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Freshwater Management Series No. 2

Phytoremediation: An Environmentally Sound Technology for
Pollution Prevention, Control and Redmediation

An Introductory Guide To Decision-Makers

Phytoremediation Processes

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.

Figure 1

Destruction of Organic Contaminants by Phyotodegradation
figure 1: Destruction of Organic Components
Enzymes in plant roots break down (degarde) organic contaminants. The fragments are incorporated 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 of plant roots and is a much slower process than phytodegradation. Micro-organisms (yeast, fungi, or bacteria) consume and digest organic substances for nutrition and 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 nutrients 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.

The 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.

photo 3: Enclosed phytoremediation tests in Canada
Photo 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.

Figure 2

Uptake of Metals (Nickel) by Phytoextraction
Figure 2: Uptake 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.

Photo 4: harvesting in the experimental crops exposed to toxic metals in Poland
Photo 4: Harvesting in the experimental crops exposed to toxic metals in Poland. (Photo: IETU)

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
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 be applied 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
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.

Photo 5: Phyto remediation experimental sunflower plantation in Silesia, Poland
Photo 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)

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