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 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. In the light of recent advances in Australia, using the ability of eucalyptus trees and cer-tain native grasses to absorb metals from the soil, the approach is to be 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 per year; success in this trial 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. Clearly, the metallurgists would have to be assured that it was a worthwhile exercise. The recycling question is a long way from being a workable solution, but potentially it could offer a highly preferable option to the currently prevalent landfill route.