The scientists found that Centella asiatica, commonly known as Indian pennywort, could effectively absorb cadmium, a toxic metal, from affected soils. Credit: NTU Singapore
Scientists in Singapore have discovered a literal green way to remove toxic heavy metals from soil by using common plants with phytoremediation abilities.
Phytoremediation is the use of plants to extract and store contaminants from soil. Although it is a slow and long-term commitment, the researchers showed that with enough growth cycles through repeated planting, phytoremediation can ultimately lead to reductions in the level of heavy metals and metalloids in the soil. Additionally, it is an eco-friendly alternative compared with existing industrial options like soil washing and acid leaching, which can be costly and may utilize harsh chemicals.
“In a small nation like Singapore, land may be repurposed to support new development plans, so it is important that we have a green and sustainable way to remediate land that is contaminated,” explained Lam Yeng Ming, co-lead of the study and chair of Nanyang Technological University’s School of Materials Science and Engineering.
For their study published in Environmental Pollution, Lam’s team collected soil and plant samples for 10 months between 2019 and 2020. A total of 46 plant species were studied as potential candidates for phytoremediation, with 12 demonstrating the ability to absorb several types of heavy metals and metalloids—including cadmium, arsenic, lead and chromium.
All these elements occur naturally in soils, but rarely at toxic levels. However, over a long period of time, heavy metal particles can accumulate due to air pollution, settling in the top layers of soil and posing a toxic threat to animals and humans. The incorrect disposal of paints, batteries and industrial waste can also result in high levels of heavy metals in soil, as well as the use of pesticides and land application of industrial sludge.
Importantly, the 12 plant species that proved effective are widely available and native or naturalized to Singapore, including cow grass (axonopus compressus), brake fern (pteris vittata) and the Indian pennywort (centella asiatica).
“Through advanced characterization techniques, we showed how some of these tropical plant species can be an environmentally friendly and literally ‘green’ way to remove contaminants in soil,” said Lam. “Phytoremediation also has benefits of cost effectiveness, simplicity of management, aesthetic advantages, and long-term applicability and sustainability. The strategy prevents erosion and metal leaching by stabilizing or accumulating heavy metals, so that helps reduce the risk of contaminant spread.”
Analyzing the soil samples collected during the experimental field survey, the research team did find regions where the levels of heavy metals and metalloids were high enough to “affect the environmental and the health of flora and fauna in Singapore,” according to study co-author Tan Swee Ngin, a professor in NTU’s National Institute of Education. “This would call for preventive actions.”
The research team is now testing phytoremediation on the plots of land that have high concentrations of heavy metals to better determine the effectiveness of the plants in a real-life urban setting. They are also testing inorganic particles that can be incorporated into plants to help stimulate growth. This would reduce the time needed for absorption of heavy metals by the plants, as well as speed remediation time.
Chronic health issues associated with long-term heavy metal exposures include mental incapacities, kidney, liver and GI tract problems, skin poisoning, central nervous system issues and more.