148 Environmental Biotechnology
adsorption onto, or precipitation around, the roots of plants. On first inspection,
the difference between these approaches is difficult to see, since in effect, phy-
tostabilisation does employ both extractive and filtrative techniques. However,
what distinguishes this particular phytoremediation strategy is that, unlike the
preceding regimes, harvesting the grown plants is not a feature of the process.
In this sense, it does not remove the pollutants, but immobilises them, delib-
erately concentrating and containing them within a living system, where they
subsequently remain. The idea behind this is to accumulate soil or groundwater
contaminants, locking them up within the plant biomass or within the rhizosphere,
thus reducing their bio-availability and preventing their migration off site. Metals
do not ultimately degrade, so it can be argued that holding them in place in this
way is the best practicable environmental option for sites where the contamina-
tion is low, or for large areas of pollution, for which large-scale remediation by
other means would simply not be possible.
A second benefit of this method is that on sites where elevated concentrations
of metals in the soil inhibits natural plant growth, the use of species which have
a high tolerance to the contaminants present enables a cover of vegetation to
be re-established. This can be of particular importance for exposed sites, min-
imising the effects of wind erosion, wash off or soil leaching, which otherwise
can significantly hasten the spread of pollutants around and beyond the affected
land itself.
Organic Phytoremediation
A wide variety of organic chemicals are commonly encountered as environ-
mental pollutants including many types of pesticides, solvents and lubricants.
Probably the most ubiquitous of these across the world, for obvious reasons,
are petrol and diesel oil. These hydrocarbons are not especially mobile, tend to
adhere closely to the soil particles themselves and are generally localised within
2 metres of the surface. Accordingly, since they are effectively in direct contact
with the rhizosphere, they are a good example of ideal candidates for phytoreme-
diation. The mechanisms of action in this respect are typically phytodegradation,
rhizodegradation, and phytovolatilisation.
Phytodegradation
Phytodegradation, which is sometimes known by the alternative name of phyto-
transformation, involves the biological breakdown of contaminants, either inter-
nally, having first been taken up by the plants, or externally, using enzymes
secreted by them. Hence, the complex organic molecules of the pollutants are
subject to biodegradation into simpler substances and incorporated into the plant
tissues. In addition, the existence of the extracellular enzyme route has allowed
this technique to be successfully applied to the remediation of chemicals as var-
ied as chlorinated solvents, explosives and herbicides. Since this process depends