Phytotechnology and Photosynthesis - Environmental Biotechnology: Theory and Application - page 146

Hyperaccumulation
Hyperaccumulation itself is a curious phenomenon and raises a number of fun-
damental questions. While the previously mentioned pteridophyte, Pteris vittata ,
tolerates tissue levels of 0.5% arsenic, certain strains of naturally occurring alpine
pennycress ( Thlaspi caerulescens ) can bioaccumulate around 1.5% cadmium, on
the same dry weight basis. This is a wholly exceptional concentration. Quite how
the uptake and the subsequent accumulation is achieved are interesting enough
issues in their own right. However, more intriguing is why so much should be
taken up in the first place. The hyperaccumulation of copper or zinc, for which
there is an underlying certain metabolic requirement can, to some extent, be
viewed as the outcome of an over-efficient natural mechanism. The biological
basis of the uptake of a completely nonessential metal, however, particularly in
such amounts, remains open to speculation at this point. Nevertheless, with plants
like Thlaspi showing a zinc removal rate in excess of 40 kg per hectare per year,
their enormous potential value in bioremediation is very clear.
In a practical application, appropriate plants are chosen based on the type of
contaminant present, the regional climate and other relevant site conditions. This
may involve one or a selection of these hyperaccumulator species, dependent on
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 unenvironmental 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