Fact sheet: Phytoremediation of Organic Compounds

From: Public Services and Procurement Canada

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Description

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).

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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 sensitive environments.

  • Projects may include:
  • Site access restrictions with fencing and signage
  • Detailed site characterization, including existing vegetation survey and mapping, climatic conditions, and consideration of depth of contamination and remediation timeframes.
  • Review of case studies and available databases for plant selection
    Pilot scale study of selected plants

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
  • Mowing, pruning (harvesting and proper handling and disposal of plant waste is generally not required with organic compounds),
  • Monitoring growth conditions and remediation performances
    • Sampling and analysis of soil and groundwater
    • Sampling and analysis of plant tissue
    • Sampling and analysis of transpiration gases

For large sites, farming and/or specialized equipment will 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.
    • Allows for the treatment of residual organic contamination in the soil, sediments, surface water, and groundwater located near the soil surface (< 1.0 m), or within the growth zone of the plant root system.
    • Deep lying contaminated groundwater may be pumped and plants may be used to treat it.
    • Can be used over large areas.
    • Can be used as a barrier system for contaminants in groundwater.

Waste and Discharges

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.

Recommended analyses for detailed characterization

Chemical analysis

  • pH
  • Organic matter content
  • Metals concentrations
  • 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)
  • Presence of receptors:
    • presence of potential environmental receptors
    • presence of above and below ground infrastructure
    • the risk of off-site migration

Phase III

  • Soil stratigraphy
  • Conceptual site model with hydrogeological and geochemical inputs
  • Characterization of the hydrogeological system including:
    • the direction and speed of the groundwater flow
    • the hydraulic conductivity
    • the seasonal fluctuations
    • the hydraulic gradient

Applications

  • Allows for the treatment of residual organic contamination in the soil, sediments, surface water, and groundwater located near the soil surface (< 1.0 m), or within the growth zone of the plant root system.
  • Deep lying contaminated groundwater may be pumped and plants may be used to treat it.
  • Can be used over large areas.
  • Can be used as a barrier system for contaminants in groundwater.

Applications to sites in northern regions

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.

Treatment type

Treatment type
Treatment typeApplies or Does not apply
In situ
Applies
Ex situ
Does not apply
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
With restrictions
Chlorobenzenes
With restrictions
Explosives
Applies
Metals
Does not apply
Monocyclic aromatic hydrocarbons
Applies
Non metalic inorganic compounds
Does not apply
Pesticides
Applies
Petroleum hydrocarbons
Applies
Phenolic compounds
Applies
Policyclic aromatic hydrocarbons
Applies
Polychlorinated biphenyls
Applies

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.

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

Notes:

This technique does not 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.

Long-term considerations (following remediation work)

There is no specific long-term consideration except that the plant system put in place may require maintenance or removal.

Secondary by-products and/or metabolites

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.

Limitations and Undesirable Effects of the Technology

  • The depth of contamination is very limited as treatment zone is determined by plant root depth (less than 1 metre depth for soil and less than 3 m depth for groundwater). The use of trees would allow the treatment of deeper contamination.
  • Treatment time extends over several years (more than 5 years)
  • High contaminant concentrations can have a toxic effect on the 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.
  • Contaminants are not always treated and can be transferred from one medium to another (for example, from soil to air during photovolatilization)
  • This technology can only be applied to 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.
  • New transport pathways may form that can affect contaminant mobility (such as amendment and cultivation practises)
  • Plant tissue testing may be necessary before disposal of any contaminated plant material
  • Institutional controls may be required to restrict access to site
  • A large land area may be required
  • There are potential threats to the food chain if plants are consumed by wildlife (bioaccumulation).

Complementary technologies that improve treatment effectiveness

Fertilizers rich in nitrogen stimulate plant growth as well as microbial activity and the rate of degradation.

Required secondary treatments

None

Application examples

The phytoremediation of organic compounds is a technique that has demonstrated its effectiveness on several sites.

Application examples are available at these addresses:

Performance

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.

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 (soil moisture, for example).
  • Consider biosafety concerns, take appropriate safeguards and follow all regulations when using genetically modified (transgenic) plants (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:
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