Soil and water worldwide are more contaminated than ever, especially with heavy metals such as copper, lead, chromium, aluminum, and oil, which have built up over decades. This pollution greatly hampers plant growth, leaving them more susceptible to threats and reducing the quality of their yield. Yet, despite the pollution caused by human activities, nature has demonstrated an incredible capacity to constantly adapt to changing conditions.[1]
Recently, scientists have been developing a new natural technique for cleaning up pollution by letting plants do the work instead. ‘Phytoremediation’ is a natural way of cleaning up chemical pollution. [2]
Despite being far less expensive than current treatments, this technique is still underutilized. In France, for instance, leading pollution control companies rely on physico-chemical methods in most cases, which do not result in complete depollution. Promoting and advancing plant-based treatment, known as phytoremediation, is therefore highly relevant today.
What Is Phytoremediation?
Toxic metal contamination of water and soil is a serious environmental issue, and most traditional remediation methods fall short of providing effective solutions. Phytoremediation is an emerging technology that involves using specially chosen plants to clean up polluted environments. In simple definition, it is a process where certain plants remove harmful chemicals from contaminated areas. These plants can help eliminate various pollutants, including metals, pesticides, explosives, and oil. However, they are most effective when contaminant levels are low, as higher concentrations can hinder plant growth and extend the cleanup time. In addition, plants can help stop contaminants from spreading to nearby areas or deeper underground through wind, rain, or groundwater.[3]
Three main phytoremediation strategies are considered:
Three subsets are applicable to toxic metal remediation:
- Phytoextraction: the use of metal-accumulating plants to remove toxic metals from soil;
- Rhizofiltration: the use of plant roots to remove toxic metals from polluted waters; and
- Phytostabilization: the use of plants to eliminate the bioavailability of toxic metals in soils.[4]
Application of Phytoremediation
Phytoremediation has been applied in various settings to address a wide range of environmental issues, such as:
- Soil and Water Remediation: This method is widely applied to treat polluted soils and aquatic ecosystems.
- Mine Sites: Phytoremediation is used to restore abandoned metal mines and areas contaminated by industrial polychlorinated biphenyls (PCBs). It is also employed to manage ongoing pollution from active coal mines, helping to mitigate the effects on soil, water, and air quality.
- Pesticides, Crude Oil, and Derivatives: Phytoremediation has been successfully implemented across the globe to reduce pollutants like metals, pesticides, solvents, explosives, and crude oil and its byproducts, using plants to absorb, degrade, or immobilize these contaminants.
- Waste Sites: Plants such as mustard, alpine pennycress, hemp, and pigweed have shown the ability to accumulate high levels of toxins at waste sites. Their natural capacity for absorption and detoxification makes them valuable for environmental cleanup efforts.
Advantages of Phytoremediation
Phytoremediation provides several benefits compared to traditional remediation methods, including:
- Environmentally Friendly Option: It is an eco-friendly approach as it can reduce pollutant exposure to the environment and ecosystems.
- Wide Applicability and Simple Disposal: This method is easy to dispose of and can be applied across a wide range of environments.
- Reduces Erosion and Contaminant Spread: By stabilizing heavy metals, it helps to minimize pollutant spread and reduces erosion and metal leaching.
Disadvantages of Phytoremediation
However, phytoremediation comes with some drawbacks, including:
- Relocation Instead of Removal: Phytoremediation moves harmful heavy metals rather than fully eliminating them from the environment.
- Limited Reach: Phytoremediation is restricted to the surface area and depth that plant roots can access.
- Slow Growth and Limited Biomass: Due to slow plant growth and limited biomass, this method requires a long-term commitment.
- Incomplete Pollution Control: Plant-based remediation methods cannot entirely prevent pollutants from seeping into groundwater.
Examples of Phytoremediation
Here are some examples of phytoremediation applications:
- Trees like poplar are used to degrade organic pollutants such as solvents.
- Willow trees are used in landfill caps to manage leachate and stabilize slopes, improving environmental conditions at landfill sites.[5]
How long will it take?
Phytoremediation can take several years to clean up a site, with the duration depending on several factors. For instance, it will take longer when:
- Contaminant levels are high.
- The contaminated area is large or deep.
- Plants with long growth cycles are used.
- The growing season is short.
These factors can differ from site to site. If plants are damaged by extreme weather they may need to be replaced, which can further extend the cleanup time.[6]
Conclusion
Soil and water pollution are critical concerns for agricultural production and food safety due to their toxic effects and rapid accumulation in the environment. To address or reduce contamination, various techniques have been developed. Phytoremediation has proven to be a promising method with high public acceptance and offers several advantages over other physicochemical techniques.
However, phytoremediation using natural hyper-accumulators still faces some limitations, as it is a slow process that can take a long time to clean contaminated areas, particularly in moderately to highly polluted sites. Improving plant performance is therefore a key step in making phytoremediation more effective. Fortunately, genetic engineering has emerged as a powerful tool to modify plants with desirable traits like faster growth and better adaptation to different climates and soil conditions.
Lastly, a single approach is neither feasible nor sufficient for effectively cleaning heavy metal-contaminated soil. Combining multiple methods will be essential for achieving highly efficient and comprehensive phytoremediation in the future.[7]
[1] https://biomede.fr/en/what-is-phytoremediation/
[2] https://www.bbc.com/learningenglish/tigrinya/features/6-minute-english_2023/ep-230713
[3] https://www.epa.gov/sites/default/files/2015-04/documents/a_citizens_guide_to_phytoremediation.pdf
[4] https://pubmed.ncbi.nlm.nih.gov/9634787/
[5] https://www.epa.gov/sites/default/files/2015-04/documents/a_citizens_guide_to_phytoremediation.pdf
[6] Op.cit.
[7] https://www.frontiersin.org/journals/plant-science/articles/10.3389/fpls.2020.00359/full
A propos de Bahram MADANI
Étudiant en master 2, droit et gestion des énergies et du développement durable, Université de Strasbourg
