154 Environmental Biotechnology
successfully used on some sites may simply serve to complicate matters on others.
One such approach which achieved commercial scale use in the USA, 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 char-
acter of the site geology, it has been suggested that this could also allow lead to
percolate downwards through the soil, and perhaps ultimately into watercourses.
While it may well be possible to overcome this potential problem, using accurate
mathematical modelling, followed by the establishment of good hydraulic con-
tainment 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 bioengineering. The
potential benefits of phytotechnology for inexpensive, large-scale land manage-
ment are clear, but the lack of quantitative field data on its efficacy, especially
compared with actively managed alternative treatment 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 requires better codification. Much
research is underway in both public and the private sectors which should throw
considerable light on these issue. Hopefully it will not be too long in the future
before such meaningful comparisons can be drawn.
One area where phytoremediation may have a particular role to play, and
one which might be amenable to early acceptance is as a polishing phase in
combination with other clean-up technologies. As a finishing process follow-
ing on from a preceding bioremediation or nonbiological method first used to
deal with ‘hot-spots’, plant-based remediation could well represent an optimal
low-cost solution. The tentative beginnings of this have already been tried in
small-scale trials and techniques are being suggested to treat deeply located con-
taminated groundwater by simply pumping to the surface and using it as the
irrigant for carefully selected plant species, allowing them to biodegrade the pol-
lutants. The lower levels of site intrusion and engineering required to achieve
this would bring clear benefits to both the safety and economic aspects of the
remediation operation.
Aquatic Phyto-Systems (APS)
Aquatic phyto-systems are principally used to process effluents of one form or
another, though manufactured wetlands have been used successfully to remediate
some quite surprising soil contaminants, including TNT residues. Though the
latter type of application will be discussed in this section, it is probably best
considered as an intergrade between the other APS described hereafter and the
TPS of the previous. Many of the aspects of the biotreatment of sewage and