Microplastics: Sink or Swim?

Plastic pollution continues to be a crucial environmental concern, with microplastics (fragments smaller than 5 millimeters) presenting a serious threat to many ecosystems.¹ New discoveries are constantly being uncovered around how plastics break down, how long they persist, and how they interact with different systems—and now, a researchers have turned their focus to how microplastics behave after being frozen and thawed in bodies of water.

Most bodies of water contain microplastics that get trapped at the surface when ice forms. And according to a recent study research published in Environmental Science & Technology, freezing could alter the environmental impact of microplastics, including their accumulation in freshwater lake or river sediments.

It has previously been reported that freezing conditions can aggregate and increase particle size, or cause microplastics to integrate with other substances in the water.² To explore this further, a team from Concordia University in Montreal, Canada, aimed to comprehensively examine the influence of freezing conditions on the environmental fate of microplastics in aqueous environments.³ Particles were characterized before and after freezing, providing insights into various polymer attributes and their behavior across different environmental conditions. The results showed that aggregation induced by freezing enhances buoyancy effects and accelerates movement in water. When allowed to stand for 24 hours, mean particle size was not affected, but 24-hour freezing resulted in a 46% increase in mean particle size. These larger aggregated particles were not dispersed by movements designed to mimic turbulent flows in natural environments. Additionally, freezing led to enhanced surface wetting alterations, thereby improving the dispersion of hydrophobic microplastics.

To simulate environmental conditions with dissolved organic matter, the authors used sodium alginate to disperse the microplastics; in this model, increased concentration and particle size were also noted after freezing. Importantly, these effects were mitigated by the presence of salt, which reduces the pressure on the microplastics while they are trapped by ice, suggesting this might not be such a big problem in seawater. However, in cold regions where bodies of fresh water freeze, this could have implications for the patterns of microplastics re-entering the water column, providing valuable insights into the behavior of microplastics released from seasonal ice.

The group have previously looked at how other factors, such as nanobubbles, contribute to the movement of microplastics in aquatic environments.⁴ Additional research in 2023 revealed that entrainment and enrichment of microplastics in ice could be critical pathways affecting their fate in cold regions,⁵ and this new study builds on that to explain some of the underlying processes.

The behavior of microplastics in ice could also have implications for the movement of other pollutants. Previous research has shown that at least 90% of heavy microplastic and oil dispersant agglomerates stay at the seawater surface, while the lighter agglomerates are more widely distributed throughout the seawater column.⁶ Taken together with the new paper, this could suggest that some of the more heavily polluted microplastics in the environment will be subject to freezing, particle aggregation, and subsequent accelerated movement, but more work is necessary to better understand long-term implications.

References

1. Deng, H. et al. Crack patterns of environmental plastic fragments. Environ. Sci. Technol. 2022, 56 (10), 6399–6414.
2. Alimi, O.S. et al. Exposure of nanoplastics to freeze-thaw leads to aggregation and reduced transport in model groundwater environments. Water Res. 2021, 189, 116533.
3. Chen, Z. et al. Revealing the Freezing-Induced Alteration in Microplastic Behavior and Its Implication for the Microplastics Released from Seasonal Ice. Environ. Sci. Technol. 2024, 58, 30, 13529–13539.
4. Wang, Z. et al. Overlooked Role of Bulk Nanobubbles in the Alteration and Motion of Microplastics in the Ocean Environment. Environ. Sci. Technol. 2023, 57, 30, 11289–11299.
5. Chen, Z. et al. Entrainment and Enrichment of Microplastics in Ice Formation Processes: Implications for the Transport of Microplastics in Cold Regions. Environ. Sci. Technol. 2023, 57, 8, 3176–3186.
6. Yang, M. et al. Transport of Microplastic and Dispersed Oil Co-contaminants in the Marine Environment. Environ. Sci. Technol. 2023, 57, 14, 5633–5645.