Newsletter and Technical Publications
Freshwater Management Series No. 2
Phytoremediation: An Environmentally Sound
Pollution Prevention, Control and Redmediation
An Introductory Guide To Decision-Makers
Phytoremediation Work at Every Site?
Phytoremediation technologies have been used to clean up metals, pesticides,
solvents, explosives, crude oil, polyaromatic hydro-carbons, and landfill leachates.
Phyto-remediation can be used in combination with other cleanup approaches
as a ‘finishing’ or ‘polishing’ step. Some phytoremediation
applications are slower than mechanical and chemical methods and are limited
to the depths that are within the reach of the plant roots.
Generally, the use of phytoremediation is limited to sites with low to medium
contaminant concentrations, and conta-mination in shallow soils where phytotoxicity
does not occur and the roots of plants can easily access the contaminant. Plants
can also be used to clean up contaminants in streams and groundwater. Researchers
are finding that the use of trees (rather than smaller plants) allows for the
treatment of contamination at greater depths, as tree roots penetrate more
deeply into the ground. Very deep contaminated ground-water may be treated
by first pumping the water out of the ground and then using plants to treat
Further research is needed to study the effects of bioaccumulation and biomagnification
in the food chain that could occur if insects and small rodents eat the plants
that are collecting contaminants and are then eaten by larger mammals. In addition,
scientists need to establish whether contaminants can collect in the leaves
and wood of trees used for phytoremediation and be released when the leaves
fall in the autumn or when firewood or mulch from the trees is used.
|Photo 6: Test and
selection of plants for phytoremediation purposes. (Photo: Environment Canada)
Has Phytoremediation Been Used?
Phytoremediation has been successfully tested in many locations, but full-scale
applications are still limited. At a U.S. Army testing facility in Maryland,
hybrid poplar trees have been planted in a one-acre site over a shallow groundwater
plume contaminated with organics from several toxic disposal pits. The poplar
trees act as hydraulic pumps to prevent the spread of contaminants to a nearby
marsh. In addition to hydraulic control, researchers have determined that phytovolatilisation
and rhizofiltration are other mechanisms by which the system is treating the
contaminated plume. After the second growing season, the trees successfully
contained the contaminated groundwater plume with rates estimated at 2-10 gallons
of water per day for each tree. Results have also indicated that the poplar
trees are degrading the contaminants in the groundwater plume.
In Canada, Environment
Canada is working with the company that operates the world’s largest
smelter to treat landfill leachate using constructed wetlands. Also, at the
Milan Ammunition Plant in Tennessee, U.S.A., a constructed wetland design
deployed submersed and emergent plants in surface and subsurface flow systems
TNT, RDX and other explosives contaminants found in groundwater onsite. By
testing ten species of aquatic plants in growth chambers and hydroponic solutions,
researchers were able to select a number of species for the lagoon (surface
flow) unit of the wetland system. The species recommended were elodea (Elodea
Michx), sago pondweed (Stuckenia pectinatus)
and water grass (Luziola fluitans).
The use of plants and soil as a vegetative cap for landfill sites is another
effective phytoremediation technique. Vegetative covers serve as a form of
control by maximising the available storage capacity of soil, as well as
the evaporation rates and transpiration processes of plants, thereby minimising
water infiltration. They can also assist in the degradation of wastes and
contained within the landfill or on land. In Slovenia, one landfill is covered
with poplar trees to prevent creation of landfill leachate that could harm
groundwater and surface water.
7: Construction for a phytoremediation test site in Canada. (Photo: Environment
In Kazakhstan, a program has begun to remediate
pesticide contamination through the use of planted systems. International
oil companies have also begun using phyto-degradation at many sites, with
over sixty sites planted or in the planning stages.
In the United Arab Emirates,
a study has been carried out examining the growth characteristics and
performance of mangroves, halophytes and other plants in soil irrigated with
Salinity is a serious threat for crop production in arid regions, where
the high rate of evaporation, combined with the demand for scarce water reserves,
causes salt build-up during the cultivation process. The study showed
that some plants have the necessary physiological mechanisms and capacities
significant concentrations of iron, manganese and magnesium, calcium,
and chloride ions, thereby reducing the overall salinity of the soil
system and potential related effects on crop physiological and growth performance.
Based on this work, it has been suggested that by introducing highly
species (i.e., conocarpus erectus, Atriplex lentiformis, etc.) that can
be irrigated with saline water, higher plant and agricultural production
levels in arid regions can be achieved.
Protection of Riparina Corridors
One of the promising aspects of phytotechnologies
is the possibility of deriving additional benefit from the planted system during
clean-up of pollution. Trees planted as a riparian corridor that protect streams
from agricultural pollution may be managed to provide forest products like
nuts, fruits, lumber or fibre. Trees can be selected that do not move toxins
from roots to above ground, thus allowing for a safe harvest while roots clean
up groundwater. In addition, contaminated urban sites may be planted to clean
up soil and groundwater while at the same time providing recreation and park
Are the Challenges Faced by Decision-Makers?
Environmentally sound technologies (ESTs) encompass technologies that have
significantly improved environmental performance relative to other technologies.
ESTs protect the envi-ronment, are less polluting, use resources in a sustainable
manner, recycle more of their wastes and products, and handle all residual
wastes in a more environmentally acceptable way than the technologies for which
they are substitutes.
8: Phytoremediation test site showing the effect of toxical plants. (Photo: Environment
The development and application of phytoremediation as an environmentally
sound technology involves a number of challenges, including the development
of local capacity to understand and apply phytoremediation technologies, and
the establishment of an effective regulatory framework. In some countries,
there is a lack of experience in the use of phytoremediation. This is often
coupled with a lack of data, performance standards and cost-benefit analysis
regarding phytoremediation technologies. Hence, there is a need for:
||appropriate phytoremediation technologies and techniques
applicable to different geographic regions with varied weather conditions
|| site characterization, clean-up and technology selection criteria
|| assessment and evaluation methods that can be applied to determine the
applicability and appropriateness of various phytoremediation techniques
|| local training for environmental reme-diation practitioners on the planning
and implementation of phyto-remediation schemes.
There are also a number of areas where research is required.
For example, the rate of biodegradation and mineralisation during phytoremediation
affected by the nature and concentrations of contaminants present, as well
as surrounding oxygen levels, soil/air moisture, pH, temperature, soil elemental
contents and their bioavailability,
and the supporting microbial media. Plant physiological and root growth expansion
studies are needed to optimise plant uptake of contaminants and to maximise
process output performance. Research is needed to determine the best density
of plants per unit area to achieve the maximum utilisation of resources (too
may plants can negatively adverse the processes, weaken the plants and be costly
during disposal processes), and to determine the proper timing for irrigation
and harvesting in order to control the amount of biomass produced and removed
at harvest. Other areas requiring further research include the effect of fertilisers
and conditioners on the soil characteristics and the fate of these compounds.