Phytoextraction of mine wastes - Options and impossibilities

Phytoextraction is a green in situ technology which aims to diminish the concentration of the chemical element(s) (often synonymous with heavy metals) of contaminated soils to such a level that the soil can be used without danger for agriculture, horticulture, forestry or amenity. To achieve this goal within a reasonable time, growth of highly metal-accumulating plants, the so-called hyperaccumulators is often recommended, ignoring their inherent low biomass production, their low ability to extract more than one or two metals and their sensitivity to other elements inclusive metals in surplus. It has been demonstrated that the success of phytoextraction depends on the degree of soil contamination and on the number of metals in surplus at the site, the metal-resistance of the plant species and its association with mycorrhizal fungi and other rhizosphere organisms, and the desired degree of the final metal concentration. Up to now, transgenic higher plants have shown no advantage in the phytoextraction process, compared to the high potential of genetically not modified plant species. Phytoextraction demands that the metals are extracted from the harvested plant material by ashing, so that the ash can be deposed or even better recycled in metal-processing industries. Disposal of the contaminated plant material without further condensation of the biomass at storage sites is not a solution, but only a transfer of the problem from one contaminated site to another, often less contaminated site. Phytovolatilization after phytoextraction is nothing more than a pollution transfer to an unpolluted environment. As soon as a soil is highly contaminated with more than one metal (polymetallic soils), the phytoextraction procedure will last more than a centennium, thus this period is too long for an economically feasible technology. At such sites, phyto stabilization by revegetation is the most relevant technology in order to ensure a stabilization of the metals in the soil, and to hamper the contamination of groundwater and/or the surrounding landscape by wind and/or water erosion of the mine waste. (c) 2005 Elsevier GmbH. All rights reserved.