Phytotechnology and Photosynthesis - Environmental Biotechnology: Theory and Application - page 154

toluene, ethylbenzene, xylenes (collectively known as BTEXs) and polycyclic
aromatic hydrocarbons (PAHs). They can also be very helpful in controlling
wind-blown dust, wash-off and erosion. The selection of appropriate plant species
for bio-engineering is not, however, limited solely to their direct ability to treat
contaminants, since the enhancement of existing conditions forms as much a part
of the potential applications of phytotechnology as bioremediation. For instance,
legumes can be of great benefit to naturally nitrogen deficient soils, since they
have the ability, via symbiotic root nodule bacteria, to directly fix nitrogen from
the atmosphere. With so much to take into consideration in plant selection, the
value of a good botanist or agronomist in any interdisciplinary team is clear.
Applications
Phytotechnology has many potentially beneficial land uses, and the applications
are still developing, but despite the increasing interest being shown in them, it
is probably fair to say that they have not entirely reached their full potential
yet. In part this may be due to some of the doubts that have been voiced, sug-
gesting that the beneficial effects of plant utilisation, particularly in respect of
phytoremediation, have been over-stated. Some have argued that the reality may
range from genuine enhancement to no effect, or even to a negative contribution
under certain circumstances and that the deciding factors have more to do with
the nature of the site than the plants themselves. In addition, some technologies
which have been successfully used on some sites may simply serve to complicate
matters on others. One such approach which achieved commercial scale use in the
United States, principally for lead remediation, required the addition of chemicals
to induce metal take up. Lead normally binds strongly to the soil particles and
so its release was achieved by using chelating agents like ethylene diamine tetra
acetic acid (EDTA), which were sprayed onto the ground. With the lead rendered
biologically available, it can be taken up by plants and hence removed. However,
dependent on the character of the site geology, it has been suggested that this
could also allow lead to percolate downwards through the soil, and perhaps ulti-
mately into watercourses. While it may well be possible to overcome this potential
problem, using accurate mathematical modelling, followed by the establishment
of good hydraulic containment as an adjunct to the process, or by running it in
a contained biopile, it does illustrate one of the major practical limitations of
plant bio-engineering. The potential benefits of phytotechnology for inexpensive,
large scale land management are clear, but the relative lack of quantitative field
data on its efficacy, especially compared with actively-managed alternative treat-
ment options, is a serious barrier to its wider adoption. In addition, the roles of
enzymes, exudates and metabolites need to be more clearly understood and the
selection criteria for plant species and systems for various contamination events
require better codification. Much research is underway in both public and the
private sectors which should ultimately throw considerable light on these issues
and allow meaningful comparisons to be drawn once and for all.