Junk-greedy Greens: phytoremediation as a new option for soil decontamination

Industrialization has led to the release of enormous quantities of toxic compounds into the environment. Industrial activities such as chemical works, garages and service stations, metal fabrication shops, paper mills , tanneries, textile plants, waste disposal sites and intensive agriculture are particularly guilty of polluting1. Pollutants can be categorized into two large classes: elemental and organic. Elemental compounds include heavy metals, like mercury and lead, non-metallic inorganic compounds such as arsenic, as well radionuclides like uranium. Organic contaminants consist of petroleum hydrocarbons, chlorinated solvents (PCBs), linear halogenated hydrocarbons (TCE) and explosives such as TNT2. There is increasing scientific evidence indicating the toxicological effects of these contaminants. Conversely, there is a growing impetus to reuse abandoned polluted sites in order to conserve remaining pieces of untouched land1. To balance these issues, governmental regulations have become increasingly strict in recent years in order to limit the release of pollutants and to adequately remediate polluted areas3. There are numerous options for the remediation of contaminated sites. Commonly used engineering techniques include excavation and landfilling, chemical treatment, vitrification and electrokinetics. These methods are extremely expensive, costing between $50 and $500 per ton of soil4. This financial burden probably plays a role in slowing down global efforts to eradicate pollution, particularly in developing countries where these techniques are clearly not affordable. As a result, it is highly desirable to develop more cost-effective remediation methods. Bioremediation technologies, which focus on living organisms as clean-up agents, are seen as an alternative with great potential for affordably remediating polluted sites. Bioremediation research is led mainly by microbiologists, who try to identify the appropriate bacteria to breakdown contaminants into harmless products. The amazing diversity of the prokaryotic world makes it an endless resource of metabolic pathways that can process organic compounds1. However, biologists have now recognized that plants also have qualities that can make them great remediating agents, and a new field called phytoremediation is receiving more attention from both academia and industry. Phytoremediation is defined as the use of plants as well as microorganisms of the rhizosphere to remove or render harmless pollutants from contaminated sites5,6. The main advantage of phytoremediation is its low cost in comparison to engineering techniques, i.e. phytoremediation only costs about $5-$40 per ton of soil4. In addition, it is generally approved by the public, primarily because of its aesthetics and eco-friendly sustainability. Bioremediation using microorganisms is often performed on extracted soil, in the controlled environment of a bioreactor. It can also be used for in situ remediation, but it has often been found that the cleaning bacteria can compete with local microbes, and that keeping them at high concentrations requires the addition of a lot of nutrients7. In comparison, phytoremediation is easier to manage because it is an autotrophic system of large biomass that requires little nutrient input1. Moreover, plants offer protection against water and wind erosion, preventing contaminants from spreading8. Phytoremediation is a broad term that comprises several techniques used for water and soil decontamination. In this review, we will focus on four