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Freshwater Management Series No. 7


A Technical Approach in Environmental Management

III. Examples of Environmental Applications of Phytotechnology >

H. Phytoremediation

Phytoremediation is the term that refers to the use of plants for cleaning up contaminants in soil, groundwater, surface water and air. The use of phytoremediation can be a non-polluting and costeffective way to remove or stabilize toxic chemicals that might otherwise be leached out of the soil by rain to contaminate nearby watercourses. It is also a way of concentrating and harvesting valuable metals that are thinly dispersed in the ground, and offers an attractive option for the remediation of brownfield sites. Phytoremediation encompasses several methodologies, including:

phytoextraction or phytoconcentration, where the contaminant is concentrated in the roots,stem and foliage of the plant,
phytodegradation, where plant enzymes help catalyze breakdown of the contaminantmolecule,
rhizosphere biodegradation, where plant roots release nutrients to microorganisms which are active in biodegradation of the contaminant molecule,
  volatilization, where transpiration of organics, selenium and mercury run through leaves of the plant,
stabilization, where the plant converts the contaminant into a form which is not bioavailable, or the plant prevents the spreading of a contaminant plume.

The principal application of phytoremediation is for lightly contaminated soils, sludges and waters where the material to be treated is at a shallow or medium depth and the area to be treated is large, so that agronomic techniques are economical and applicable for both planting and harvesting. In addition, the site owner must be prepared to accept a longer remediation period.

Plants used to decontaminate soils must do one or more of the following:

take up contaminants from soil particles and/or soil liquid into their roots,
bind the contaminant into their root tissue, physically and/or chemically,
transport the contaminant from their roots into growing shoots,
prevent or inhibit the contaminant from leaching out of the soil.

Farmer's field with leafy vegeatbles growing

The plants should not only accumulate, degrade or volatilize the contaminants, but should also grow quickly in a range of different conditions and lend themselves to easy harvesting. If the plants are left to die in situ, the contaminants will return to the soil. For complete removal of contaminants from an area, the plants must be cut and disposed of elsewhere in a nonpolluting way. Some examples of plants used in phyoremediation practices are water hyacinths (Eichhornia crassipes), poplar tress (Papulus spp), forage kochia (Kochia spp), alfalfa (Medicago sativa), Kentucky bluegrass (Poa pratensis), Scirpus spp, coontail (Ceratophyllum demersvm L.), American pondweed (Potamogeton nodosus) and the emergent common arrowhead (Sagittaria latifolia) amongst others.

Typically, researchers look for suitable phytoremediation properties among both cultivated and wild varieties of plants. If suitable wild species are not available, researchers can try to improve the effectiveness of phytoremediation by introducing different genetic varieties. One way this is done is by soaking seeds in a mutation-producing chemical, then screening the germinated seedlings for contaminant tolerance in artificial solutions containing various concentrations of the particular contaminant(s) of concern. Testing is carried out in batches of at least 50,000 seedlings at a time. The most tolerant and vigorously growing plants are analyzed for their contaminant content and the best of them are bred to produce a line of improved plants.

Although phytoremediation has not been used extensively, it has many advantages:

It is low cost in comparison to current “mechanical” methods for soil remediation.
It is passive and solar.
It is faster than natural attenuation.
  The amount of contaminated material going to landfills can be greatly reduced.
  Energy can be recovered from the controlled combustion of the harvested biomass.
  It is low impact and public acceptance of phytoremediation is expected to be high.

A major barrier to the implementation of phytoremediation is that it is new and not fully developed. There is little regulatory experience with phytoremediation and it has to be considered on a site by site basis. Furthermore, the intrinsic characteristics of phytoremediation limit the size of the niche that it occupies in the site remediation market.

Some of the other limitations to phytoremediation are as follows:

  It is generally slower than most other treatment methods and is climate dependent.
  In most cases, the contamination to be treated must be shallow.
  It usually requires nutrient addition, and mass transfer is limited.
  High metal and other contaminant concentrations can be toxic to the plants, although some plants have greater adaptation to toxicity than others.
  Access to the site must be controlled, as the plants may be harmful to livestock and the general public.
  The contaminants being treated by phytoremediation may be transferre d across media (i.e., they may enter groundwater or may bioaccumulate in animals).
  For mixed contaminant sites (i.e., organic and inorganic) more than one phytoremediation methodology may be required.
  The site must be large enough to utilize agricultural machinery for planting and harvesting.

Momentum for the use of phytoremediation as a cleanup technique is building, particularly in application niches where other technologies are less suitable or do not exist. There will also likely be combined applications of bioremediation and phytoremediation. Table 7 illustrates how other remediation techniques compare to phytoremediation.

Table 7: Comparing Other Remediation Techniques to Phytoremediation

Treatment Name Advantages Compared to Phytoremediation Disadvantages Compared to Phytoremediation
Solidification / Stabilization Not seasonally dependent; well established; rapid; applicable to most metals and organics; simple to operate during treatment. Site is not restored to original form; leaching of the contaminant is a risk; can result in a significant volume increase.
Soil Flushing / Soil Washing Not seasonally dependent, except in cold climates; methods well established for several types of sites and contamination. Removal of metals using water flushing requires pH change;additional treatment steps and chemical handling add complexity and cost; possible lengthy period of treatment.
Bioremediation Established and accepted; a bioreactor can be utilized for exsiting work; may be faster than phytoremediation. Requires nutrient addition at a much greater level than phytoremediation; applicable to organics only.
Electrokinetics Not seasonally dependent; can be used in conjunction with phytoremediation to enhance rhizosphere biodegradation. Useful for soil only, not wetlands; uniformity of soil conditions is required.
Chemical Reduction / Oxidation Not seasonally dependent; relatively short treatment time frame; usually off site. Requires excavation; uses chemical additives; fertility of the soil after treatment may be damaged.
Excavation / Disposal Rapid, immediate solution for site owner. Transfers contaminants to landfill; does not treat.
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