Phytoextraction is an environmental cleanup technique that uses plants to remove harmful substances from soil or water. This approach harnesses the natural ability of certain plant species to absorb and concentrate pollutants within their tissues. As a subprocess of phytoremediation, phytoextraction draws contaminants into the plant’s harvestable parts, such as shoots and leaves. After absorption, plants are harvested and safely disposed of, effectively removing contaminants from the site. This offers a sustainable way to address contaminated sites using biological processes instead of disruptive physical or chemical methods.
How Plants Clean Up Contamination
Plants perform phytoextraction by absorbing contaminants through their root systems, similar to how they take up essential nutrients. First, the targeted pollutant must dissolve in the soil’s water or surrounding aquatic environment, making it available for root uptake. Once absorbed by the roots, these contaminants transport throughout the plant, primarily moving from the roots to the shoots (stems and leaves).
Their movement within the plant is regulated by various internal mechanisms. Plants can chelate metals with organic compounds to protect themselves from toxicity and facilitate their movement. Some plants also use enzymes, such as peroxidases and laccases, to transform toxic substances into less harmful forms. The absorbed contaminants are stored within the plant’s tissues, often sequestered in structures like vacuoles or cell walls, which helps the plant tolerate high concentrations without significant damage.
What Phytoextraction Targets
Phytoextraction primarily targets inorganic pollutants, particularly heavy metals, which cannot be chemically degraded in the environment. These metals, such as lead (Pb), cadmium (Cd), arsenic (As), chromium (Cr), mercury (Hg), nickel (Ni), zinc (Zn), and copper (Cu), pose significant environmental and health threats even at low concentrations.
Beyond heavy metals, phytoextraction also addresses radionuclides, which are radioactive isotopes released from nuclear activities, and certain organic pollutants. For radionuclides like cesium-137 and strontium-90, plants can absorb and store them in their tissues. While less commonly applied to organic pollutants, plants can accumulate some unaltered organic contaminants in their above-ground parts. However, many organic compounds are metabolized or volatilized by plants rather than accumulated through phytoextraction.
Plants That Do the Work
The plants most effective for phytoextraction are known as “hyperaccumulators.” They are specialized plant species that can absorb and tolerate exceptionally high concentrations of metals in their above-ground tissues—often 100 to 1,000 times more than non-accumulating plants—without showing signs of toxicity. Over 450 plant species have been identified as hyperaccumulators, with many belonging to families like Brassicaceae, Lamiaceae, and Euphorbiaceae.
Specific examples include Thlaspi caerulescens (alpine pennycress), known for its ability to hyperaccumulate zinc and cadmium, thriving on soils contaminated with these metals. Another is Alyssum murale, a nickel hyperaccumulator accumulating up to 3% nickel by dry weight in its tissues. Sunflowers (Helianthus annuus) have also been used, particularly for radionuclides like cesium and strontium, absorbing these elements, especially from waterlogged soils. Ideal phytoextraction plants also possess traits like fast growth rates, high biomass production, and deep root systems, increasing contaminant removal efficiency.
Where Phytoextraction is Applied
Phytoextraction finds practical application in various environmental cleanup scenarios, primarily for contaminated land and wastewater treatment. It is used to remediate industrial sites, former mining areas, and agricultural lands affected by pollutant accumulation. It cleans up large areas of soil contaminated with heavy metals, preventing spread and making land suitable for other uses.
In wastewater treatment, phytoextraction is employed in systems like constructed wetlands, with aquatic plants absorbing pollutants. This method is particularly effective for removing heavy metals and radionuclides from contaminated water streams. Phytoextraction offers advantages, including less disruption to soil structure, cost-effectiveness compared to traditional excavation, and aesthetic improvement through vegetation.