From: Public Services and Procurement Canada
Phytoremediation of organic compounds is the use of plants to remediate organic contaminants in soil, groundwater, surface water, or sediments. Depending on site conditions, phytoremediation can be an effective and economical technique for the treatment of organic, bioavailable contaminants; however, this technique requires considerable space and relatively long treatment times.
The phytoremediation of organic compounds consists of three main mechanisms: phytodegradation, rhizodegradation and phytovolatilization.
Phytodegradation (also known as phytotransformation) consists of the uptake and transformation or metabolization of organic compounds within plant tissues (for example, roots, leaves, shoots). Plant compounds such as enzymes secreted by the roots or other tissues can also play a role in phytodegradation. Target contaminants include chlorinated solvents, herbicides, insecticides, pentachlorophenol (PCP), polychlorinated biphenyls (PCBs) and munitions constituents.
Rhizodegradation (or phytostimulation) occurs when plant roots produce natural substances (sugars, amino acids, organic acids, plant growth promoters, etc.) that promote microbial growth in the rhizosphere, the area immediately surrounding the root. When their growth is enhanced by these substances, microorganisms (yeast, fungi or bacteria) can digest organic compounds in the rhizosphere (such as fuels or solvents). Root growth further promotes the growth of microorganisms by creating channels and preferential paths that facilitate soil aeration and water transport in the root zone. This phytoremediation mechanism is a much slower process than phytodegradation. Target contaminants include petroleum hydrocarbons, BTEX, PAHs, pesticides, chlorinated solvents, PCP and PCBs.
By the process of phytovolatilization certain hydrophylic organic contaminants can be taken up by plant roots. These compounds or a modified form are subsequently released into the atmosphere through the plant leaves via transpiration. Target contaminants include volatile organic compounds (gasoline and trichloroethene).
The primary challenge with phytoremediation is the selection of appropriate plants in consideration of the type of contaminant, climate factors, and soil and groundwater conditions. A pilot scale study may be required to verify the effectiveness of plants selected. Another factor to consider is the ecological impact of introducing a new plant species into sensitive environments.
For large sites, farming and/or specialized equipment will be required for installation and maintenance.
There are relatively minimal requirements for materials storage.
Plants are not harvested to remove contaminants since they are degraded or volatilized by the plants. However, if plants are cut for other reasons (such as maintenance) they could contain some accumulated contaminants. Verification of the presence of contaminants may be performed. If contaminant concentrations in plant tissues do exceed regulatory limits, the cut plant material or litter will need to be treated as a hazardous waste and disposed of in a proper waste disposal facility. Incineration or composting may be considered to help reduce the volume and mass of material that ultimately needs to be disposed.
The phytovolatilization process can transfer contaminants from soil to the atmosphere through plant leaves during transpiration.
If fertilizers or other soil amendments are used, the potential for contamination of surface runoff or groundwater by soil amendments should be considered in the project design. Excessive use of fertilizer or soil amendments may result in unanticipated mobilization of contaminants through pH change or soluble-metal organic complexes.
When selecting plants, consideration must be given to plant growth in environments with specific climate conditions such as cold weather and shorter growing seasons in northern sites. The ecological sensitivity of northern and remote environments must also be considered when evaluating the use of phytoremediation, particularly if the use of non-native plants is planned. Consideration must also be given to the potential for attracting birds, or other wildlife, and their impact on surrounding sites (like airports). Maintenance and irrigation may be difficult in isolated areas. Selected plants species should require little maintenance/irrigation.
Notes:
This technique doesn’t apply to aliphatic chlorinated hydrocarbons that contain 4 to 5 chlorine atoms. It is easier for plants to absorb and transfer organic compounds when the log of the octanol-water partition coefficient (log Kow) is between 0.5 and 0.3.
There is no specific long-term consideration except that the plant system put in place may require maintenance or removal.
Generally, phytoremediation of organic compounds doesn’t generate deleterious secondary by-products or metabolites. However, some contaminants may be transformed by microorganisms in the rhizosphere and may generate metabolites which are more toxic than the initial compound. For example, the bacterial transformation of trichloroethene and dichloroethene may produce vinyl chloride.
Fertilizers rich in nitrogen stimulate plant growth as well as microbial activity and the rate of degradation.
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The phytoremediation of organic compounds is a technique that has demonstrated its effectiveness on several sites.
Application examples are available at these addresses:
The time required for completion of phytoremediation treatment varies according to the type of contaminants, the selected plants, the rhizosphere population and activity (for rhizodegradation only) and the physical and chemical conditions of the contaminated site.
Unavailable for this fact sheet
Composed by : Magalie Turgeon, National Research Council
Updated by : Jennifer Holdner, M.Sc., Public Works Government Services Canada
Updated Date : March 1, 2015
Latest update provided by : Marianne Brien, P.Eng., Christian Gosselin, P.Eng., M.Eng., Golder Associés Ltée
Updated Date : March 31, 2018
