HyperaccumulatorHyperaccumulators are phytoremediative plants. Phytoremediation is a subfield of biological remediation techniques (Bioremediation ) and generally refers to the remediation of polluted and contaminated soil or groundwater with the help of plants. This is a so-called in-situ process, as the treatment of the soil or water takes place on site. Phytoremediation is being continuously developed. A distinction is made between different methods:.
Phytoremediation involves the complete restoration of polluted soils to a state close to the functioning of a natural soil (Bradshaw 1997). This subdivision of phytoremediation uses plants native to the area where the phytoremediation work is carried out. This is to achieve the full rehabilitation of the original natural ecosystem, from soil to plant communities. As Peer et al (2005) point out, compared to other phytoremediation techniques, phytoremediation highlights the issue of the level of decontamination required and sufficient. There is a big difference between decontaminating a soil to a legally satisfactory level so that it can be used again and fully restoring an area to pre-contamination conditions. When referring to wastewater phytoremediation, this is a recent process involving the use of the natural self-purification properties of plants (Dabouineau et al., 2005). Used in this sense, phytoremediation becomes synonymous with the term phytodepuration. This type of process includes water purification by macrophytes. In this case, it is the bacteria living in the root zone of the macrophytes that are responsible for the depollution, the plants simply serving as a growth substrate for the micro-organisms (see Honfleur plant).
The etymology comes from the Greek "phyton" = plant, and the Latin "remedium" = restoration of balance, remediation. Phytoremediation is not a new concept, since 3,000 years ago humans were already using the purifying capacities of plants to treat water. Since the 1970s, this practice has found renewed interest, particularly for the treatment of pesticides and metals.
Phytoremediation is a set of technologies using plants to reduce, degrade or immobilise organic pollutants (natural or synthetic) in soil, water or air resulting from human activities. This technique can also be used to treat inorganic pollution, such as trace metals or radionuclides.
Soil: This technique is used to biologically decontaminate soil polluted by metals and metalloids, pesticides, solvents, explosives, crude oil and its derivatives, radionuclides and various contaminants. Wastewater and liquid effluents: Phytoremediation is also used to decontaminate water loaded with organic matter or various contaminants (metals, hydrocarbons, organochlorines, pesticides). In this case, we consider treatments on soil in place (the effluent is spread) or directly in a wet environment. Air: This may also involve cleaning up indoor air or recycling water using depolluting plants. In addition to depollution, phytoremediation allows the recovery of polluted sites such as industrial wastelands. Moreover, it is integrated in the socio-economic interest because of its low cost and its interest in landscaping. Also, thanks to phytoextraction, the metals stored in the leaves and stems can be reused as an ecocatalyst in pharmaceutical and chemical processes.
Principle of Phytoremediation
Phytoremediation is essentially based on the interactions between plants, soil and micro-organisms. The soil is a complex matrix that supports the development of plants and micro-organisms that feed on the organic or inorganic compounds that make up the soil. When some of these compounds are in excess of the initial state of the soil, the soil is said to be contaminated (this also applies to water and air, which are fluids). The excess compounds can then be used as an energy source by plants and micro-organisms. In the plant-soil-microorganism system, bacterial biodegradation is often upstream of root uptake. Plants and micro-organisms have co-evolved to have a mutually beneficial strategy to manage phytotoxicity where micro-organisms benefit from root exudates, while the plant benefits from the degradation capabilities of rhizospheric micro-organisms to reduce phytotoxicity stress. Finally, the plant is the essential agent for the export of a contaminant from the surrounding environment.
Phytoremediation is mainly based on the interactions between plants, soil and micro-organisms. Soil is a complex structure that supports the development of plants and micro-organisms that feed on the organic or inorganic compounds of which it is composed. When some of these compounds are in excess of the initial state of the soil, the soil is described as contaminated soil (this also applies to water and air, unlike soil, which are fluids). The excess compounds can be used as an energy source by plants and micro-organisms. In the plant-soil-micro-organism system, bacterial biodegradation is often independent of root uptake. Plants and micro-organisms have co-evolved to adopt a reciprocal uptake strategy to withstand phytotoxicity, whereby the micro-organisms take advantage of the root exudates and also the plant benefits from the degradation capacity of the rhizospheric micro-organisms to reduce stress due to phytotoxicity. Ultimately, the plant is the essential agent in exporting a contaminant out of its environment.
Effect on the rhizosphere
The rhizosphere is the volume of soil under the influence of root activity. This soil volume is more or less important and varies depending on the plants and soil type. The processes occurring in the root zone are essential for phytoremediation. Microbial activity and biomass are much higher there than in soil without roots. Roots release natural substances into the soil where they grow, through root exudates. They promote and maintain the development of microbial colonies, providing them with 10-20% of the sugar produced by the photosynthetic activity of the plant. Many compounds are released, e.g. hormones, enzymes, oxygen and water. The rhizospheric micro-organisms, in turn, promote plant growth (reduction of pathogens, provision of nutrients...). Theoretically, the greater the abundance of roots, the more abundant they will provide an important development area for the microflora and microfauna of the rhizosphere. In fact, root exudates promote the biodegradation of organic pollution, stimulating microbial activity.
Principle of Decontamination
Briefly, plants will either absorb the contaminant for metabolism or storage, or reduce or prevent the release of the contaminant into other environmental compartments (phytostabilisation). In most cases, organic compounds (xenobiotic or not) can be degraded and metabolised for plant growth. The pollutant is then eliminated. In the case of inorganic pollutants (metals, metalloids or radionuclides), only phytostabilisation or phytoextraction can take place, as these types of pollutants are not biodegradable.
Avantages and Limitations
Advantages of Phytoremediation
- The cost of phytoremediation is much lower than traditional in situ and ex situ procedures.
- They are particularly useful for application on large areas, with relatively immobile contaminants, or with relatively low levels of contamination;
- By forming a vegetative cover, it improves the physical and chemical properties of the soil;
- They do not require transportation of the contaminated substrate, thus avoiding the spread of contaminants through air or water;
- Plants can be easily monitored;
- Recovery and reuse of valuable metals, biomass and water (companies specialising in phytomining);
- Does not require specialised personnel for management, because conventional agronomic practices are used;
- Does not require electrical power;
- It avoids excavation and heavy traffic;
- It is the least destructive method, as it uses natural organisms and preserves the natural state of the environment (compared to the use of chemical processes, there is no negative impact on soil fertility);
- Phytoremediation is limited to the surface and depth occupied by roots (note that many metal-based contaminants also remain in the topsoil);
- Slow growth and low biomass require considerable investment in time, or/and sometimes the addition of chelating agents or other substances (for inorganic contaminants such as heavy metals). Can be used with fast growing plants - see hyperaccumulator tables, which show a wide range of choice for most contaminants of all types;
- It is not possible, with a plant-based remediation system, to completely prevent the passage of contaminants to the water table (this is only possible by total soil removal). An experience in Iowa (USA), however, shows that poplar trees planted between a corn field and a stream significantly reduce the nitrate concentration in the surface water: the edge of the field contained 150 mg/l nitrate, while between the poplar trees the nitrate content was only 3 mg/l;4
- The level and type of contamination affects the phytotoxicity of the pollutants. In some cases, plant growth or survival may be impaired;
- Possible bioaccumulation of contaminants through the food chain, from the level of primary to secondary consumers. It is essential to dispose of plants responsibly, and not to consume plants used for land decontamination.
Phytoaccumulation is related to plant phyto-tolerance towards contaminants. The toxicity of some pollutants can be reduced by chemical reduction of the elements involved, which are thus transformed into less polluting substances, and/or by the incorporation of organic components (another form of biotransformation).
For this purpose, pollutants can be chelated with specific ligands that reduce the amount of free ions.
Experiments in electrokinetics have been carried out: the soil is subjected to a direct current to promote the movement of ions in the soil.
The interaction between phytoremediation and in situ bioremediation (use of micro-organisms, or their enzymes, in the soil) is also being studied.
The field of phytoremediation-oriented genetic engineering is developing rapidly.
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