Phytotechnology and Photosynthesis 145In effect, phytoremediation may be defined as the directin situuse of living
green plants for treatment of contaminated soil, sludges or groundwater, by the
removal, degradation, or containment of the pollutants present. Such techniques
are generally best suited to sites on which low to moderate levels of contamination
are present fairly close to the surface and in a relatively shallow band. Within
these general constraints, phytoremediation can be used in the remediation of land
contaminated with a variety of substances including certain metals, pesticides,
solvents and various organic chemicals.
Metal Phytoremediation
The remediation of sites contaminated with metals typically makes use of the
natural abilities of certain plant species to remove or stabilise these chemicals by
means of bioaccumulation, phytoextraction, rhizofiltration or phytostabilisation.
Phytoextraction
The process of phytoextraction involves the uptake of metal contaminants from
within the soil by the roots and their translocation into the above-ground regions
of the plants involved. Certain species, termed hyperaccumulators, have an innate
ability to absorb exceptionally large amounts of metals compared to most ordinary
plants, typically 50–100 times as much (Chaneyet al. 1997, Brookset al. 1998)
and occasionally considerably more. The original wild forms are often found
in naturally metal-rich regions of the globe where their unusual ability is an
evolutionary selective advantage. Currently, the best candidates for removal by
phytoextraction are copper, nickel and zinc, since these are the metals most
readily taken up by the majority of the varieties of hyperaccumulator plants. In
order to extend the potential applicability of this method of phytoremediation,
plants which can absorb unusually high amounts of chromium and lead are also
being trialled and there have been some recent early successes in attempts to
find suitable phytoextractors for cadmium, nickel and even arsenic. The removal
of the latter is 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. There have been
some advances like the application of bipolar electrolysis to oxidise arsenite
into arsenate, which reacts with ferric ions from an introduced iron anode, 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, a specific arsenic-tolerant plant selectively pulling the metal from the soil
would be a great breakthrough. One attempt to achieve this which has shown
some promise involves the Chinese ladder brake fern,Pteris vittata, which has
been found to accumulate arsenic in concentrations of 5 grams per kilogramme
of dry biomass. Growing very rapidly and amassing the metal in its root and
stem tissue, it is easy to harvest for contaminant removal.