Phytoremediation
Phytoremediation is the use of plants to bioremediate contaminants. Metals can be phytoremediated by several different processes including phytoextraction, rhizofiltration, and phytostabalization. Phytoremediation can also be used to clean up hydrocarbons and nitrogenous compounds.
This is a type of phytoremediation whereby plants absorb pollutants and concentrate them in their tissues. Certain plants known as hyperaccumulators have evolved the capacity to extract particularly high levels of toxic substances. Some known hyperaccumulators are pictured below.
Alyssum murale; Pteris vitatta; Thlaspi caerluscens
Absorption: Plants first release special chelates known as phytosiderophores that bond to metals in the rhizosphere and allow them to be absorbed by the plant.
Translocation: Next, metals are either stored in the root or transported to the chute through the xylem tube. Normally metals would have difficulty traveling through the high cation exchange capacity of the xylem, but they are once again chelated to facilitate this process.
Storage: Once the metals arrive at the chute they are stored in plant cell vacuoles. Inside the vacuole, they are once again chelated to prevent further damage to the plant.
How is phytoextraction implemented?
Chelate application: In many cases, phytoextraction is limited by the plants inability to absorb the metal. Thus adding an artificial chelate, such as ethylenediaminetetraacetic acid (EDTA), will greatly increase the plant’s absorption potential.
EDTA is an excellent chelate since it has 4 ligand sites that bind to the metal.
Continuous phytoextraction: Without artificial chelates, this the gradual absorption of toxins by the hyperaccumulator and relies solely on the physiological capabilities of the plant. In order to improve these capabilities, researchers are exploring genetic engineering to increase the enzyme production in rate limiting steps such as chelate production and plant growth rate.
Chelate application vs. continuous phytoextraction
Toxins can be filtered from groundwater by plant roots. With this technique, metals are deposited on roots via precipitation, ion exchange, or chelation. This strategy has been successful for a wide range of pollutants including Cu, Cd, Cr, Ni, Pb, Zn, and U.
Sunflowers (Asteracaea spp.) have successfully been implemented for rhizofiltration at Chernobyl to remediate uranium contamination.
In phytovolatilization, plants incorporate metals into volatile organic compounds which are then lost to the atmosphere. Phytovolatilization has been successful in remediating Se, As, and Hg. While there is still some uncertainty about the mechanism for this action, some studies suggest that volatilization in plants is closely tied to microbial activity.
Astragalus racemosus
Sources:
Dushenkov (1995); Lasat (2002); McGrath (2003); Raskin (1997); Salt (1998)