From green to clean: options for phytoremediation and phytoextraction of organic pollutants

The use of green plants specifically chosen or tailored for the remediation of polluted soils and brownfields, the treatment of industrial wastewater or the removal of volatile organic compounds from indoor or outdoor air is becoming essential for sustainable development. Phytoremediation options are less expensive compared to physico-chemical approaches, even if the time scale required reaching the target end-points can be a limiting factor. Phytotechnologies are now offering efficient tools and environmentally friendly solutions for the cleanup of soils and water contaminated by organic pollutants. However this goal requires a sound understanding of how plants can specifically accumulate, detoxify and metabolise xenobiotic compounds. Significant progress has been made recently to better understand the soil, rhizosphere and plant mechanisms involved in the phytotreatment of organic pollutants; to implement phytoextraction, phytodegradation, rhizofiltration, and rhizodegradation as instruments for the removal and/or management of organic contaminants present in soil and water; and to identify critical points in overcoming bottlenecks in plant removal and detoxification of xenobiotics. Depending on the physico-chemical properties of the organic pollutant to be removed or detoxified, as well as on the specific plant physiology and biochemistry, different phytotreatments are available to decontaminate water and soils. For example, aquatic macrophytes or even terrestrial plants can be grown under hydroponic conditions or in constructed wetlands to remove many xenobiotic compounds, e.g. sulphonated anthraquinones and azo dyes present in wastewater from the dye and textile industries. Advances have also been made to remediate soils contaminated with hydrophobic compounds like PCBs, petroleum hydrocarbons or pesticides (e.g. lindane or DDT), highlighting the respective roles of plants, endophytic and rhizospheric bacteria, and the importance of their interactions. These examples illustrate the potential of plants for the phytotreatment of xenobiotics, as well as their ability to cooperate with microorganisms (phytostimulation). Exploring and exploiting the biological diversity of plant metabolism is necessary. Consideration of plant taxonomy and phytochemistry should be the first steps in the appropriate use of the huge potential of plant species, since plants often produce natural chemicals whose structure is close to foreign compounds. Finally, the success of phytoremediation also largely depends on the ability of plants to tolerate the pollutant(s) to be removed. The possible effects of these contaminants on plant physiology and biochemistry must be addressed, since they will determine the ability of a specific plant to deal with specific pollutants or a mixture of them, and thus affect the efficiency of any phytotreatment. It is thus of utmost importance to determine the maximal possible amount of the xenobiotic compounds that can be accumulated and detoxified without injury, critical stress and disruption of metabolism or redox processes in the plant species under consideration. Such information will help to maintain the plant wellness, a key factor to correctly design and size the constructed wetland to remove the foreign compounds present in a particular wastewater or to estimate the time required to clean up a contaminated soil.