These plant species are a cost-effective solution to heavy metal pollution.
Rapid development and modernization, pollution from chemicals, heavy metals and other contaminants has become a huge problem across the world. These contaminants pollute and degrade soil, making it inhospitable to vegetation and soil-inhabiting organisms. There are conventional, energy-intensive solutions to extract pollutants from soil, such as soil washing, excavation and disposal, electrokinetic remediation, thermal desorption and vitrification. However, a promising solution exists in nature that predates humans and our disruptive behaviour - phytoremediation. By using plants, we can clean up contaminated soils and water, while being cost-effective. This method has been gaining traction globally, and in India the potential is immense.
What is phytoremediation?
Phytoremediation (phyto-plants, remediation-remedy) is a procedure where plants are deployed To absorb, degrade, or stabilize contaminants in soil and water. These contaminants can include heavy metals, toxic chemicals, pesticides, and even radiation. Such pollutants can persist in soil and water for decades as they are non-biodegradable. Phytoremediation is a cheaper and easier alternative to standard remediation techniques like soil excavation or chemical treatment, both of which are costly and disrupt ecosystems.
Phytoremediation is sometimes viewed as a ‘new’ discovery, but that’s far from the truth. We have known about this technology for at least 300 years. And we have known about the ability of certain plants to tolerate metals in soil since the 16th century. However, this technology was formally recognised in 1991 by Ilya Raskin from Rutgers University in the United States when the government enforced a law on industries to take accountability for their exploitative and polluting practices.
Mechanisms of Phytoremediation
Certain plants can be classified as hyperaccumulators. As they grow in their habitat, absorbing nutrients and water, they also take up contaminants and store them in their roots, stems, and leaves. Some plants even release enzymes that break down these substances into less harmful substances. Once the contaminants are sufficiently broken down the plants can be harvested and must be disposed of methodically. In some cases, we may even be able to extract and reuse metals from the plants.
Phytoremediation involves different processes depending on the type of contaminant and the plant species used. Here are the five main types:
Phytoextraction
Some plants have the ability to "suck up" harmful metals or chemicals from the soil or water through their roots. These substances then travel up into the plant's stems, leaves, or roots, where they get stored. The contaminants don't harm the plant as much because certain plants are naturally tolerant to them. After a period of time, these plants can be harvested and removed, taking the harmful pollutants with them.
Explain It Like I'm 5: Think of it like a sponge soaking up water. But here, the plant is soaking up metals like lead or mercury from the soil.
2. Phytodegradation
Some plants can break down harmful chemicals into safer substances. They do this either inside their own cells or by interacting with bacteria and fungi around their roots. These microorganisms help break down complex pollutants into simpler, less toxic forms. The plant helps speed up this process by releasing certain substances through its roots that boost microbial activity.
EILI5: The plant acts as a "detox machine," taking a harmful substance and turning it into something that’s no longer a problem.
3. Phytostabilization
Certain plants are great at trapping harmful substances in the soil, preventing them from spreading to other areas (like into nearby water bodies). The plant’s roots hold the soil and pollutants in place, acting as a barrier. The plants don't remove the contaminants but keep them stable, so they don’t move around and cause more harm.
EILI5: Think of it as a safety net. The plant doesn’t remove the pollutants but keeps them from spreading.
4. Rhizofiltration
The plants can clean water by absorbing pollutants through their roots. The plants are placed in water, and their roots act like a filter, pulling contaminants like heavy metals out of the water. These contaminants are trapped in the roots, making the water cleaner.
EILI5: Similar to a water filter, the plant roots "suck up" the pollutants, making the water cleaner.
5. Rhizodegradation
Plants create an environment around their roots, which encourages certain bacteria in the soil to break down harmful substances. These bacteria digest pollutants, and the plant helps them grow and work better.
EILI5: The plant acts as a host, feeding and encouraging helpful bacteria to clean up the soil for it.
6. Phytovolatilization
Some plants absorb pollutants from the soil or water and convert them into gases. These gases are then released into the air through the plant’s leaves in a less harmful form. While this process doesn’t completely remove the pollutants, it can reduce the concentration of harmful chemicals in the soil or water.
EILI5: It’s like the plant "breathing out" a pollutant after pulling it out of the ground or water.
Social, Economic and Ecological Benefits of Phytoremediation
Social Benefits
Public Health: By reducing pollutants, phytoremediation creates a healthier living environment, especially for communities living near contaminated areas.
Minimal Disruption: Traditional soil and water remediation methods require excavation and displacement or chemical treatment, which can disrupt local ecosystems and communities. Phytoremediation, on the other hand, is non-invasive and allows the land to stay usable during the process.
Aesthetic Value: Using plants for remediation enhances the landscape. Green spaces contribute to beautification and enhance biodiversity.
Economic Benefits
Cost-Effective: Phytoremediation is significantly cheaper than traditional methods. It requires less labour, equipment, and energy.
Metal Recovery: In some cases, the heavy metals absorbed by plants can be harvested and reused, adding an economic incentive to the process. This process is known as phytomining, it has been done to extract nickel from plants in Albania.
Low Maintenance: Once the plants are established, phytoremediation requires little maintenance, making it a long-term, sustainable solution for contamination.
Ecological Benefits
Environmentally Friendly: Phytoremediation relies on natural processes, reducing the need for harmful chemicals and conserving the integrity of the ecosystem.
Increased Biodiversity: By planting native species, phytoremediation can restore ecological balance and encourage the growth of other plants and wildlife.
Erosion Prevention & Carbon Sequestration: Many of the plants used in phytoremediation also help reduce soil erosion, which protects the environment from further damage. Additionally, these plants can capture and store carbon.
Limitations
Phytoremediation is not a silver bullet, it has certain limitations that must be accounted for when choosing the right solution for removing contaminants.
Time-Consuming: The process is not quick. It may take several years for plants to significantly reduce pollutant levels, especially for deeply embedded contaminants.
Depth Limitations: Plant roots can only reach a certain depth, making it difficult to remediate contaminants that are deeply buried in the soil.
Climatic Restrictions: The success of phytoremediation depends on the climate. Some plant species used in the process may not thrive in all regions, Limiting its application in certain areas. India has about 400 species of plants that can be used for phytoremediation, but they must be compatible with the environment and pollutants that need to be addressed.
Adverse Effects on Wildlife: There is a risk that animals might consume contaminated plants, which could cause harm. This must be carefully managed, particularly in wildlife-rich areas.
Examples of phytoremediation
In Coimbatore, Tamil Nadu, vetiver grass (Chrysopogon zizanioides) has been successfully tested in a pilot project for the removal of chromium from tannery effluents using rhizofiltration. Similarly, in North Carolina, USA to absorb lead and arsenic on agricultural lands to treat agricultural chemical residues through phytodegradation.
Other phytoremediation techniques, such as rhizofiltration were confirmed to have removed radioactive radiation from groundwater in Chernobyl, Ukraine using a specific variety of sunflower (Helianthus annuus). At a mining site in China, a native plant commonly known as ramie (Boehmeria nivea) was used along with biochar and organic fertilisers to manage and reduce cadmium and lead contamination from the soil.
Plants such as tobacco, Indian mustard, rye and spinach are also popular choices to remove heavy metals such as lead, cadmium, zinc, copper and chromium. India itself has about 400 species of plants that have properties to carry out phytoremediation of different organic pollutants.
Case Study in India: Hebbal, Mysuru
Hebbal Industrial Area, near Mysuru, Karnataka, is home to several types of industries that release wastewater into the surrounding water bodies. This water is used for irrigation in fields and over time has led to the buildup of heavy metals like iron, copper, cadmium, lead and zinc into the soil. These elements have adverse effects on plants, people, and animals that live in the region. Conventional methods like soil excavation and disposal, soil washing or vitrification are complicated and expensive, especially on a large scale. Soil samples were taken from five locations to study the effectiveness of Indian mustard (Brassica juncea) and radish (Raphanus sativus) in removing heavy metals from soil.
In this study, researchers employed phytoextraction, where heavy metals would be absorbed from the roots of the plant and stored in the roots and leaves. The researchers planted mustard and radish in the contaminated soil for 80 days to test the effectiveness of these plants to remove contaminants from the soil. They compared the results to areas with no plants and monitored the growth and health of plants during this period.
Both plants were successful at reducing the contamination level from the soil. Indian mustard was effective at removing copper, lead, zinc and cadmium. Radish was better at absorbing iron, copper, lead and cadmium. The results are promising, but it’s not a perfect solution. Firstly, the process is slow, it would take several planting cycles to remove a significant amount of pollutants from the soil. Even then, the plants can’t absorb all the metals, the result depends on the type of metal, plant species and soil condition. Once the plants absorb the metals, prudent disposal is key to avert the metals from returning to the environment. Radish and Indian mustard are promising candidates for treating contaminated soils. It’s not a quick fix but is a low-cost, environmentally friendly solution.
In the anthropocene, nature has its way to undo the damage we impose on the planet. It is humanity that needs saving, not the planet. Phytoremediation is starting to gain traction globally now, despite being around for quite some time. It is a cost-effective, ecologically friendly method compared to conventional methods. In India, the potential is immense, with 100s of native species capable of cleaning up the environment, but it has to be implemented with the utmost care. Selecting the right species for the environment and conditions along with sustained efforts are key to leverage plants as a means to restore habitats.
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