Phytoremediation

Phytoremediation involves the use of plants to remove, transfer, stabilise and/or degrade organic and inorganic contaminants in soil, sediment and water. The technique is emerging as an attractive remedial method due to its simplicity, relatively low cost and in situ "green clean" approach. In addition to the removal of contaminants, the technique offers containment of leachates from soil and landfill, while also facilitating the improvement (or at least maintenance) of soil structure and fertility and the possibility of contaminant recycling. It is particularly suited to large expanses of soils, on which excavation is not possible, and the treatment of ground and waste waters that are transported into shallow ponds or bioreactors. It can be used in conjunction with other technologies, especially where vegetation is used as a final "polishing treatment", and is applicable to a wide range of contaminants, at varying concentrations.
Phytoremediation includes five main methods, described below.

Phytodegradation
Phytodegradation is the enzyme-catalysed metabolism of contaminants, typically organics, within plant tissues. To date this process has mainly been used on surface and ground waters, since the contaminant must be readily taken up by the plant and absorption is often faster from aqueous media.

Phytostabilisation
Phytostabilisation utilises the plant production of compounds which immobilise contaminants at the interface of roots and soil, or roots and water. An example of this method is where root exudates cause the precipitation of metals, reducing their bioavailability.

Rhizofiltration
Rhizofiltration is based on a combination of phytoextraction and phytostabilisation methods and was originally developed for metal and radionuclide polluted water (surface, ground or wastewater). Contaminants are absorbed and concentrated by plant roots, then precipitated as phosphates and carbonates.

Despite a focus towards inorganic contaminants, rhizofiltration with aquatic and wetland plants has recently been applied to waters that contain organics such as tetrachloroethane, trichloroethylene, metolachlor, atrazine, nitrotoluenes, anilines, dioxanes and various petroleum hydrocarbons. Hydroponically grown terrestrial plants are often used (e.g. the sunflower, Helianthus annuus L.), as they tend to have larger root systems and greater biomass compared to aquatic ones. Numerous species of Populus have been particularly useful in groundwater treatment as they have penetrating, water-loving roots that are tolerant to high contaminant concentrations

Enhanced rhizosphere biodegradation
Enhanced rhizosphere biodegradation is stimulated by plants whose roots release substances that are nutrients for microorganisms, such as fungi and bacteria. The presence of these microbial nutrients enhances biological activity in the area immediately surrounding the roots, thereby accelerating the digestion of organic substances in the upper soil layers and/or groundwater.

Phytoaccumulation
Phytoaccumulation involves the uptake of soil contaminants by plant roots, followed by their translocation through the xylem and accumulation in the shoots and leaves. While some contaminants, such as selenium, mercury and volatile organics, can be released through the leaves into the atmosphere (i.e. phytovolatilisation), the aerial parts of the plant must be collected to complete the remediation.

For the process to be economically viable a cultivated plant must hyperaccumulate the contaminant(s) and produce large biomass. Other factors, such as growth rate, element selectivity, resistance to diseases, method of harvesting and disposal, are also important. Although taxonomically widespread, a significant number of known hyperaccumulators are members of the Brassicaceae family, including Brassica juncea, Alyssum and Thlaspi species, and various grasses.

Postharvest processing of the plant materials currently incorporates thermal, microbial and/or chemical treatments to reduce the biomass. Recovery of constituents, such as the trace metals, is currently under investigation.