Phytoextraction (or phytoaccumulation) uses plants or algae to remove contaminants from soils, sediments or water into harvestable plant biomass (organisms that take larger-than-normal amounts of contaminants from the soil are called hyperaccumulators). Phytoextraction has been growing rapidly in popularity worldwide for the last twenty years or so. In general, this process has been tried more often for extracting heavy metals than for organics. At the time of disposal, contaminants are typically concentrated in the much smaller volume of the plant matter than in the initially contaminated soil or sediment. 'Mining with plants', or phytomining, is also being experimented with.
The plants absorb contaminants through the root system and store them in the root biomass and/or transport them up into the stems and/or 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 usually be repeated through several crops to achieve a significant cleanup. After the process, the cleaned soil can support other vegetation.
Advantages: The main advantage of phytoextraction is environmental friendliness. Traditional methods that are used for cleaning up heavy metal-contaminated soil disrupt soil structure and reduce soil productivity, whereas phytoextraction can clean up the soil without causing any kind of harm to soil quality. Another benefit of phytoextraction is that it is less expensive than any other clean-up process.
Disadvantages: As this process is controlled by plants, it takes more time than anthropogenic soil clean-up methods.
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. In many cases natural hyperaccumulators are metallophyte plants that can tolerate and incorporate high levels of toxic metals.
Examples of phytoextraction (see also 'Table of hyperaccumulators'):
- Arsenic, using the Sunflower (Helianthus annuus), or the Chinese Brake fern (Pteris vittata), a hyperaccumulator. Chinese Brake fern stores arsenic in its leaves.
- Cadmium, using willow (Salix viminalis): In 1999, one research experiment performed by Maria Greger and Tommy Landberg suggested willow has a significant potential as a phytoextractor of Cadmium (Cd), Zinc (Zn), and Copper (Cu), as willow has some specific characteristics like high transport capacity of heavy metals from root to shoot and huge amount of biomass production; can be used also for production of bio energy in the biomass energy power plant.
- Cadmium and zinc, using Alpine pennycress (Thlaspi caerulescens), a hyperaccumulator of these metals at levels that would be toxic to many plants. On the other hand, the presence of copper seems to impair its growth (see table for reference).
- Lead, using Indian Mustard (Brassica juncea), Ragweed (Ambrosia artemisiifolia), Hemp Dogbane (Apocynum cannabinum), or Poplar trees, which sequester lead in their biomass.
- Salt-tolerant (moderately halophytic) barley and/or sugar beets are commonly used for the extraction of sodium chloride (common salt) to reclaim fields that were previously flooded by sea water.
- Caesium-137 and strontium-90 were removed from a pond using sunflowers after the Chernobyl accident.
- Mercury, selenium and organic pollutants such as polychlorinated biphenyls (PCBs) have been removed from soils by transgenic plants containing genes for bacterial enzymes.
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