Before heavy metals become the next generation's problem to solve, we need to come up with an answer.
Most metals do not degrade unlike other organic contaminants and are in the soil for a longer period of time. That’s what makes them one of the most persistent pollutants in the environment, as compared to organic pollutants. Soils are the main sink for heavy metals which are released into the environment by human activities. In 2019, the Indian government spent 30 billion INR to battle pollution. There is a much easier and more cost-effective way to combat pollution than expensive man-made techniques and resource-intensive technologies - it’s nature-based solutions.
Phytoextraction is one such nature-based solution. Phytoextraction is the ability of plants or algae to remove contaminants from soil or water and turn them into harvestable plant biomass. The idea of using plants to remove metals from soils came from the discovery of different wild plants, often endemic to naturally mineralized soil, that accumulate high concentrations of metals in their foliage.
Heavy metal contamination of soil is hazardous to humans and the ecosystem through direct ingestion or contact with contaminated soil, contaminated groundwater, reduction in food quality (safety and marketability) via phytotoxicity, reduction in land usability for agricultural production causing food insecurity, and land tenure problems.
Industries responsible for Heavy metal contamination:
Mining, Smelter, and Metallurgical Industries- In several countries, heavy metal contamination has been recorded in areas near mines and smelters. High concentrations of Mn, Zn, and Cr were recorded near metal industries.
Chemical and Petrochemical Industries- Chemical and petrochemical industries have been identified as major emitters of a multitude of contaminants, including metals. In a study near a sulphuric acid plant in Bangladesh, extremely high levels of Zn, Mn, Ni, and Cu were found in soil samples.
Textile Industries- Textile industry may be one of the most significant sources of metal emissions in the environment. There is evidence that textile factories have released large quantities of trace metals into the surrounding soil.
Leather Industries-In most developing countries, tannery effluents are directly discharged to nearby land where they adversely affect the quality of both soil and groundwater
Non-Metallic Mineral Industries- Non-metallic mineral industries such as cement, ceramic, and battery manufacturing facilities can act as a major source of trace metal pollution in soil. In Jordan, high Pb, Zn, Mn, and Ni concentrations were detected in soil samples near cement plants Ni and Zn were found in high levels, in the soil samples of ceramic industry sites in Bangladesh. High levels of Pb and Zn were also found near battery manufacturing facilities, which are suspected to pollute the soil in the industrial area of Baoji city, China.
Heavy metals can last for over 1000 years in the soil. Heavy metals cannot be destroyed biologically but are only transformed from one to another. As a consequence of the alteration of its state, the metal may become either: (i) more water-soluble and is removed by leaching, (ii) inherently less toxic, (iii) less water-soluble so that it precipitates and then becomes less bioavailable or removed from the contaminated site, or (iv) volatilized and removed from the polluted area.
Through phytoextraction, the plant takes up metal contaminants from the soil through absorption by plant roots. Plants intended for metal extraction are called hyperaccumulators. Plants that are hyperaccumulators have a higher tolerance to heavy metals than other plants and are capable of absorbing larger amounts of metal. The contaminated particles (metals) travel through the roots and are stored there or up into the stems or leaves. The plants will carry on absorbing contaminants until harvested to either be disposed of by incineration or be composted to recycle metals. After the harvest, the soil will contain a lower concentration of the contaminant. Research has shown that some plant species absorbed toxic heavy metals of up to several per cent of their dried shoot biomass. These hyperaccumulators had toxic element levels in the leaf and stalk biomass of about 100 times than those of non-accumulator plants in the same soil and in some cases more than a thousand times.
Benefits of phytoextraction
It is cost-effective,
The contaminant is permanently removed from the soil,
It is environment friendly,
It reduces the amount of rock waste that must be disposed of after traditional mining,
Due to incineration, the volume of harvested plant biomass that requires disposal is dramatically reduced,
In some cases, an additional source of revenue can be obtained by the extraction of metals from metal-rich ash; therefore, it can be used to offset the cost of remediation.
Limitations of phytoextraction
Effective extraction of toxic metals by hyperaccumulators is limited to shallow soil depths of up to 24 inches.
At present, existing hyperaccumulators lack appropriate biomass production, physiological adaptability to varying climatic conditions, and adaptability to current agronomic techniques
It is slower than traditional soil clean-up methods.
Insects/herbivores feeding on the plant can cause bioaccumulation and biomagnification of the metal back into the food chain.
Phytoextraction has shown outstanding results in metal extraction, especially in the case of Lead. Together with a high extraction rate by roots, the success of phytoextraction depends on substantial increases in the transfer of metals to shoots. Phytoextraction appears a very promising technology for the removal of metal pollutants from the environment and maybe, at present, approaching commercialization.
(Also Read: Aloevera Batteries: A Green Source of Energy)
References
Suman, J. (2018). Phytoextraction of Heavy Metals: A Promising Tool for Clean-Up of Polluted Environment? Frontiers. https://www.frontiersin.org/articles/10.3389/fpls.2018.01476/full
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Phytoextraction. (n.d.). Hawaii.Edu. https://www.hawaii.edu/abrp/Technologies/phyextr.html
Garbisu, C., & Alkorta, I. (2001). Phytoextraction: a cost-effective plant-based technology for the removal of metals from the environment. Bioresource Technology, 77(3), 229–236.
Nascimento, C. W. A. D. (n.d.). Phytoextraction: a review on enhanced metal availability and plant accumulation. Scielo.Br.
Biological methods of metal extraction - Higher - Material resources - AQA Synergy - GCSE Combined Science Revision - AQA Synergy. (n.d.). BBC Bitesize.
Wuana, R. A. (2011, October 24). Heavy Metals in Contaminated Soils: A Review of Sources, Chemistry, Risks and Best Available Strategies for Remediation. Hindawi.Com.