDTA easily
percolates through soil profile and causes ground water pollution. Studies are
thus going on to find a better biodegradable organic chelate.
Detoxification as Bioremediation Strategy:
Once inside the plant metal needs to be detoxified.
Detoxification of lead occurs by its binding to some chemical groups e.g. to
cell wall, to polyphosphates bodies in some cyanobacteria and in most cases by
binding to specific peptides called phytochelatins.
Phytochelatins (PCs) are low molecular weight peptides
having the general structure (
-Glu-Cys)n-Gly
where n=2-11. These are enzymatically synthesized by the enzyme Phytochelatin
synthase (PC synthase) and their synthesis is induced by the entry of metal.
Induction of PCs by Pb has been reported in algae, lichens, aquatics and cells
cultures. Phytochelatin responses to lead have been extremely sensitive and
these detoxifying peptides are synthesized even at 1 nM Pb2+ levels in a marine
alga, Thalassiosira weissflogii. PCs bind Pb via thiolate coordination.
PC2 and PC3 can bind one Pb molecule per peptide molecule whereas PC4 forms two
distinct species with stoichiometries for binding one or two Pb ions per
peptide molecule respectively.
In case of cadmium (Cd), S2- incorporation stabilizes the
PC-Cd complexes. The PC-metal complexes are finally transported to vacuole via
ATP-binding cassette (ABC) type transporter where metal causes no harm. Though
mechanism has not yet been elucidated for Pb, however, as PC-Pb complexes are
formed, it is likely that these are sequestrated in vacuole in a similar way.
A future prospective of the problem is to genetically
engineer high biomass non-edible plants for better hyperaccumulation of lead.
There is one report of transfer of PC synthase gene (TaPCS1) of wheat to
Nicotiana glauca (Shrub tobacco), where it doubled the accumulation of Pb.
Biosorption of Lead:
Biosorption is another emerging potentially economic
technology for metal removal and recovery. It consists of several mechanisms
including ion exchange, chelation, adsorption, and ion entrapment in structural
polysaccharide network. This also uses dead or inactive microbial biomass for
sorption of metal by chemically active surface groups. Due to high surface
area, microbial biomass eg. algae like Oscillatoria, Anabaena, Eudorina etc.
fungi e.g. Aspergillus and bacteria like Pseudomonas can
adsorb high amount of lead on their surfaces and can achieve quick remediation.
Conclusion:
Thus phytoremediation is best-suited technology in present
context to clean up Pb contaminated sites and it is cost effective as well.
Since there are no known high biomass plants which could hyperaccumulate lead,
research is going on to genetically engineer the plants, which produce more
biomass in short time and then incorporating genes for hyperaccumulation and
detoxification of lead into them to achieve the goal. Genetic engineering
efforts thus may strengthen phytoremediation as a much better applied
technology in recent future and remediation will be achieved in quicker time at
low cost.
Dr. R.D.Tripathi is a senior Scientist and Group Leader in
Ecotoxicology and Bioremediation Group at National Botanical Research
Institute, Lucknow, India; E-mail: [email protected]
Mr. Sudhakar Srivastava and Ms. Seema Mishra are Junior Research
Fellows in Ecotoxicology and Bioremediation Group at National Botanical
Research Institute, Lucknow, India
