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Wednesday, June 29, 2022

Explained: How scientists plan to use plants to remove toxic metals from soil

Toxic heavy metals are absorbed by food crops and other plants before they eventually make their way into our food chain, directly affecting human life along with ecology.

Written by Sethu Pradeep , Edited by Explained Desk | Thalassery |
Updated: May 31, 2022 11:23:33 am
Phytoremediation refers to the usage of “hyperaccumulator” plants to absorb the toxic materials present in the soil and accumulate in their living tissue. (Representational image: Pixabay)

Soil contamination can happen due to a variety of reasons, including manufacturing, mineral extraction, accidental spills, illegal dumping, leaking underground storage tanks, pesticide and fertiliser use etc. A study published in the JNKVV research journal in 2015 concluded that heavy metal pollution of soil is “emerging at a speedy rate” in India due to industrialisation.

These toxic heavy metals are then absorbed by food crops and other plants before they eventually make their way into our food chain, directly affecting human life along with ecology. Many technologies have emerged to remediate this soil pollution but according to a paper published in the journal Agriculture, these methods have been “deemed lacking in terms of sustainability as they come with a large cost and have adverse effects themselves.”

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Turning toward more sustainable and eco-friendly technologies, scientists have developed methods of “Phytoremediation”, a remediation method that uses living organisms like plants, microalgae, and seaweeds. One particular way to remove toxic heavy metals from the soil includes the use of “hyperaccumulator” plants that absorb these substances from the soil.

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A paper published in the MDPI Agriculture journal titled, “Utilizing Mediterranean Plants to Remove Contaminants from the Soil Environment: A Short Review”, studies the development of this method of remediation.

What are hyperaccumulator plants?

Phytoremediation refers to the usage of “hyperaccumulator” plants to absorb the toxic materials present in the soil and accumulate in their living tissue. Even though most plants do sometimes accumulate toxic substances, hyperaccumulators have the unusual ability to absorb hundreds or thousands of times greater amounts of these substances than is normal for most plants.

Most discovered hyperaccumulator plants typically accumulate nickel and occur on soils that are rich in nickel, cobalt and in some cases, manganese. These hyperaccumulator species have been discovered in many parts of the world including the Mediterranean region (mainly plants of the genus Alyssum), tropical outcrops in Brazi, Cuba, New Caledonia (French territory) and Southeast Asia (mainly plants of the genus Phyllanthus).

How can phytoremediation with hyperaccumulators be used to remove toxic metals from the soil?

Suitable plant species can be used to ‘pick up’ the pollutants from the soil through their roots and transport them to their stem, leaves and other parts. After this, these plants can be harvested and either disposed or even used to extract these toxic metals from the plant.

This process can be used to remove metals like silver, cadmium, cobalt, chromium, copper, mercury, manganese, molybdenum, nickel, lead and zinc; metalloids such as arsenic and selenium; some radionuclides; and non-metallic components such as boron. But it cannot be used to remove organic pollutants from the ground due to metabolic breakdown.

What are the advantages of phytoremediation with hyperaccumulators?

One of the primary advantages of phytoremediation is the fact that it is quite cost-effective in comparison with other remediation methods. The only major costs attached are related to crop management (planting, weed control, watering, fertilisation, pruning, fencing, harvesting etc.). This method is also relatively simple and doesn’t require any new kinds of specialised technology. Also, no external energy source is required since the plants grow with the help of sunlight.

Another important advantage of this method is that it enriches the soil with organic substances and microorganisms which can protect its chemical and biological qualities. Also, while the plants are growing and accumulating toxic heavy metals, they protect the soil from erosion due to wind and water.

What are the disadvantages of phytoremediation with hyperaccumulators?

For all its advantages, this kind of phytoremediation with hyperaccumulators has a big drawback: it is a very slow and time-consuming process. The restoration of an area with this process can take up to 10 years or more. This means that the area in question cannot be used for any other purposes for up to ten years, including growing other crops and allowing animals to graze it. This comes with a large economic cost, proportional to the size of the area under rehabilitation.

But it also comes with another major drawback. The plants to conduct this rehabilitation must be carefully selected based on a large number of characteristics or they could act as an invasive species, growing out of control and upsetting the delicate ecological balance of not just the area under rehabilitation, but also the entire region it is part of.

Due to this reason, scientists only propose using species that are native to the region where the phytoremediation project is undertaken. This also has other benefits: these plants will already be acclimatised to the region and there will be no legal problems concerning the procurement, transport and use of seeds.

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