REVIEWS AND ANALYSES Phytoextraction of Toxic Metals: A Review of Biological Mechanisms

plants capable of accumulating uncommonly high Zn levels. In 1935, Byers documented the accumulation of Remediation of sites contaminated with toxic metals is particularly selenium in Astragalus spp. One decade later, Minguzzi challenging. Unlike organic compounds, metals cannot be degraded, and the cleanup usually requires their removal. However, this energyand Vergnano (1948) identified plants capable of hyperintensive approach can be prohibitively expensive. In addition, the accumulating up to 1% Ni in shoots. Following the idenmetal removing process often employs stringent physicochemical tification of these and other hyperaccumulator species, agents which can dramatically inhibit soil fertility with subsequent a great deal of research has been conducted to elucidate negative impacts on the ecosystem. Phytoremediation has been prothe physiology and biochemistry of metal hyperaccumuposed as a cost-effective, environmental-friendly alternative technollation in plants. Significant results have been obtained, ogy. A great deal of research indicates that plants have the genetic and the understanding of metal accumulating mechapotential to remove many toxic metals from the soil. Despite this nisms substantially advanced. However, a better undpotential, phytoremediation is yet to become a commercially available erstanding of the biological processes is needed if phytechnology. Progress in the field is hindered by a lack of understanding toextraction is to become a reliable, commercially of complex interactions in the rhizosphere and plant-based mechanisms which allow metal translocation and accumulation in plants. In available technology. this paper, four research areas relevant to metal phytoextraction from The success of phytoextraction, as an environmental contaminated soil are reviewed. The review concludes with an assesscleanup technology, depends on several factors includment of the current status of technology deployment and suggestions ing the extent of soil contamination, metal availability for future phytoremediation research. for uptake into roots (bioavailability), and plant ability to intercept, absorb, and accumulate metals in shoots (Ernst, 1996). Ultimately, the potential for phytoextracP the use of plants for environmention depends on the interaction between soil, metal, and tal restoration, is an emerging cleanup technology. plant. The complexity of this interaction, controlled by To exploit plant potential to remediate soil and water climatic conditions, argues against generic and in favor contaminated with a variety of compounds, several techof a site specific phytoremediating approach. This undernological subsets have been proposed. Phytoextraction lines the importance of understanding the mechanisms is the use of higher plants to remove inorganic contamiand processes that govern metal uptake and accumulanants, primarily metals, from polluted soil. In this aption in plants. In this review, four research areas, releproach, plants capable of accumulating high levels of vant to soil and plant interaction as it relates to metal metals are grown in contaminated soil. At maturity, phytoextraction, have been identified. The significance metal-enriched aboveground biomass is harvested and of these areas is briefly discussed below. a fraction of soil metal contamination removed. Plants have a natural propensity to take up metals. Some, such Soil Microorganisms and Metal Phytoextraction as Cu, Co, Fe, Mo, Mn, Ni, and Zn, are essential mineral Soil microorganisms have been shown to possess sevnutrients. Others, however, such as Cd and Pb, have no eral mechanisms capable of altering metal bioavailabilknown physiological activity. Perhaps not surprisingly, ity for uptake into roots. For example, microbes have phytoremediation as an environmental cleanup technolbeen documented to catalyze redox reactions leading ogy was initially proposed for the remediation of metalto changes in metal mobility in soil and propensity for contaminated soil (Utsunamyia, 1980; Chaney, 1983; uptake into roots. In addition, root mycorrhizal associaBaker et al., 1991). The identification of metal hyperactions have been shown to affect the rate of metal uptake. cumulators, plants capable of accumulating extraordinarily high metal levels, demonstrates that plants have Metal Bioavailability for Uptake into Roots the genetic potential to clean up contaminated soil. As early as the 19th century, Baumann (1885), identified For most metals, uptake into roots takes place from the aqueous phase. Strong binding to soil particles and/ U.S. Environmental Protection Agency, Office of Research and Deor precipitation renders a significant soil metal fraction velopment, National Center for Environmental Research (8722R), insoluble, and largely unavailable for plant uptake. Low 1200 Pennsylvania Ave. NW, Washington, DC 20460. Received 6 Feb. soil bioavailability is a major factor limiting the potential 2001. *Corresponding author ([email protected]). for phytoextraction of significant metal contaminants such as, lead. A major objective of current phytoremediPublished in J. Environ. Qual. 31:109–120 (2002).