221 Phytoremediation of metals: using plants to remove pollutants from the environment Ilya Raskin * , Robert D Smith and David E Salt Phytoremediation uses plants to remove pollutants from the environment. The use of metal-accumulating plants to clean soil and water contaminated with toxic metals is the most rapidly developing component of this environmentally friendly and cost-effective technology. The recent discovery that certain chelating agents greatly facilitate metal uptake by soil-grown plants can make this technology a commercial reality in the near future. Addresses AgBiotech Center, Cook College, Rutgers University, PO Box 231, New Brunswick, NJ 08903-0231, USA * e-mail: raskin@aesop.rutgers.edu Correspondence: Ilya Raskin Current Opinion in Biotechnology 1997, 8:221–226 Electronic identifier: 0958-1669-008-00221  Current Biology Ltd ISSN 0958-1669 Abbreviation EDTA ethylenediaminetetraacetic acid Introduction The basic idea that plants can be used for environmental remediation is very old and cannot be traced to any particular source; however, a series of fascinating scientific discoveries combined with an interdisciplinary research approach have allowed the development of this idea into a promising environmental technology called phytoreme- diation. Phytoremediation is defined as the use of green plants to remove pollutants from the environment or to render them harmless. Phytoremediation is being devel- oped as a potential remediation solution for thousands of contaminated sites in the US and abroad. Soil and water contaminated with metals pose a major environmental and human health problem that is still in need of an effective and affordable technological solution. Nonradioactive As, Cd, Cu, Hg, Pb and Zn and radioactive Sr, Cs and U (referred to here as toxic metals) are the most environmentally important metallic pollutants. Microbial bioremediation has been somewhat successful for the degradation of certain organic contaminants, but is ineffective at addressing the challenge of toxic metal con- tamination, particularly in soil. Although organic molecules can be degraded, toxic metals can only be remediated by removal from soil. The current state-of-the-art technology for the clean-up of toxic metal-contaminated soils is the excavation and burial of the soil at a hazardous waste site at an average cost of $1 000 000 per acre. In the US 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 when large areas are contaminated with radionuclides, for example, areas surrounding the Chernobyl nuclear reactor. The phytoremediation of metals is a cost-effective ‘green’ technology based on the use of metal-accumulating plants to remove toxic metals, including radionuclides, from soil and water. Phytoremediation has recently become a subject of intense public and scientific interest and a topic of many recent reviews [1 •• ,2,3 •• ,4 • ]. Phytoremediation takes advantage of the fact that a living plant can be considered a solar-driven pump, which can extract and concentrate particular elements from the environment. Phytoremediation is becoming possible because of 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 Pb, Cd, Cr, As 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. Discussed in this review are several specific subsets of metal phytoremediation being developed: phytoextrac- tion, in which 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 agricul- tural methods [5 • ]; phytofiltration, in which plant roots (rhizofiltration) [6 • ] or seedlings (blastofiltration) grown in aerated water precipitate and concentrate toxic metals from polluted effluents; phytovolatilization, in which plants extract volatile metals (e.g. Hg and Se) from soil and volatilize them from the foliage; and phytostabilization, in which plants stabilize pollutants in soils, thus rendering them harmless. Phytostabilization, which is related to soil reclamation, is a less-developed area of phytoremediation research; therefore, it will not be discussed in this review. Phytoextraction The phytoextraction of heavy metals and radionuclides represents one of the largest economic opportunities for phytoremediation because of the size and scope of environmental problems associated with metal-contami- nated soils, and the competitive advantage offered by a