Phytotechnology and Photosynthesis 147been subject to diffuse contamination, or former mines where further traditional
methods are no longer practical. The manner proposed for this is essentially phy-
toextraction and early research seems to support the economic case for drying
the harvested biomass and then recovering the nickel. Even where the actual
post-mining residue has little immediate worth, the application of phytotechno-
logical measures can still be of benefit as a straightforward clean-up. In the light
of recent advances in Australia, using the ability of eucalyptus trees and cer-
tain native grasses to absorb metals from the soil, the approach is to be tested
operationally for the decontamination of disused gold mines (Murphy and Butler
2002). These sites also often contain significant levels of arsenic and cyanide
compounds. Managing the country’s mining waste is a major expense, costing
in excess of Aus$30 million per year; success in this trial could prove of great
economic advantage to the industry.
The case for metals with intermediate market values is also interesting. Though
applying a similar approach to zinc, for instance, might not result in a huge com-
mercial contribution to the smelter, it would be a benefit to the metal production
and at the same time, deal rationally with an otherwise unresolved disposal issue.
Clearly, the metallurgists would have to be assured that it was a worthwhile
exercise. The recycling question is a long way from being a workable solution,
but potentially it could offer a highly preferable option to the currently prevalent
landfill route.
Rhizofiltration
Rhizofiltration is the absorption into, or the adsorption or precipitation onto,
plant roots of contaminants present in the soil water. The principal difference
between this and the previous approach is that rhizofiltration is typically used
to deal with contamination in the groundwater, rather than within the soil itself,
though the distinction is not always an easy one to draw. The plants destined
to be used in this way are normally brought on hydroponically and gradually
acclimatised to the specific character of the water which requires to be treated.
Once this process has been completed, they are planted on the site, where they
begin taking up the solution of pollutants. Harvesting takes place once the plants
have become saturated with contaminants and, as with the phytoextraction, the
collected biomass requires some form of final treatment. The system is less widely
appreciated than the previous technology, but it does have some very important
potential applications. Sunflowers were reported as being successfully used in a
test at Chernobyl in the Ukraine, to remove radioactive uranium contamination
from water in the wake of the nuclear power station accident.
Phytostabilisation
In many respects, phytostabilisation has close similarities with both phytoextrac-
tion and rhizofiltration in that it too makes use of the uptake and accumulation by,