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Phytovolatilisation involves the uptake of the contaminants by plants and their release into the atmosphere, typically in a modified form. This phytoremediation biotechnology generally relies on the transpiration pull of fast-growing trees, which accelerates the uptake of the pollutants in groundwater solution, which are then released through the leaves. Thus the contaminants are removed from the soil, often being transformed within the plant before being voided to the atmo-sphere. One attempt which has been explored experimentally uses a genetically modified variety of the Yellow Poplar, Liriodendron tulipifera, which has been engineered by the introduction of mercuric reductase gene (mer A) as discussed. This confers the ability to tolerate higher mercury concentrations and to convert the metal’s ionic form to the elemental and allows the plant to withstand contaminated conditions, remove the pollutant from the soil and volatilise it. The advantages of this approach are clear, given that the current best available technologies demand extensive dredging or excavation and are heavily disruptive to the site.
The choice of a poplar species for this application is interesting, since they have been found useful in similar roles elsewhere. Trichloroethylene (TCE), an organic compound used in engineering and other industries for degreasing, is a particularly mobile pollutant, typically forming plumes which move beneath the soil’s surface. In a number of studies, poplars have been shown to be able to volatilise around 90% of the TCE they take up. In part this relates to their enor-mous hydraulic pull, a property which will be discussed again later. Acting as large, solar-powered pumps, they draw water out of the soil, taking up contaminants with it, which then pass through the plant and out to the air.
The question remains, however, as to whether there is any danger from this kind of pollutant release into the atmosphere and the essential factor in answering that must take into account the element of dilution. If the trees are pumping out mercury, for instance, then the daily output and its dispersion rate must be such that the atmospheric dilution effect makes the prospect of secondary effects, either to the environment or to human health, impossible. Careful investigation and risk analysis is every bit as important for phytoremediation as it is for other forms of bioremediation.
Using tree species to clean up contamination has begun to receive increasing interest. Phytoremediation in general tends to be limited to sites where the pollu-tants are located fairly close to the surface, often in conjunction with a relatively high water table. Research in Europe and the USA has shown that the deeply penetrating roots of trees allows deeper contamination to be treated. Once again, part of the reason for this is the profound effect these plants can have on the local water relations.
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