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.
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 (Chaney et al. 1997, Brooks et 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.
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