Phytoremediation
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Phytoremediation describes the treatment of environmental problems (bioremediation) through the use of plants.
The term phytoremediation describes a range of processes mediated by plants that are useful in treating environmental problems:
- Phytoextraction - uptake and concentration of substances from the environment into plant biomass.
- Phytostabilization - reducing the mobility of substances in the environment, for example by limiting the leaching of substances from the soil.
- Phytotransformation - chemical modification of environmental substances as a direct result of plant metabolism, often resulting in their inactivation, degradation (phytodegradation) or immobilization.
- Phytostimulation - enhancement of soil microbial activity for the degradation of contaminants, typically by organisms that associate with roots. This process is also known as rhizosphere degradation.
- Phytovolatilization - removal of substances from soil or water with release into the air, sometimes as a result of phytotransformation to more volatile substances.
- Rhizofiltration - filtering water through a mass of roots to remove toxic substances or excess nutrients.
Phytoextraction
Phytoextraction (or phytoaccumulation) uses plants to remove contaminants from soils, sediments or water into harvestable plant biomass. Phytoextraction is growing rapidly in popularity world-wide. Generally this process has been more effective for extracting heavy metals than organics. It is clean, efficient, inexpensive and less environmentally desruptive than processes that require excavation of soil. At the time of disposal contaminants are typically concentrated in a much smaller volume of plant matter than an inital contaminated soil or sediment.
After the plant absorbs contaminants through the root system it will either store them in the root biomass or transport them up into the stems and leaves. A living plant may continue to absorb contaminants until it is harvested. After harvest a lower level of the contaminant will remain in the soil, so the growth/harvest cycle must be repeated through several crops to achieve a significant cleanup. After the process, the soil usually is fertile and can support other vegetation.
Two versions of phytoextraction:
- natural hyper-accumulation, where plants naturally take up the contaminants in soil unassisted, and
- induced or assisted hyper-accumulation, in which a conditioning fluid containing a chelator or another agent is added to soil to increase metal solubility or mobilization so that the plants can absorb them more easily.
- Arsenic, using the Sunflower (Helianthus annuus), or the bracken fern, a hyperaccumulator. Bracken can store arsenic in its leaves, as much as 200 times that present in the soil.
- Cadmium and zinc, using alpine pennycress (Thlaspi caerulescens), a hyperaccumulator of these metals at levels that would be toxic to many plants.
- Lead, using Indian Mustard (Brassica juncea), Ragweed (Ambrosia artemisiifolia) or Hemp Dogbane (Apocynum cannabinum).
- Sodium chloride, common salt. Salt-tolerant (moderately halophytic) barley and/or sugar beets are commonly used to reclaim fields that were previously been flooded by sea water.
- Uranium, using sunflowers, as used after the Chernobyl accident.
Phytotransformation
In the case of organic pollutants, such as pesticides, explosives, solvents, industrial chemicals, and other xenobiotic substances, certain plants render these substances non-toxic by their metabolism. In other cases, microorganisms living in association with plant roots may metabolize these substances in soil or water.
The Role of Genetics
Breeding programs and genetic engineering are powerful methods for enhancing natural phytoremediation capabilities, or for introducing new capabilities into plants. Genes for phytoremediation may originate from a microorganism or may be transferred from one plant to another variety better adapted to the environmental conditions at the cleanup site.
See also
- Biodegradation
- Bioremediation
- John Todd - cofounder of the [New Alchemy Institute]
External links
- [What is Phytoremediation?] A good overview.
- [International Journal of Phytoremediation] - devoted to the publication of current laboratory and field research describing the use of plant systems to remediate contaminated environments.
- [Using Plants To Clean Up Soils] - from [Agricultural Research magazine]
- [Phytoremediation website hosted by the Missouri Botanical Garden] - Review Articles, Conferences, Phytoremediation Links, Research Sponsors, Books and Journals, and Recent Research.
Bibliography
[“Getting the lead out”, June 1995. Vegetarian Times.]
[“Phytoextraction”, February 2000. Brookhaven National Laboratory 2000.]
[“Phytoextraction of Metals from Contaminated Soil” April 18th, 2001. M.M. Lasat]
[July 2002. Donald Bren School of Environment Science & Management.]
[“Phytoremediation” October 1997. Department of Civil Environmental Engineering.]
[“Phytoremediation” June 2001, Todd Zynda.]
[“Technology Profile: Phytoextraction” 1997. Environmental Business Association.]
[“The role of EDTA in lead transport and accumulation by Indian mustard” June 1998.]
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