Phytoremediation improves metal-contaminated sites by the extraction of contaminating metals (phytoextraction), or their immobilisation (phytostabilisation). Phytoextraction removes metals from the soil by repeated crops of plants that accumulate large amounts of one or more target metals in their above-ground biomass. The harvested plant material is removed from the site. Despite more than ten years of research, there are few examples of successful phytoextraction. This technology is limited by the long period required for cleanup, the restricted number of target metals that can be extracted, the limited depth that can be accessed by roots, and the difficulty of producing a high-biomass crop of the desired species. There is also concern about metal-accumulating plants providing an exposure pathway for toxic elements to enter the food chain. The addition of chelants to enhance plant-metal uptake, invariably increases the risk of metal leaching. Phytostabilisation exploits transpiration and root-growth to immobilise contaminants by reducing leaching, controlling erosion, creating an aerobic environment in the root-zone, and adding organic matter to the substrate that binds metals. Soil amendments can promote plant growth and enhance metal immobilisation. Phytostabilisation requires the establishment of tailored vegetation on the site that is left there in perpetuity. A succession of plant species may be used to establish the desired climax vegetation. Unlike phytoextraction, there are numerous examples of successful phytostabilisation on metal-contaminated sites.
Phytoremediation technology is site specific due to the plethora of environmental variables that affect plant growth and metal mobility. Most contaminated sites contain a heterogeneous mixture of several elemental and organic contaminants. Plant-growth may be limited by other environmental variables, such as low pH, low nutrient availability, salinity, insufficient aeration or low water availability. The commercial success of phytoremediation is thus dependent on convincing decision makers that phytoremediation can satisfy environmental regulations. Obviously, field demonstrations at each site are not practical; therefore validated mechanistic models are required to calculate the effect of phytoremediation on metal fluxes. Central to such models is an understanding of root-metal interactions in these typically heterogeneous media.