Nanoplastics threaten plants’ ability to clean up industrial waste

There’s a lot of research on constructing artificial wetlands to clean up industrial waste. Now a new study published in the Journal of Hazardous Materials conducted by Huawei Jia and colleagues in China has revealed that the aquatic plant Pontederia crassipes, (also known as Eichhornia crassipes or Water Hyacinth), often used to remove contaminants from wastewater sources, may be facing problems. The scientists have tested Water Hyacinth’s ability to adsorb pollutants when water is contaminated with nanoplastics. They have found that these plastic fragments, smaller than microplastics, enter the plant’s body, causing stress and reducing its ability to function.

Microplastics are a recognised environmental problem, but Jia and colleagues say that nanoplastics, plastic fragments below one millimetre in size, could be even more of a problem. They cite Luo and colleagues, who state, “It has been estimated that the environmental level of NPs might be 17 orders of magnitude higher than that of MPs and show an increasing trend over time.” This is a bold claim. Going back to the source of that claim, Besseling and colleagues actually write: “However, environmental number concentrations of nanoplastic may potentially become 17 orders of magnitude higher than those of microplastic in the future, due to fragmentation of larger plastic particles (calculation based on 5 mm spheres fragmenting into 100 nm spheres).” Whatever the current reality, the smaller nanoplastics have a larger surface area compared to their volume than microplastics, making some chemical interactions easier. Their smaller size means they can enter the food chain more easily.

The team examined how the Water Hyacinth Pontederia crassipes, referred to as Eichhornia crassipes in the article, could remove polystyrene nanoplastics. Water Hyacinths are fast-growing floating aquatic plants that can be used to clean pollutants like antibiotics and heavy metals from wastewater. Jia and colleagues placed Water Hyacinths into wastewater with differing concentrations of nanoplastics and then studied how the plants grew.

The team found nanoparticles in the Water Hyacinths’ leaves, roots and petioles. The team believes that roots are an obvious entry point for nanoparticles, but there might be other entry points. They refer to research on seagrasses that show nanoparticles can also penetrate the epidermis of leaves.

The nanoparticles had a measurable effect on how the plants grew. At low concentrations, petioles actually increased in diameter, possibly to compensate for the impact of the nanoparticles. However, at high nanoparticle concentrations, there was a strong negative effect on plant growth. The plant growth reflected the plant’s ability to remove pollution. The scientists found that at lower concentrations, the plants remained extremely effective at removing some pollutants, reducing chemical oxygen demand by 99% and NH4-N by 96%. The removal of chemical oxygen demand reduced to 31% when there was a high concentration of nanoparticles.

The research opens up new avenues of investigation for scientists working on phytoremediation, using plants to clean up waste. A fairly obvious route is to ask if it’s possible to breed Water Hyacinths that can cope better with nanoplastics, to maintain the efficiency of artificial wetlands. Jia and colleagues highlight another possibility. They note that the Water Hyacinths immobilise nanoplastics in their organs. So is there a possibility to use plants to help clean up some plastic pollution? If Besseling and colleagues prediction comes to pass, there’ll be a gargantuan amount of pollution that will need cleaning up.

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Jia, H., Yu, H., Li, J., Qi, J., Zhu, Z. and Hu, C. (2023) “Trade-off of abiotic stress response in floating macrophytes as affected by nanoplastic enrichment,” Journal of Hazardous Materials, (131140), p. 131140. Available at: https://doi.org/10.1016/j.jhazmat.2023.131140.