Slow-flow sand filtration can efficiently remove nanoplastics from water at scale
Microplastic contamination and its potential threats to ecosystems and human health have become a major cause of public concern. The discovery of microscopic plastic fragments within sediments, water bodies, and even within human and animal organs1 has attracted much attention in non-scientific outlets and social media, but realistic ways of dealing with existing plastic pollution have been lacking. A recent study2 published in the Journal of Hazardous Materials offers a promising outlook for the detection and elimination of this pollution.
From nano- to micro-scale
In the 1970s, American marine biologist Ed Carpenter noticed pollution by tiny plastic fragments hundreds of kilometers from the shore, drawing attention to what would prove to be a growing problem worldwide.3
In 2004, the marine biologist Richard Thompson coined the term “microplastic” for the minute plastic fragments, usually smaller than 5mm in diameter, found in the sediments around Plymouth, UK.4
Smaller still are nanoplastics, which measure under one micrometer in length.5 These plastics are so small that they can even become incorporated into the lipid membranes of living cells,6 damaging their natural function and causing stress responses in organisms.7 Worryingly, they have even been found within human organs and blood, although the long-term risks remain unclear.8
Our addiction to plastics
Plastics are ubiquitous in everyday life, owing to their many advantages. One of the greatest strengths of plastics is their longevity, but this advantage can be ruinous to our environment. A high-density polyethylene water bottle may last you for a few months or years, but if it is disposed of carelessly and the fragments end up in the sea, they may take anywhere from decades to thousands of years to degrade.9
Recently, bioplastics derived from plant matter have gained attention, owing to their biodegradability and ability to sequester atmospheric carbon. Polylactic acid is one such bioplastic that has seen increased use. However, its low heat resistance and brittleness10 make it less than ideal compared to plastics such as polyethylene terephthalate. Moreover, its vaunted biodegradability is quite poor outside of optimal conditions,11 such as in an industrial composter.
Even with improvements, it will be difficult to eliminate all plastic waste in the world. Larger plastic fragments will eventually degrade to nanoplastics and the existing nanoplastic waste will remain in the environment for thousands of years without human intervention.
Bioactive sand filtration may be a key breakthrough
The previously mentioned paper by Pulido-Reyes et al.,2 due to be published in August 2022, details their success in removing around 99.9% of nanoplastic contamination, both within the laboratory and on a larger scale at Zurich Water Works. Potable water is often subjected to ozonation, which effectively eliminates harmful microorganisms, but this treatment shows little activity against nanoplastics found in the water. However, slow-flow sand filtration allows microbial biofilms on the aged sand to effectively retain these small particles, achieving a 99.97% retention of nanoplastics across a multistep process.
While this process is only one countermeasure against nanoplastic pollution, this experiment demonstrates that humans can still undo some of the damage we have inflicted on the environment and lessen our exposure.
References
1. Carrington, D. & editor, D. C. E. Microplastics found in human blood for first time. The Guardian (2022).
2. Pulido-Reyes, G. et al. Nanoplastics removal during drinking water treatment: Laboratory- and pilot-scale experiments and modeling. J. Hazard. Mater. 436, 129011 (2022).
3. Earth Has a Hidden Plastic Problem—Scientists Are Hunting It Down - Scientific American. https://www.scientificamerican.com/article/microplastics-earth-has-a-hidden-plastic-problem-mdash-scientists-are-hunting-it-down/.
4. Thompson, R. et al. Lost at Sea: Where Is All the Plastic? Science 304, 838 (2004).
5. Harrison, R. M. & Hester, R. E. Plastics and the Environment. (Royal Society of Chemistry, 2018).
6. Hollóczki, O. & Gehrke, S. Can Nanoplastics Alter Cell Membranes? Chemphyschem 21, 9–12 (2020).
7. Liu, Z. et al. Polystyrene nanoplastic exposure induces immobilization, reproduction, and stress defense in the freshwater cladoceran Daphnia pulex. Chemosphere 215, 74–81 (2019).
8. Leslie, H. A. et al. Discovery and quantification of plastic particle pollution in human blood. Environ. Int. 163, 107199 (2022).
9. Chamas, A. et al. Degradation Rates of Plastics in the Environment. ACS Sustain. Chem. Eng. 8, 3494–3511 (2020).
10. Razavi, M. & Wang, S.-Q. Why Is Crystalline Poly(lactic acid) Brittle at Room Temperature? Macromolecules (2019) doi:10.1021/acs.macromol.9b00595.
11. Teixeira, S., Eblagon, K. M., Miranda, F., R. Pereira, M. F. & Figueiredo, J. L. Towards Controlled Degradation of Poly(lactic) Acid in Technical Applications. C 7, 42 (2021).
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