Fact sheet: Phytoremediation of Inorganic Compounds

From: Public Services and Procurement Canada

On this page

Description

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.

Internet links:

Implementation of the technology

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:

  • Site access restrictions with fencing and signage
  • Detailed site characterization, including climatic conditions and depth of contamination, an existing vegetation survey and mapping and consideration of remediation timeframes.
  • Review of case studies and available databases for plant selection
  • Laboratory and/or pilot scale study of selected plant
  • Full-scale design and implementation
    • Map/drawings of the final planted layout;
    • Soil preparation (tilling, fertilizing and planting)
    • Irrigation system. A water supply may be required near or on-site depending on the plants’ requirements and climatic conditions;
    • Monitoring device/wells;
  • Site maintenance
    • Fertilization and irrigation
    • Weed and pest control
    • Monitoring growth conditions and remediation performances
      • Sampling and analysis of soil and groundwater
      • Sampling and analysis of plant tissue
    • Mowing, pruning, harvesting, and proper handling, and disposal of plant waste

For large sites, farming and/or specialized equipment may be required for installation and maintenance.

Materials and storage

There are relatively minimal requirements for materials storage.

  • Plant seeds or tree/plant cuttings
  • Fertilizers and other amendments
  • Water for irrigation: may be stored on-site if a water supply is not available on-site. Depending on the size of the project and climate, large quantities of irrigation water may be required.

Waste and Discharges

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.

Recommended analyses for detailed characterization

Chemical analysis

  • pH
  • Organic matter content
  • Metals concentrations
  • Metals speciation
  • Tessier's sequential extraction for metals
  • Contaminant concentrations present in the following phases:
    • adsorbed
    • dissolved
    • free
  • Nutrient concentrations including:
    • ammonia nitrogen
    • total Kjeldahl nitrogen
    • nitrates
    • nitrites
    • total phosphorus
  • Salinity/conductivity

Physical analysis

  • Soil water content
  • Soil granulometry
  • Evaluation of biological conditions and ecological factors

Recommended trials for detailed characterization

Biological trials

  • Seed germination toxicity test
  • Root elongation toxicity test
  • Greenhouse trials

Other information recommended for detailed characterization

Phase II

  • Contaminant delineation (area and depth)

Phase III

  • Soil stratigraphy
  • Conceptual site model with hydrogeological and geochemical inputs

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

Applications

  • Allows for treatment of inorganic contamination in soil, sediments, surface water and groundwater
  • Residual contamination must be located near the surface (deep lying contaminated groundwater may, however, be pumped and plants may be used to treat it)
  • Suitable for sites with low to intermediate concentrations of contaminants
  • Can be used over large areas

Applications to sites in northern regions

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.

Treatment type

Treatment type
Treatment typeApplies or Does not apply
In situ
Applies
Ex situ
Applies
Biological
Applies
Chemical
Applies
Control
Applies
Dissolved contamination
Applies
Free Phase
Does not exist
Physical
Applies
Residual contamination
Applies
Resorption
Applies
Thermal
Does not exist

State of technology

State of technology
State of technologyExist or Does not exist
Testing
Does not exist
Commercialization
Exist

Target contaminants

Target contaminantsApplies, Does not apply or With restrictions
Aliphatic chlorinated hydrocarbons
Does not apply
Chlorobenzenes
Does not apply
Explosives
Does not apply
Metals
Applies
Monocyclic aromatic hydrocarbons
Does not apply
Non metalic inorganic compounds
Applies
Pesticides
Does not apply
Petroleum hydrocarbons
Does not apply
Phenolic compounds
Does not apply
Policyclic aromatic hydrocarbons
Does not apply
Polychlorinated biphenyls
Does not apply

Treatment time

Treatment time
Treatment timeApplies or Does not apply
Less than 1 year
Does not apply
1 to 3 years
Does not apply
3 to 5 years
Does not apply
More than 5 years
Applies

Long-term considerations (following remediation work)

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.

Secondary by-products and/or metabolites

Phytoremediation of inorganic compounds does not produce any secondary by-products or metabolites.

Limitations and Undesirable Effects of the Technology

  • Depth of contamination should be limited as treatment zone is determined by plant root depth (less than 1 metre from the surface for soil and less than 3 metres for in situ treatment of ground water). The use of trees would allow the treatment of deeper contamination.
  • Treatment times are relatively long, lasting several years
  • High contaminant concentrations can have a toxic effect on plants
  • Plant growth is limited by the geographic location (climate/season), and soil characteristics,
  • Presence of buildings or underground structures interferes with or excludes planting and the technology cannot be used at those locations
  • The bioavailability of the contaminants can inhibit the application of the technology
  • Human and wildlife access to the site must be restricted to some extent. The potential for bioaccumulation in the food chain must be considered when evaluating wildlife access control. 
  • May require a large open area in which the topography is conducive to cultivation of the chosen plant species.
  • Plant tissue testing may be necessary before disposal of any potentially contaminated plant material
  • This technology can only be applied at sites with a low potential risk to human and environmental health, for example, extended remediation times are permissible and the bioconcentration of toxic contaminants in plants is not an important risk factor
  • Irrigation may affect groundwater flow and (hydraulic) displacement.
  • Cultivation practises and the use of soil amendments may cause the formation of new transport pathways that can affect contaminant mobility.
  • Plant residues with inorganic contaminants must be properly harvested and stored for later disposal in secure area. Plant tissue testing may be necessary before disposal of any contaminated plant material.

Complementary technologies that improve treatment effectiveness

  • The addition of a chelating agent can increase the bioavailability of certain metals (e.g. lead).
  • The addition of citric acid and ammonium nitrate can be used to enhance the bioavailability of uranium and radio-cesium 137 respectively.

Required secondary treatments

  • A management program for plant residues must be implemented. 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.

Application examples

The phytoremediation of inorganic compounds is a technique that has demonstrated its effectiveness at several sites.

Application examples are available at these links:

Performance

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.

Measures to improve sustainability or promote ecological remediation

  • Optimize fertilizer and water addition through plant specific considerations, soil nutrient studies and drip irrigation systems.
  • Consider means to optimize maintenance and monitoring programs such as automated irrigation systems combined with telemetry (for example, soil moisture).
  • Consider biosafety concerns and take appropriate safeguards and follow all regulations when using genetically modified (transgenic) plants (for example, consider cultivation methods, rooting, flowering, etc).

Potential impacts of the application of the technology on human health

Unavailable for this fact sheet

References

Author and update

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

Version:
1.2.1