Also called phytostabilization. Many different processes fall under this category which can involve absorption by roots, adsorption to the surface of roots or the production of biochemicals by the plant that are released into the soil or groundwater in the immediate vicinity of the roots, and can sequester, precipitate, or otherwise, immobilize nearby contaminants.
This takes place in the soil or groundwater immediately surrounding the plant roots. Exudates from plants stimulate rhizosphere bacteria to enhance biodegradation of soil contaminants.
Use of deep-rooted plants (usually trees) to contain, sequester, or degrade groundwater contaminants that come into contact with their roots. In one example of this, poplar trees were used to contain a groundwater plume of methyl-tert-butyl-ether (MTBE).
Hong et al. 2001. Environmental Science and Technology 35(6):1231-1239.
Also known as phytoaccumulation. Plants take up—or hyper-accumulate—contaminants through their roots and store them in the tissues of the stem or leaves. The contaminants are not necessarily degraded but are removed from the environment when the plants are harvested. This is particularly useful for removing metals from soil. In some cases, the metals can be recovered for reuse by incinerating the plants in a process called phytomining.
Plants take up volatile compounds through their roots, and transpire the same compounds, or their metabolites, through the leaves, thereby releasing them into the atmosphere.
Contaminants are taken up into the plant tissues where they are metabolized, or biotransformed. Where the transformation takes place depends on the type of plant and can occur in roots, stem or leaves.
Six Types of Phytoremediation
Phytoremediation is a form of bioremediation and applies to all chemical or physical processes that involve plants for degrading or immobilizing contaminants in soil and groundwater. The word comes from the Greek word phyto, meaning plant, and the Latin word remedium, meaning restoring balance. When put together, the two words refer to technologies that use living plants to clean up soil, air, and water contaminated with hazardous chemicals.
Why People Use Phytoremediation
Phytoremediation is a cost-effective, plant-based approach to remediation that takes advantage of the ability of plants to concentrate elements and compounds from the environment and to metabolize various molecules in their tissues. It refers to the natural ability of certain plants called hyperaccumulators to bioaccumulate, degrade, or render harmless contaminants in soils, water, or air. Toxic heavy metals and organic pollutants are the major targets for phytoremediation. Since the late 20th century, knowledge of the physiological and molecular mechanisms of phytoremediation have begun to emerge together with biological and engineering strategies designed to optimize and improve phytoremediation. In addition, several field trials confirmed the feasibility of using plants for environmental cleanup.
While the technology is not new, current trends suggest its popularity is growing.
Areas of Concern
Because phytoremediation is relatively new in practice, there still are questions about its broader environmental impact. The Center for Public Environmental Oversight (CPEO), for example, says more research is needed to understand the impact of various compounds on the entire ecosystem that plants might be a part of.
For example, are plants introducing harmful chemicals into the food chain? Do contaminants collect in the leaves and wood of trees that might be used for mulch or for burning? If plants do contain high levels of contaminants, how should disposal of those plants be handled?