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Fytoremediering af forurening med olie- og tjæreprodukter
Phytoremediation is an experimental biotechnology for cleaning polluted soil and
groundwater. Plants mainly used are trees (poplar, willow), grasses (ryegrass) and legumes
(alfalfa, Medicágo satíva). The plants change the physical, chemical and
biological conditions in the soil surrounding their roots, and impact the site's water
balance through their transpiration.
Various organic pollutants have been shown to be eliminated from soil using plants. The
mechanisms responsible range from enhanced microbial degradation to plant uptake and
volatilization, and have not been identified in all cases reported. However, mathematical
modelling allows to pinpoint the main route of contaminant elimination, their
physico-chemical properties of the individual contaminant being the decisive parameters.
Most of the studies have been performed in the US, but work has also been done in
Denmark and other European countries. Although phytoremediation is an in situ
technique, not a single controlled in situ study has shown remediation starting
from "contaminated" down to "clean" (meeting soil quality criteria).
Apparently the technology is still younger than the time span reqired for complete
remediation. This leaves an important gap of knowledge, as laboratory "controls"
are not nontreated (soil has been manipulated) and, therefore, degradation in the absence
of plants will be faster than in situ. On the other hand, the effect of vegetation
may also be overestimated in laboratory models, as bioavailability in situ is less,
due to the contaminants' age, higher sorption and uneven spatial distribution. Until
results from in situ studies are available, the outcome of phytoremediation cannot
be predicted with certainty.
Only a few studies target the phytoremediation of BTEX and light-chain alkanes. These
compounds are not persistent in the environment and can be quickly degraded by bacteria,
if oxygen supply is sufficient. Vegetation will accelerate elimination of BTEX by
improving aeration, and by uptake and volatilization.
The elimination of MTBE, a quite recalcitrant
compound, can be stimulated by vegetation, the major route of dissipation being plant
uptake and evapotranspiration. If volatilization of MTBE is considered environmentally
acceptable, this contaminant may be the most promising candidate for phytoremediation, as
plants simply serve as a pump removing the contaminated water. The limitation of the
technology is, of course, the maximum depth of tap roots.
In numerous projects, accelerated elimination of PAH
from soil in the presence of vegetation has been shown. The elimination of the low
molecular weight PAHs, which also are more biodegradable, is significantly enhanced by
vegetation, while the removal of high molecular weight PAHs (e.g., benzo(a)pyrene), which
tend to be bio-unavailable and less biodegradable, was only improved in a few cases.
Oil pollution in top soil is approximately reduced by 40% in two years. In a particular
oil spill on surface soil, degradation of the oil residues was by bacteria living on the
roots of Compositae plants.
One of the major disadvantages of phytoremediation is that it requires large time spans
in the order of years. For one, establishing a vegetation cover in the field takes at
least one growing season, and measurable effects should not be expected before the second
year. And secondly, removal rates shown have in all cases extrapolated to several seasons
for reaching soil quality criteria.
Another principal limitation of phytoremediation relates the rooting depth of plants.
In practice one would choose the plant species according to the depth of the
contamination, e.g., grasses and other small plants for surface soil, deep-rooting
species, e.g., trees and alfalfa, for subsoil contamination. The maximum depth of
effective soil decontamination presently should be considered to be about 2 m. In the
future, however, water-soluble compounds, e.g., MTBE
and BTEX, may be shown to be extractable by tap roots, exceding 10 m in depth.
There does not appear to be a biological "compatibility" of plant species to
chemical contaminant classes. Therefore, the choice of plant for a phytoremediation
project should be guided by agronomic considerations and rooting depth. All successful
studies utilized locally grown crop or forestry plants, or wild plants adapted to soil and
climate. In Denmark willow is the fastest growing plant, but may be limited by
insufficient water supply at some sites, in which case its close relative, the poplar,
should be considered. However, several successes with PAHs seem to point at alfalfa as the
plant of choice.
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