Phytoremediation, meaning "plant", and Latin remedium, meaning "restoring
balance") describes the treatment of environmental problems through the use
of plants that mitigate the environmental problem without the need
to excavate the contaminant material and dispose of it elsewhere.
Phytoremediation is a cost-effective plant-based approach to remediation that
takes advantage of the ability of plants to concentrate elements and compounds
from the environment and to metabolize various molecules in their tissues.
Toxic heavy metals and organic pollutants are the major targets for
phytoremediation. Knowledge of the physiological and molecular mechanisms
of phytoremediation began to emerge in recent years together with biological
and engineering strategies designed to optimize and improve phytoremediation.
In addition, several field trials confirmed the feasibility of using
plants for environmental cleanup. Application
Phytoremediation may be applied wherever the soil or static water environment has
become polluted or is suffering ongoing chronic pollution. Examples where
phytoremediation has been used successfully include the restoration of
abandoned metal mine workings, and sites where polychlorinated biphenyls have
been dumped during manufacture and mitigation of ongoing coal mine
discharges reducing the impact of contaminants in soils, water, or air.
Contaminants such as metals, pesticides, solvents, explosives, and crude oil and
its derivatives, have been mitigated in phytoremediation projects worldwide.
Many plants such as mustard plants, alpine pennycress, hemp, and pigweed
have proven to be successful at hyperaccumulating contaminants at toxic
waste sites. Over the past 20 years, this technology
has become increasingly popular and has been employed at sites with soils
contaminated with lead, uranium, and arsenic. While it has the advantage that
environmental concerns may be treated in situ; one major disadvantage of
phytoremediation is that it requires a long-term commitment, as the process is
dependent on a plant's ability to grow and thrive in an environment that is not
ideal for normal plant growth. Phytoremediation refers to the natural
ability of certain plants called hyperaccumulators to bioaccumulate,
degrade, or render harmless contaminants in soils, water, or air.
Advantages and limitations Advantages:
the cost of the phytoremediation is lower than that of traditional processes
both in situ and ex situ the plants can be easily monitored
the possibility of the recovery and re-use of valuable metals
it is potentially the least harmful method because it uses naturally
occurring organisms and preserves the environment in a more natural state.
Limitations: phytoremediation is limited to the
surface area and depth occupied by the roots.
slow growth and low biomass require a long-term commitment
with plant-based systems of remediation, it is not possible to completely prevent
the leaching of contaminants into the groundwater
the survival of the plants is affected by the toxicity of the contaminated land
and the general condition of the soil. bio-accumulation of contaminants,
especially metals, into the plants which then pass into the food chain, from
primary level consumers upwards or requires the safe disposal of the
affected plant material. Various phytoremediation processes
A range of processes mediated by plants or algae are useful in treating
environmental problems: "Phytosequestration"-Phytochemical
complexation in the root zone, reduce the fraction of the contaminant that is
bioavailable.Transport protein inhibition on the root
membrane-preventing contaminants from entering the plant.Vacuolar storage in
the root cells -Contaminants can be sequestered into the vacuoles of root
cells, Phytoextraction — uptake and
concentration of substances from the environment into the plant biomass.
Phytostabilization — reducing the mobility of substances in the
environment, for example, by limiting the leaching of substances from the
soil. Phytotransformation — chemical
modification of environmental substances as a direct result of plant metabolism,
often resulting in their inactivation, degradation, or immobilization.
Phytostimulation — enhancement of soil microbial activity for the degradation
of contaminants, typically by organisms that associate with roots. This process
is also known as rhizosphere degradation. Phytostimulation can also
involve aquatic plants supporting active populations of microbial degraders, as
in the stimulation of atrazine degradation by hornwort.
Phytovolatilization — removal of substances from soil or water with
release into the air, sometimes as a result of phytotransformation to more
volatile and/or less polluting substances.
Rhizofiltration — filtering water through a mass of roots to remove toxic
substances or excess nutrients. The pollutants remain absorbed in or
adsorbed to the roots. = Phytoextraction=
Phytoextraction uses plants or algae to remove contaminants from soils,
sediments or water into harvestable plant biomass. Phytoextraction has been
growing rapidly in popularity worldwide for the last twenty years or so. In
general, this process has been tried more often for extracting heavy metals
than for organics. At the time of disposal, contaminants are typically
concentrated in the much smaller volume of the plant matter than in the
initially contaminated soil or sediment. 'Mining with plants', or phytomining, is
also being experimented with: The plants absorb contaminants through
the root system and store them in the root biomass and/or transport them up
into the stems and/or leaves. A living plant may continue to absorb
contaminants until it is harvested. After harvest, a lower level of the
contaminant will remain in the soil, so the growth/harvest cycle must usually be
repeated through several crops to achieve a significant cleanup. After the
process, the cleaned soil can support other vegetation.
Advantages: The main advantage of phytoextraction is environmental
friendliness. Traditional methods that are used for cleaning up heavy
metal-contaminated soil disrupt soil structure and reduce soil productivity,
whereas phytoextraction can clean up the soil without causing any kind of harm to
soil quality. Another benefit of phytoextraction is that it is less
expensive than any other clean-up process.
Disadvantages: As this process is controlled by plants, it takes more time
than anthropogenic soil clean-up methods.
Two versions of phytoextraction: natural hyper-accumulation, where plants
naturally take up the contaminants in soil unassisted.
induced or assisted hyper-accumulation, where a conditioning fluid containing a
chelator or another agent is added to soil to increase metal solubility or
mobilization so that the plants can absorb them more easily. In many cases
natural hyperaccumulators are metallophyte plants that can tolerate
and incorporate high levels of toxic metals.
Examples of phytoextraction: Arsenic, using the Sunflower, or the
Chinese Brake fern, a hyperaccumulator. Chinese Brake fern stores arsenic in its
leaves. Cadmium, using willow: In 1999, one
research experiment performed by Maria Greger and Tommy Landberg suggested
willow has a significant potential as a phytoextractor of Cadmium, Zinc, and
Copper, as willow has some specific characteristics like high transport
capacity of heavy metals from root to shoot and huge amount of biomass
production; can be used also for production of bio energy in the biomass
energy power plant. Cadmium and zinc, using Alpine
pennycress, a hyperaccumulator of these metals at levels that would be toxic to
many plants. On the other hand, the presence of copper seems to impair its
growth. Lead, using Indian Mustard, Ragweed,
Hemp Dogbane, or Poplar trees, which sequester lead in their biomass.
Salt-tolerant barley and/or sugar beets are commonly used for the extraction of
sodium chloride to reclaim fields that were previously flooded by sea water.
Caesium-137 and strontium-90 were removed from a pond using sunflowers
after the Chernobyl accident. Mercury, selenium and organic pollutants
such as polychlorinated biphenyls have been removed from soils by transgenic
plants containing genes for bacterial enzymes.
= Phytostabilization= Phytostabilization focuses on the long
term stabilization and containment of the pollutant.For example, the plant's
presence can reduce wind erosion; or the plant's roots can prevent water erosion,
immobilize the pollutants by adsorption or accumulation, and provide a zone
around the roots where the pollutant can precipitate and stabilize. Unlike
phytoextraction, phytostabilization focuses mainly on sequestering
pollutants in soil near the roots but not in plant tissues. Pollutants become
less bioavailable, and livestock, wildlife, and human exposure is reduced.
An example application of this sort is using a vegetative cap to stabilize and
contain mine tailings. = Phytotransformation=
In the case of organic pollutants, such as pesticides, explosives, solvents,
industrial chemicals, and other xenobiotic substances, certain plants,
such as Cannas, render these substances non-toxic by their metabolism. In other
cases, microorganisms living in association with plant roots may
metabolize these substances in soil or water. These complex and recalcitrant
compounds cannot be broken down to basic molecules by plant molecules, and,
hence, the term phytotransformation represents a change in chemical
structure without complete breakdown of the compound. The term "Green Liver" is
used to describe phytotransformation, as plants behave analogously to the human
liver when dealing with these xenobiotic compounds. After uptake of the
xenobiotics, plant enzymes increase the polarity of the xenobiotics by adding
functional groups such as hydroxyl groups.
This is known as Phase I metabolism, similar to the way that the human liver
increases the polarity of drugs and foreign compounds. Whereas in the human
liver enzymes such as cytochrome P450s are responsible for the initial
reactions, in plants enzymes such as peroxidases, phenoloxidases, esterases
and nitroreductases carry out the same role.
In the second stage of phytotransformation, known as Phase II
metabolism, plant biomolecules such as glucose and amino acids are added to the
polarized xenobiotic to further increase the polarity. This is again similar to
the processes occurring in the human liver where glucuronidation and
glutathione addition reactions occur on reactive centres of the xenobiotic.
Phase I and II reactions serve to increase the polarity and reduce the
toxicity of the compounds, although many exceptions to the rule are seen. The
increased polarity also allows for easy transport of the xenobiotic along
aqueous channels. In the final stage of
phytotransformation, a sequestration of the xenobiotic occurs within the plant.
The xenobiotics polymerize in a lignin-like manner and develop a complex
structure that is sequestered in the plant. This ensures that the xenobiotic
is safely stored, and does not affect the functioning of the plant. However,
preliminary studies have shown that these plants can be toxic to small
animals, and, hence, plants involved in phytotransformation may need to be
maintained in a closed enclosure. Hence, the plants reduce toxicity and
sequester the xenobiotics in phytotransformation. Trinitrotoluene
phytotransformation has been extensively researched and a transformation pathway
has been proposed. Role of genetics
Breeding programs and genetic engineering are powerful methods for
enhancing natural phytoremediation capabilities, or for introducing new
capabilities into plants. Genes for phytoremediation may originate from a
micro-organism or may be transferred from one plant to another variety better
adapted to the environmental conditions at the cleanup site. For example, genes
encoding a nitroreductase from a bacterium were inserted into tobacco and
showed faster removal of TNT and enhanced resistance to the toxic effects
of TNT. Researchers have also discovered a mechanism in plants that allows them
to grow even when the pollution concentration in the soil is lethal for
non-treated plants. Some natural, biodegradable compounds, such as
exogenous polyamines, allow the plants to tolerate concentrations of pollutants
500 times higher than untreated plants, and to absorb more pollutants.
Hyperaccumulators and biotic interactions
A plant is said to be a hyperaccumulator if it can concentrate the pollutants in
a minimum percentage which varies according to the pollutant involved.
This capacity for accumulation is due to hypertolerance, or phytotolerance: the
result of adaptative evolution from the plants to hostile environments through
many generations. A number of interactions may be affected by metal
hyperaccumulation, including protection, interferences with neighbour plants of
different species, mutualism, commensalism, and biofilm.
= Table of hyperaccumulators= Hyperaccumulators table – 1 : Al, Ag,
As, Be, Cr, Cu, Mn, Hg, Mo, Naphthalene, Pb, Pd, Pt, Se, Zn
Hyperaccumulators table – 2 : Nickel Hyperaccumulators table – 3 :
Radionuclides, Hydrocarbons, Organic Solvents.
Phytoscreening As plants are able to translocate and
accumulate particular types of contaminants, plants can be used as
biosensors of subsurface contamination, thereby allowing investigators to
quickly delineate contaminant plumes. Chlorinated solvents, such as
trichloroethylene, have been observed in tree trunks at concentrations related to
groundwater concentrations. To ease field implementation of phytoscreening,
standard methods have been developed to extract a section of the tree trunk for
later laboratory analysis, often by using an increment borer. Phytoscreening
may lead to more optimized site investigations and reduce contaminated
site cleanup costs. See also
Phytoremediation plants Phytotreatment
Biodegradation Bioremediation
Bioaugmentation References
Bibliography “Phytoremediation Website” — Includes
reviews, conference announcements, lists of companies doing phytoremediation, and
bibliographies. “An Overview of Phytoremediation of Lead
and Mercury” June 6 2000. The Hazardous Waste Clean-Up Information Web Site.
“Enhanced phytoextraction of arsenic from contaminated soil using sunflower”
September 22 2004. U.S. Environmental Protection Agency.
“Phytoextraction”, February 2000. Brookhaven National Laboratory 2000.
“Phytoextraction of Metals from Contaminated Soil” April 18, 2001. M.M.
Lasat July 2002. Donald Bren School of
Environment Science & Management. “Phytoremediation” October 1997.
Department of Civil Environmental Engineering.
“Phytoremediation” June 2001, Todd Zynda.
“Phytoremediation of Lead in Residential Soils in Dorchester, MA” May, 2002. Amy
Donovan Palmer, Boston Public Health Commission.
“Technology Profile: Phytoextraction” 1997. Environmental Business
Association. Vassil AD, Kapulnik Y, Raskin I, Salt
DE, "The Role of EDTA in Lead Transport and Accumulation by Indian Mustard",
Plant Physiol. 117: 447–53, doi:10.1104/pp.117.2.447, PMC 34964,
PMID 9625697. Salt, D. E.; Smith, R. D.; Raskin, I..
"Phytoremediation". Annual Review of Plant Physiology and Plant Molecular
Biology 49: 643–668. doi:10.1146/annurev.arplant.49.1.643.
PMID 15012249. K. Oh, T. Li, H. Y. Cheng, Y. Xie, and
S. Yonemochi., “Development of Profitable Phytoremediation of
Contaminated Soils with Biofuel Crops,” Journal of Environmental Protection,
vol. 4, pp. 58–64, 2013. X. J. Wang, F. Y. Li, M. Okazaki, and M.
Sugisaki, “Phytoremediation of contaminated soil”, Annual Report CESS,
vol. 3, pp. 114–123, 2003. External links
Missouri Botanical Garden: Phytoremediation website — Review
Articles, Conferences, Phytoremediation Links, Research Sponsors, Books and
Journals, and Recent Research. International Journal of
Phytoremediation — devoted to the publication of current laboratory and
field research describing the use of plant systems to remediate contaminated
environments. Using Plants To Clean Up Soils — from
Agricultural Research magazine New Alchemy Institute — co-founded by
John Todd