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 (which does not roll off the tongue) actually makes sense. It comes from the Greek word phyto meaning “plant” and the Latin word remedium meaning “restoring balance.” When put together, the two words refer to the 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 of 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. Knowledge of the physiological and molecular mechanisms of phytoremediation began to emerge in recent years 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. The following is a list of six different types of phytoremediation with explanations describing how they work.
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 and, 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.