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PHYTOREMEDIATION
- A Cost-Effective Remediation Method

Phytoremediation refers to the use of plants for cleaning up contaminants in soil, groundwater, surface water and air. It encompasses: 1) phytoextraction or phytoconcentration, where the contaminant is concentrated in the roots, stem and foliage of the plant; 2) phytodegradation, where plant enzymes help catalyze breakdown of the contaminant molecule; 3) rhizosphere biodegradation, where plant roots release nutrients to microorganisms which are active in biodegradation of the contaminant molecule; 4) volatilisation, where organics are transpired through plant leaves; and, 5) stabilisation, 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.

Although phytoremediation has not been used extensively, it has many advantages: 1) It is low cost compared to current "mechanical" methods for soil remediation. 2) It is passive and solar driven. 3) It is faster than natural attenuation. 4) The amount of contaminated material going to landfills can be greatly reduced. 5) Energy can be recovered from controlled combustion of harvested biomass. 6) And, overall, it is low impact and public acceptance is expected to be high. Still, phytoremediation 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 its application.

Some other limitations of phytoremediation are:

• 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 some plants.
• Access to the site must be controlled, as contaminants being treated by phytoremediation may be transferred across media (i.e., may enter groundwater or may bioaccumulate in animals).
• For mixed contaminant sites (i.e., organic and inorganic) more than one phytoremediation method may be required.
• The site must be large enough to utilise agricultural machinery for planting and harvesting.

Although phytoremediation as a cleanup technique is not yet widely applied, momentum for its use is expected to build, 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. Figure 1 illustrates how other remediation techniques compare to phytoremediation.

Figure 1: A comparison of phytoremediation and other remediation techniques.

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