Phytotechnology and Photosynthesis - Environmental Biotechnology: Theory and Application - page 147

circumstances. After the plants have been permitted to grow for a suitable length
of time, they are harvested and the metal accumulated is permanently removed
from the original site of contamination. If required, the process may be repeated
with new plants until the required level of remediation has been achieved. One of
the criticisms commonly levelled at many forms of environmental biotechnology
is that all it does is shift a problem from one place to another. The fate of
harvested hyperaccumulators serves to illustrate the point, since the biomass thus
collected, which has bioaccumulated significant levels of contaminant metals,
needs to be treated or disposed of itself, in some environmentally sensible fashion.
Typically the options are either composting or incineration. The former must
rely on co-composting additional material to dilute the effect of the metal-laden
hyperaccumulator biomass if the final compost is to meet permissible levels; the
latter requires the ash produced to be disposed of in a hazardous waste landfill.
While this course of action may seem a little un-environmental in its approach,
it must be remembered that the void space required by the ash is only around a
tenth of that which would have been needed to landfill the untreated soil.
An alternative that has sometimes been suggested is the possibility of recy-
cling metals taken up in this way. There are few reasons, at least in theory, as
to why this should not be possible, but much of the practical reality depends on
the value of the metal in question. Dried plant biomass could be taken to pro-
cessing works for recycling and for metals like gold, even a very modest plant
content could make this economically viable. By contrast, low value materials,
like lead, for example would not be a feasible prospect. At the moment, nickel
is probably the best studied and understood in this respect. There has been con-
siderable interest in the potential for biomining the metal out of sites which have
been 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, as recent
work in Thailand to improve conditions for rice growing around zinc mines has
established (Phaenark et al ., 2009).
In the light of pilot scale investigations in Australia, using the ability of euca-
lyptus trees and certain native grasses to absorb metals from the soil, a similar
approach was then 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/year; developing a success-
ful methodology suitable for deployment on a large scale 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.