24 T. Macek et al. Biotechnology Advances 18 2000 23–34
Cost comparisons of phytoremediation to other remediation technologies have recently been made. The consensus cost of phytoremediation has been estimated at 25–100 per ton
of soil treatment and 0.60–6.00 per 1000 gallons for treatment of aqueous waste streams. In both cases, the remediation of organic contaminants can be expected to fall at the lower
end of these ranges and remediation of heavy metals to fall at the higher end. In each ap- proach discussed, the expenses of phytoremediation represent less than half of the price
needed for any other effective treatment. According to 1997 U.S. EPA estimates, the cost of using phytoremediation in the form of an alternative cover vegetative cap ranges from
10000 to 30000 per acre, which is thought to be two- to five-fold less expensive than tradi- tional capping [3].
2. Advantages of phytoremediation
Phytoremediation has made tremendous gains in market acceptance in recent years. In ad- dition to its favorable economics, according to various authors [1,4–6] the main advantages
of phytoremediation in comparison with classical remediation methods can be summarized as follows:
It is far less disruptive to the environment. There is no need for disposal sites.
It has a high probability of public acceptance. It avoids excavation and heavy traffic.
It has potential versatility to treat a diverse range of hazardous materials.
Considering these factors and the much lower cost expected for phytoremediation, it ap- pears that it may be used in much larger scale clean-up operations than is possible by other
methods. The process is relatively inexpensive, because it uses the same equipment and sup- plies that are generally used in agriculture.
3. Disadvantages of phytoremediation
Like other methods of environmental remediation, phytoremediation has its disadvan- tages. The use of phytoremediation is limited by the climatic and geological conditions of the
site to be cleaned, temperature, altitude, soil type, and accessibility by agricultural equip- ment. Other disadvantages vary with the application and type of contamination for which the
method will be used. As discussed by others [1,4–6], the main concerns come from the fol- lowing problems:
Formation of vegetation may be limited by extremes of environmental toxicity. Contaminants collected in leaves can be released again to the environment during litter fall.
Contaminants can be accumulated in fuel woods. The solubility of some contaminants may be increased, resulting in greater environmental
damage andor pollutant migration. It may take longer than other technologies.
T. Macek et al. Biotechnology Advances 18 2000 23–34 25
4. Uptake and biotransformation of pollutants in plants
Phytoremediation is currently divided into following areas as summarized by Salt et al. [7]. • Phytoextraction: the use of pollutant-accumulating plants to remove metals or organics
from soil by concentrating them in the harvestable parts. • Phytodegradation: the use of plants and associated microorganisms to degrade organic
pollutants. • Rhizofiltration: the use of plant roots to absorb and adsorb pollutants, mainly metals,
from water and aqueous waste streams. • Phytostabilization: the use of plants to reduce the bioavailability of pollutants in the en-
vironment. • Phytovolatilization: the use of plants to volatilize pollutants.
• The use of plants to remove pollutants from air. Various mechanisms are involved in each of the processes listed above. Plants remediate
organic compounds by direct uptake of contaminants, as summarized by Schnoor et al. [5], followed by subsequent transformation, transport, and their accumulation in a nonphytotoxic
form which does not necessarily mean nontoxic for humans. In addition, plants support bioremediation by release of exudates and enzymes that stimulate both microbial and bio-
chemical activity in the surrounding soil and mineralization in the rhizosphere. The use of plants as a final water treatment step and for the disposal of sludge resulting from waste wa-
ter treatment is centuries old [8]. These processes either ‘decontaminate’ the soil, or ‘stabi- lize’ the pollutant within it i.e. preventing its migration to a site of actual danger to human
health. Specifically, two subsets of phytoremediation are nearing commercialization. First is phytoextraction, in which high biomass metal-accumulating plants and appropriate soil
amendments are used to transport and concentrate metals from the soil into the harvestable part of roots and aboveground shoots, which are harvested with conventional agricultural
methods [2,7]. The other is rhizofiltration, in which plant roots grown in water absorb, con- centrate, and precipitate toxic metals and organics from polluted effluents [2,7].
5. Role of rhizospheric microbial communities
