Section 3:  General Phytoremediation

  Key Concepts

• Phytoremediation
• Phytostabilization
• Phytodecontamination
• Rhizosphere Degradation
• Phytoaccumulation and Phytoextraction

Phytoremediation is a general term used to describe various mechanisms by which living plants alter the chemical composition of the soil matrix in which they are growing. Essentially, it is the use of green plants to clean-up contaminated soils, sediments, or water. The word "phytoremediation" is from the Greek prefix phyto- meaning "plant" and the Latin root word remidium- meaning "to correct or remove an evil". In soil, the "evil" could be anthropogenic (man-made) contaminants such as organic solvents, heavy metals, pesticides, or radionuclides.
 

The research available to implement phytoremediation has been in existence for more than 20 years in industrial fields, yet phytoremediation is a new technology for remediation of urban, residential areas.

Phytoremediation Advantages:

• phytoremediation cost’s are much less than traditional in situ and ex situ processes
• plants can be easily monitored to ensure proper growth
• valuable metals can be reclaimed and reused through phytoremediation
• the least destructive method of remediation because it utilizes natural organisms
• preserves the natural state of the environment

       Phytoremediation Limitations:

• phytoremediation is confined to the area covered by the depths of the roots.
• slow growth rate and small biomass = large time commitment involved
• leeching of contaminants into groundwater cannot be fully prevented by plant based remediation systems
• plant survival is at risk due to the toxicity of the contaminated soil as well as general soil condition.
• threat of bioaccumulation of contaminants such as heavy metals, from primary to secondary consumers in the food chain

 

One phytoremediation process is phytostabilization

     Definition:

       Phytostabilization is the use of plants to stabilize the soil matrix itself and immobilize the contaminant from future migration
 

Phytostabilization techniques are most appropriate for relatively immobile materials and large surface areas, and may work better with heavier textured soils. The soil conditions may be toxic to plants at sites of contamination, resulting in isolated or scattered plant cover. However, phytostabilization can provide a dense vegetative ground cover which can greatly reduce soil erosion and human exposure to contaminants (such as Pb) via dermal contact and inhalation (Cunningham, 1997).
 

 
 

       Definition:

The contaminants can be degraded or removed from the soil matrix, through the process of rhizosphere degradation and      phytoaccumulation/phytoextraction, respectively.

 
These processes are different from phytostabilization, which only binds the contaminant and reduces its mobility. Through rhizosphere degradation, the contaminant can be degraded to other byproduct compounds that may be less toxic, persistent, and reactive. Phytoextraction removes the contaminant from the soil matrix by concentrating it in the plant biomass, which requires the plant to be removed, or releasing it to the atmosphere, which is known as phytovolatization.

Rhizosphere degradation:

The rhizosphere soil is the soil adhering to the root and contains higher microbial numbers, biomass, and activity than surrounding root free soil. Phytoremediation relies heavily on rhizosphere processes. Plant roots provide a large surface area for microbial colonies, enhancing the foundation for rhizosphere development. The root system can also enhance the microorganisms’ ability to biodegrade hazardous persistent organic compounds such as PAHs into harmless compounds, through the natural release of nutrients, enzymes, and oxygen. However, only contaminants in close vicinity of the root system have the possibility to be degraded. In addition for rhizosphere degradation to be successful, the contaminants must be biologically available for adsorption to the plant roots or associated microorganisms.
 
 

This process involves the translocation of the heavy metals or inorganics from the rhizosphere to the easily harvestable shoots, which must be disposed of properly after they are harvested. Most plants that grow on metal-contaminated soil can accumulate and store heavy metals in their roots and shoots to some extent. However, metal hyperaccumulators are plants that can accumulate and concentrate metals in their shoots to levels of at least 0.1% metal/dry biomass. Such hyperaccumulators can be used to reduce inorganic levels to levels that are environmentally acceptable and meet regulatory requirements. While lead is not a known essential nutrient for higher nutrients, there is evidence indicating that various plant species have the ability to absorb lead through the roots and translocate the metal to the shoots.