In many locations, toxins can drain easily into the soil, but removing the trapped pollutants creates a difficult challenge. For example, cleansing soil through chemical extraction, filtering, or washing techniques can remove the nutrient-rich topsoil, and can be extremely expensive. Fortunately, some plants can clean up pollutants in soil or water through a process called phytoremediation, which can cost much less than conventional approaches. The Federal Remediation Technologies Roundtable's Remediation Technologies Screening Matrix and Reference Guide Version 3.0 includes a section on Phytoremediation, which states: "the cost for phytoremediation of one acre of lead-contaminated soil to a depth of 50 cm was $60,000 to $100,000, whereas excavating and landfilling the same soil volume was $400,000 to $1,700,000." Consequently, many academic and commercial laboratories study this approach.

Phytoremediation covers an array of techniques. The Web site for Ilya Raskin's laboratory at Rutgers explains:

Phytoremediation takes advantage of the fact that a living plant operates as a solar driven pump, which can extract and concentrate particular elements from the environment. We have exploited this property to develop a method for enhancing the ability of specially selected and/or engineered plants to remove toxic metals such as lead, cadmium, and various radionuclides from soil and water and concentrate these metals in harvestable parts.

To get an overview of this field, visit the Environmental Protection Agency's report A Citizen's Guide to Phytoremediation (in PDF format). This site describes various aspects of phytoremediation, including phytoextraction, which is described as "the uptake and translocation of metal contaminants in the soil by plant roots into the above ground portions of the plants." Plants can also break down contaminants through so-called phytodegradation. A variety of other plant characteristics can be used to stabilize contaminants in an area, remove them from water, and more.

Beyond the various approaches to phytoremediation, investigators must also consider which plants to use for different contaminants. The Agency for Toxic Substances and Disease Registry provides a list of the Top 20 Hazardous Substances, which gives an indication of the diversity of toxins that can pollute soil and water. The most harmful hazardous substances, however, poison not only humans, but other higher organisms, including plants. Nevertheless, some plants survive even when taking up high concentrations of toxins.

Some plants, though, need a little help through bioengineering before they can be employed in phytoremediation. Bioengineering techniques, for instance, can modify metal-binding proteins in plants to help them accumulate heavy metals without damage. A plant can also be engineered with different transport processes in its roots or leaves. Investigators also tinker with a variety of other plant characteristics in hopes of making a better tool for phytoremediation. Just imagine, a researcher could combine a transgenic plant with a suitable genetically modified bacteria or fungi to detoxify a specific substance.

To see what investigators are trying, visit some of the research laboratories online. For example, scientists in the Pilon-Smits Lab at Colorado State University and members of the Phytoremediation Research Lab at the University of California at Berkeley collaborate on studies of heavy-metal tolerance and accumulation in Brassica juncea, or Indian mustard. Moreover, investigators in the Soil Protection and Remediation program at the Institute of Arable Crops Research (IACR) in Rothamsted explore the accumulation of heavy metals in Thlaspi caerulescens, or Alpine pennycress.

The Web also supplies several comprehensive scientific review articles on phytoremediation. For example, agronomist Rufus Chaney and his colleagues posted Phytoremediation of Soil Metals, which Current Opinion in Biotechnology published. Moreover, the Phytoremediation section from the Missouri Botanical Garden provides links to a variety of articles.

In addition, the Electronic Journal of Biotechnology includes many articles on phytoremediation, for instance, Feasible Biotechnological and Bioremediation Strategies for Serpentine Soils and Mine Spoils and Plant Biotechnology in the 21st Century: The Challenges Ahead. Some strictly online resources also supply information on phytoremediation, including the Restoration and Reclamation Review, which is a student online journal from the University of Minnesota.

Although phytoremediation remains experimental in many cases, a variety of sites describe ongoing practical applications. For example Phytoremediation: Natural Attenuation That Really Works describes work at the Brookhaven National Laboratory that uses Brassica juncea and Brassica oleracea, or cabbage, to clean up cesium-137 and strontium-90. In addition, the Remediation Technologies Development Forum (RTDF) and Brownfieldstech.org provide reports on phytoremediation field studies. The application of phytoremediation also occupies scientists at various consulting firms, including The Bioengineering Group, D. Glass Associates, Edenspace Systems Corporation, PHYTOkinetics, and Verdant Technologies.

Nevertheless, phytoremediation remains limited to sites of lower toxin concentrations. Moreover, its success depends on location, soil characters, and climate. Currently, most plants used for phytoremediation produce very little biomass, which limits the total amount of toxins taken up. As a result, more work lies ahead, but the breadth of toxic problems makes this approach worth the while.