From: Public Services and Procurement Canada
Phytoremediation is the use of plants to remediate or contain contaminants in soil, sludges, sediments, groundwater, surface water or wastewater through several mechanisms depending on the type of contaminant and contaminated media. Depending on-site conditions, phytoremediation can be an effective and economical technique for the treatment of inorganic, bioavailable contaminants; however, this technique requires considerable space and relatively long treatment times.
Phytoremediation of inorganic compounds includes three mechanisms: phytoextraction (also known as phytoaccumulation); rhizofiltration; and phytostabilization (phytosequestration).
During the process of phytoextraction, heavy metals from soil are absorbed by plant roots, and are translocated towards various above ground parts where they accumulate. The contaminants are not degraded, but rather remain in the plant roots and/or in aerial plant tissues. Once harvested, plants that contain inorganic contaminants must be stored in a secure disposal area. They can also be incinerated or composted to recycle the metals. If plants are incinerated, the ash is considered a hazardous waste and must be disposed accordingly. Compared to the volume of contaminated soil to manage with a remediation technique involving excavation of the site, the volume of contaminated plant material or ash to manage is relatively small. Easily bioavailable metals for plant uptake include cadmium, nickel, zinc, arsenic, selenium and copper. Moderately bioavailable metals include cobalt, manganese and iron.
During rhizofiltration, inorganic contaminants are either adsorbed or precipitated onto the plant roots or absorbed into the plant roots. Rhizofiltration differs from phytoaccumulation in that it applies to dissolved contaminants in surface water, groundwater, or wastewater. The plant roots are harvested once saturated with contaminants. Since all reactions involve the plant root, it is important to select a plant with a fast-growing and extensive root system in order to maximize the contact area between the contaminants and the plants. Rhizofiltration can be used as an ex situ groundwater treatment method where contaminated water to be treated is used inside a greenhouse as irrigation water for the plants.
Phytostabilization refers to the immobilization of contaminants in the soil and groundwater by absorption and accumulation by roots, adsorption onto roots or precipitation within the root zone (rhizosphere) of plants. This mechanism is used to reduce the mobility of the contaminants and prevent their migration which also helps in preventing their entry into the food chain.
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 a potentially sensitive environment.
Projects may include:
For large sites, farming and/or specialized equipment may be required for installation and maintenance.
There are relatively minimal requirements for materials storage.
Plant residues after harvesting require proper handling, storage, and disposal. 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.
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.
Notes:
A treatment wetland may require considerable surface area depending on contaminant concentrations and the volume of water. The site should also be relatively flat.
High concentrations of certain contaminants, even temporarily, can disable the system on a long-term basis
When selecting plants, consideration must be given to plant growth in environments with specific climatic 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 (e.g. airports). Maintenance and irrigation may be difficult in isolated areas. Plants species should require little maintenance/irrigation.
Plant residues may need to be properly treated in case of phytoaccumulation of contaminant in plant roots or tissues, or sorption to plant roots through rhizofiltration.
Phytoremediation of inorganic compounds does not produce any secondary by-products or metabolites.
The phytoremediation of inorganic compounds is a technique that has demonstrated its effectiveness at several sites.
Application examples are available at these links:
The time required for completion of phytoremediation treatment varies according to the type of contaminants, selected plants, rhizosphere microbial population and activity—and 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
