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

Considerable advances have also been made over the last decade in the
phytoextraction of arsenic. Its removal poses a big challenge, since arsenic
behaves quite differently from other metal pollutants, typically being found
dissolved in the groundwater in the form of arsenite or arsenate, and does not
readily precipitate. The application of bipolar electrolysis to oxidise arsenite
into arsenate, which reacts with ferric ions from an introduced iron anode,
represented a step forward in the wider sphere of remediation, but generally
conventional remediation techniques aim to produce insoluble forms of the
metal's salts, which, though still problematic, are easier to remove. Clearly,
then, the prospect of a specific arsenic-tolerant plant selectively pulling the
metal from the soil was long recognised as a highly advantageous potential
addition to the bioremediator's tool-kit. The earliest candidate for the role was
the Chinese ladder brake fern, Pteris vittata , which had then been found to
accumulate arsenic in concentrations of 5 g/kg of dry biomass. Growing very
rapidly and amassing the metal in its root and stem tissue, it is easy to harvest
for contaminant removal, making it eminently suitable from a practical point of
view. Subsequent work (Gonzaga, Santos and Ma, 2008) has further established
the efficacy of this species for the task, having been shown to be able to
accumulate up to 2.3% arsenic in its biomass (Gonzaga, Santos and Ma, 2006).
Other plant species have also recently been investigated as potentially useful
arsenic phytoextractors, including Vetiveria zizanioides , known as Khus grass
(Singh et al ., 2007) and another fern Pityrogramma calomelanos (Gonzaga,
Santos and Ma, 2006).
Hyperaccumulation
Hyperaccumulation itself is a curious phenomenon and raises a number of fun-
damental questions. The previously mentioned pteridophyte, P. vittata , tolerates
tissue levels of 2.3% arsenic and certain strains of naturally occurring alpine pen-
nycress ( Thlaspi caerulescens ) can bioaccumulate around 1.5% cadmium, on the
same dry weight basis. These are wholly exceptional concentrations. Quite how
the uptake and the subsequent accumulation are 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 non-essential metal, however, particularly in
such amounts, remains open to speculation at this point. Never-the-less, with a
plant like T. caerulescens showing a zinc removal rate in excess of 40 kg/ha/year,
and suggestions that the entire Pteris genus could possibly be used in phytoex-
traction of arsenic (Gonzaga, Santos and Ma, 2006), 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