The basic idea that plants can be used for environmental remediation is certainly very old and cannot be traced to any particular source. However, a series of fascinating scientific discoveries combined with interdisciplinary research approaches have led to the development of this idea into a promising environmental technology called phytoremediation. Phytoremediation is defined as the use of green plants to remove pollutants from the environment or to render them harmless. Research into phytoremediation may offer the remediation solution for at least 30,000 contaminated sites in the U.S.

Soils and waters contaminated with toxic metals pose a major environmental and human health problem which is still in need of an effective and affordable technological solution. Microbial bioremediation has been somewhat successful for the degradation of certain organic contaminants, but ineffective at addressing the challenge of toxic metal contamination, particularly in soils. While organic molecules can be degraded, toxic metals are remediated by removal from soil. Therefore, the state of the-art technology for cleanup of toxic metal contaminated soils is the excavation and burial of the soil at a hazardous waste site at costs that can reach $1,000,000 per acre. In the U.S alone, the cost of cleaning up sites contaminated with toxic and radioactive metals is estimated to be $300 billion. The problem is even more acute abroad, particularly if large areas contaminated with radionuclides as a result of the Chernobyl nuclear disaster are taken into account.

Phytoremediation of metals is a cost-effective "green" technology based on the use of specially selected metal-accumulating plants to remove toxic metals, including radionuclides, from soils and water. Phytoremediation takes advantage of the fact that a living plant can be compared to a solar driven pump, which can extract and concentrate particular elements from the environment. Phytoremediation is becoming possible because of successful basic and applied research -- much of it conducted with the productive interdisciplinary cooperation of plant biochemists, molecular biologists, soil chemists, agronomists, environmental engineers, and federal and state regulators. The metals targeted for phytoremediation include lead, cadmium, chromium, arsenic and various radionuclides. The harvested plant tissue, rich in accumulated contaminant, is easily and safely processed by drying, ashing or composting. The volume of toxic waste produced as a result is generally a fraction of that of many current, more invasive remediation technologies, and the associated costs are much less. Some metals can be reclaimed from the ash, which further reduces the generation of hazardous waste and generates recycling revenues.

Specifically, several subsets of metal phytoremediation exist including phytoextraction, rhizofiltration, phytostabilization and phytovolatilization.

(I) Phytoextraction -- high biomass metal-accumulating plants and appropriate soil amendments are used to transport and concentrate metals from the soil into the above-ground shoots, which are harvested with conventional agricultural methods, (II) Rhizofiltration -- plant roots grown in aerated water, precipitate and concentrate toxic metals from polluted effluents, (III) Phytostabilization -- plants stabilize the pollutants in soils thus rendering them harmless, and (IV) Phytovolatilization -- plants extract volatile metals (e.g. mercury and selenium) from soil and volatilize them from the foliage.

Phytoextraction of metals represents one of the largest economic opportunities because of the size and scope of environmental problems associated with metal-contaminated soils and the competitive advantage offered by a plant-based technology. The initial scientific efforts in this area focused on the selection of the best "phytoextracting" plants and on the optimization of their laboratory and field performance. Certain varieties of wild and crop-related brassicas (mustards) were selected for their ability to accumulate metals in the above ground (harvestable) parts. Some of these plants concentrated toxic heavy metals (lead, copper, nickel) up to several percent of their dried shoot biomass. This is more than 100 times the concentration in soil. Recently, scientists began a search for genes from various plant, bacterial and animal sources which can further enhance the metal accumulating potential of plants in which these genes are inserted. Understanding the biological mechanisms of metal accumulation and transport in plants will undoubtedly lead to the production of superior phytoextracting varieties.

However, since metals may be bound too tightly to soil components, genetic potential to accumulate metals does not always translate into effective phytoextraction. Metal chelators and other soil amendments, which release metals to plant roots and facilitate metal uptake and translocation are extremely effective in improving phytoextraction in the field and make this process cost effective. The diagram of metal phytoextraction is shown on figure 1.

Figure 1. Phytoextraction of lead from soils

Rhizofiltration represents another major opportunity for using plants to clean up the environment. After screening hundreds of plant species, certain varieties of sunflowers were identified as having the highest metal removal capacity of all the plants tested. Within several hours small amounts of roots of hydroponically grown sunflower plants were able to remove various heavy metals (lead, copper, uranium, strontium, cesium, cobalt, zinc ) from water to reach concentrations meeting accepted water standards. Innovative engineering solutions, agronomic practices and nutritional amendments, which increase root production and the overall efficiency of rhizofiltration, are currently being developed. In addition, work is in progress to bioengineer plants to enhance rhizofiltration performance. The diagram of a rhizofiltration system is shown in figure 2.

Figure 2. Flow-through rhizofiltration system. The system contains 8-12 week-old sunflower plants with roots immersed in flowing contaminated water.

For the last two years, Rutgers University scientists undertook an extensive field demonstration program that focused primarily on lead contaminated soils in the US and on the radionuclide contaminated soils in the Chernobyl region of Ukraine. The results were very promising and showed measurable decreases in soil pollutants. Rhizofiltration was also successfully tested in the summer of 1995 at two locations; Ashtabula, Ohio, a DOE site contaminated with 100-400 ppb uranium in ground and surface water and in a small pond within 1 km of the Chernobyl nuclear power plant in the Ukraine. The field results demonstrated that rhizofiltration is a practical way to treat radionuclide-contaminated water. Successful transfer of phytoremediation from the laboratory to the field is a crucial step in the development of this technology. A growing number of scientists are directing their efforts to understanding the physiological, biochemical and molecular mechanisms involved in metal uptake, accumulation, and resistance. Novel molecular genetic approaches to improving phytoremediation are also being developed.

While major opportunities for phytostabilization and phytovolatilization of metals also exist, these technologies are relatively less developed than those described above. This situation should improve as more researchers become interested in this area. In addition, phosphates and nitrates, essential for plants but considered water pollutants, can be treated by plants. Finally, plant-based land and shore restoration is another application close to phytoremediation, which has been gaining momentum recently.

Phytoremediation of organics

Phytoremediation of organic pollutants takes advantage of the fact that living plants carry out a plethora of chemical reactions energized by sunlight, which metabolize or mineralize organic molecules. Targets of this technology are PCBs (polychlorinated biphenyls), TCEs (trichloroethylenes), PAHs (polyaromatic hydrocarbons), pesticide residues, various explosives and other toxic organic pollutants deposited in soils and waters around us by industry. Plants and associated microorganisms can degrade these pollutants, or at least limit their spread in the environment. Various enzymes that are capable of metabolizing pollutants were identified in plants. In general, our understanding of the mechanisms of degradation of organic pollutants by plants lags behind our knowledge of bacteria-assisted bioremediation. It is also difficult to assess the contribution of rhizospheric microorganisms (bacteria and fungi associated with plant roots) to the overall success of plant-assisted phytoremediation of organics. Nevertheless, the initial progress in utilizing plants for the cleanup and containment of organic pollutants warrants serious evaluation of this area of research.

Conclusions

Crops are among our most inexpensive products. It costs only several hundred dollars to grow a hectare of soybeans or corn which can yield over 20 tons of dry biomass. Growing plants is also several hundred times cheaper than growing an equivalent weight of bacterial biomass. The main reason is that unlike bacteria, plants do not require sterile conditions or organic nutrients and are easier to propagate and harvest. However, until recently, bacteria attracted much more interest in remediation of water and soil than plants. Research results show significant potential to use plants for remediation of metal and organic pollutants and to develop molecular approaches which further improve this process.

by Professor Ilya Raskin
AgBiotech Center, Cook College, Rutgers University
P.O. Box 231, New Brunswick NJ, 08903-0231

Professor Raskin's paper on "Phytoremediation: Using plants to remove pollutants from the environment" is part of the American Society of Plant Biologists (ASPB) Invited Author Series on Leading Plant Research. For more information on phytoremediation or other plant science research, contact the ASPB Public Affairs office by phone at 301-251-0560; by mail at 15501 Monona Drive, Rockville, MD 20855 or visit the ASPB web site at http://www.aspb.org