6 Types of Phytoremediation
The word phytoremediation comes from the Greek word phyto (plant), and the Latin word remedium (restoring balance). The technology is a form of bioremediation (the use of organisms to clean up contaminated soil) and applies to all chemical or physical processes that involve plants for degrading or immobilizing contaminants in soil and groundwater.
The Concept of 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 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 soil, 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 has 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.
Also referred to as phytostabilization, there are many different processes that fall under this category. They can involve absorption by roots, adsorption to the surface of roots, or the production of biochemicals by a plant that is released into the soil or groundwater in the immediate vicinity of the roots and can sequester, precipitate, or otherwise, immobilize nearby contaminants.
This process takes place in the soil or groundwater immediately surrounding the plant roots. Exudates (excretions) 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. For example, poplar trees were used to contain a groundwater plume of methyl-tert-butyl-ether (MTBE).
This term is also known as phytoaccumulation. Plants take up or hyper-accumulate contaminants through their roots and store them in the tissues of stems 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, stems, or leaves.
Some Areas of Concern
Because phytoremediation is relatively new in practice, there still are questions about its broader environmental impact. According to the Center for Public Environmental Oversight (CPEO), more research is needed to understand the effect of various compounds on the entire ecosystem of which plants may be a part
Depending on the concentration of contaminants in the soil, phytoremediation may be limited to less concentrated areas since plants are limited in the amount of waste they can uptake and process.
Additionally, the CPEO warns that large amounts of surface area are needed for phytoremediation treatments to be successful. Some contaminants might be transferred across different mediums (soil, air, or water), and some contaminants are not compatible with the treatment (such as polychlorinated biphenyls, or PCBs).