Newsletter and Technical Publications
Freshwater Management Series No. 2
Phytoremediation: An Environmentally
Sound Technology for
Pollution Prevention, Control and Redmediation
An Introductory Guide To Decision-Makers
Depending on the underlying processes, applicability, and type of contaminant,
phyto-remediation can be broadly categorised as:
||Phytodegradation: use of plants to uptake, store and
degrade contaminants within its tissue
|| Phytostimulation or rhizodegradation: use of rhizospheric associations
between plants and symbiotic soil microbes to degrade contaminants.
|| Phytovolatilisation: use of a plant's ability to uptake contaminants
from the growth matrix and subsequently transform and volatilise contaminants
into the atmosphere
||Phytoextraction: use plants to absorb, translocate and store toxic contaminants
from a soil matrix into their root and shoot tissue
|| Rhizofiltration: use of roots to uptake also store contaminants
from an aqueous growth matrix
|| Phytostabilisation: plant-mediated immobilisation or binding of
contami-nants into the soil matrix, thereby reducing their bioavailability
The Use of Phytoremediation to Treat Organic Contaminants
Organic contaminants (specifically hydro-carbons that contain carbon and
hydrogen atoms) are common environmental pollutants. There are several ways
that plants can be used for the phytoremediation of these contaminants: phytodegradation,
rhiz-odegradation, and phytovolatilisation.
Phytodegradation, also called phyto-transformation, is the breakdown of contaminants
taken up by plants through metabolic processes within the plant, or the breakdown
of contaminants surrounding the plant through the effect of compounds (such
as enzymes) produced by the plants. Complex organic pollutants are degraded
into simpler molecules and are incorporated into the plant tissues to help
the plant grow faster (Figure 1). Plants contain enzymes (complex chemical
proteins) that catalyse and accelerate chemical reactions. Some enzymes break
down and convert ammunition wastes, others degrade chlorinated solvents such
as trichloroethylene (TCE), and others degrade herbicides.
of Organic Contaminants by Phyotodegradation
in plant roots break down (degarde) organic contaminants. The fragments
into new plant material.
Rhizodegradation, also called phyto-stimulation or plant-assisted bioreme-diation/degradation,
is the breakdown of contaminants in the rhizosphere (soil surrounding the
roots of plants) through microbial activity that is enhanced by the presence
roots and is a much slower process than phytodegradation. Micro-organisms
(yeast, fungi, or bacteria) consume and digest organic substances for
energy. Certain micro-organisms can digest organic substances such as fuels
or solvents that are hazardous to humans and break them down into harmless
products in a process called biodegradation. Natural substances released
by the plant roots – sugars, alcohols, and acids – contain
organic carbon that provides food for soil microorganisms and the additional
enhance their activity. Biodegradation is also aided by the way plants
loosen the soil and transport water to the area.
Phytovolatilisation is the uptake and transpiration of a contaminant by a
plant, with release of the contaminant or a modified form of the contaminant
from the plant to the atmosphere. Phytovolatilisation occurs as growing trees
and other plants take up water and the organic contaminants. Some of these
contaminants can pass through the plants to the leaves and evaporate, or volatilise,
into the atmosphere. Poplar trees at one particular study site have been shown
to volatilise 90% of the TCE they take up.
Use of Phytoremediation to Treat Metal Contaminants
At sites contaminated with metals, plants can be used to either stabilise
or remove the metals from the soil and groundwater through three mechanisms:
phytoextraction, rhizofiltration, and phytostabilisation.
3: Enclosed phytoremediation tests in Canada. (Photo: Envi-ronment Canada)
Phytoextraction, also called phytoaccumulation, refers to the uptake of metals
from soil by plant roots into above-ground portions of plants (Figure 2). Certain
plants, called hyperaccumulators, absorb unusually large amounts of metals
in comparison to other plants. After the plants have been allowed to grow for
some time, they are harvested and either incinerated or composted to recycle
the metals. This procedure may be repeated as necessary to bring soil contaminant
levels down to allowable limits. If plants are incinerated, the ash must be
disposed of in a hazardous waste landfill, but the volume of ash will be less
than 10% of the volume that would be created if the contaminated soil itself
were dug up for treatment. Metals such as nickel, zinc, and copper are the best
candidates for removal by phytoextraction because
the majority of the approximately 400 known plants that absorb unusually large
amounts of metals have a high affinity for accumulating these metals. Plants
that absorb lead and chromium are currently being studied and tested.
of Metals (Nickel) by Phytoextraction
Nickel is removed from soil by moving up into plant roots, stems, and
leaves. The plantr is then harvested and disposed of and the site replanted
until the nicel in the soil is lowered to acceptable levels.
Rhizofiltration (‘rhizo’ means ‘root’) is the adsorption
or precipitation onto plant roots (or absorption into the roots) of conta-minants
that are in solution surrounding the root zone. Rhizofiltration is similar
to phytoextraction, but the plants are used to clean up contaminated groundwater
rather than soil. The plants to be used for cleanup are raised in greenhouses
with their roots in water. Contaminated water is either collec-ted from a
waste site and brought to the plants or the plants are planted in the contaminated
area, where the roots then take up the water and the contaminants dissolved
in it. As the roots become saturated with contaminants, they are harvested.
For example, sunflowers were successfully used to remove radioactive contaminants
from pond water in a test at Chernobyl, Ukraine.
Phytostabilisation is the use of certain plant species to immobilise contaminants
in the soil and groundwater through absorption and accumulation by roots, adsorption
onto roots, or precipitation within the root zone of plants (rhizosphere).
This process reduces the mobility of the contaminant and prevents migration
to the groundwater or air, and also reduces bioavailability for entry into
the food chain. This technique can be used to re-establish a vegetative cover
at sites where natural vegetation is lacking due to high metal concentrations
in surface soils or physical disturbances to surficial materials. Metal-tolerant
species can be used to restore vegetation to the sites, thereby decreasing
the potential migration of contamination through wind erosion and transport
of exposed surface soils and leaching of soil contamination to groundwater.
4: Harvesting in the experimental crops exposed to toxic metals in Poland. (Photo:
The Use of Phytoremediation for Hydraulic Control of Contaminants
Plants can act as hydraulic pumps when their roots reach down toward the
water table and establish a dense root mass that takes up large quantities
of water. Poplar trees, for example, can transpire between 50 and 300 gallons
of water per day out of the ground. The water consumption by the plants decreases
the tendency of surface contaminants to move towards groundwater and into drinking
water. The use of plants to rapidly uptake large volumes of water to contain
or control the migration of subsurface water is known as hydraulic control.
There are several applications that use plants for this purpose, such as riparian
corridors/buffer strips and vegetative caps.
Riparian corridors (the term ‘riparian’ means ‘located
on the bank of a river’) or buffer strips are applications of phytoremediation
that may also incorporate aspects of phytodegradation, phytovolatilisation,
and rhizodegradation to control, intercept, or remediate contamination
entering a river or groundwater plume. In a riparian corridor, plants may
along a stream or river bank, while buffer strips may be applied around
the perimeter of landfills. Applications of these systems prevent contamination
from spreading into surface water and/or groundwater.
Vegetative cover (or a vegetative cap) is a long-term, self-sustaining cap
composed of soil and plants growing in and/or over waste in a landfill. This
type of cover is an alter-native to composite clay or plastic layer caps. Plants
control erosion and minimise seepage of water that could otherwise percolate
through the landfill and form contaminated leachates. In addition, a vegetative
cap can be designed not only to control erosion and seepage of water, but also
to enhance the degradation of underlying materials in the landfill.
5: Phytoremediation experimental sunflower plantation in Silesia, Poland. Heavy
metals are directly applied on the top soil by dispenser designed at the Institute
for Ecology of Industrial Areas - Kotowice, Poland. (Photo: IETU)