Most bodies of water contain tiny plastic particles trapped at the surface when ice forms. However, as the ice melts, these microplastics change. In a recent study published in Environmental Science & Technology, researchers from Concordia University suggested that depending on the type of polymer, the thawed particles may be larger and sink or float more quickly.
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According to the study's authors, freezing may change how microplastics affect the environment, including how they accumulate in the sediments of freshwater lakes and rivers.
Different behaviors of the polymers that create microplastics in the environment are observed. For example, polyethylene (PE) floats because it is less dense than water, but polyurethane (PU) and polytetrafluoroethylene (PTFE) sink toward lake beds and seafloors because they are denser than water. Particle size, salinity, buoyancy, and other environmental factors influence whether microplastics remain at the water's surface or sink to the bottom of the sediment.
Chunjiang An and associates performed studies in the lab using fresh and saltwater to determine how freezing affects PE, PU, and PTFE particles.
The research team began with 6–10 µm particles of each polymer in different solutions ranging from freshwater to seawater's salinity levels. The samples were frozen for a full day before being fully thawed by the team. When frozen, the particles of all three polymers in freshwater grew larger than those of a control that was kept cold for a full day.
In particular, the team found that the polymer PE, which repels water, changed more (46%) than the polymer PU, which attracts water (9%). According to the researchers, this difference may have contributed to the particles' increased dispersion after freezing. However, comparing freezing to cool-temperature controls had no effect on any polymer's particle size as the samples' salinity levels rose.
According to the researchers' theory, the ice structures' brine channels provide trapped particles with spaces to prevent compressing and joining to form larger clumps.
Following the freeze-thaw cycle, more PTFE and PU microplastics settled at the bottom of the beakers than before. The researchers next looked into the forces causing these particle movements.
The researchers estimated that after freezing and thawing from ice, particles would rise (PE) or sink (PFTE and PU) in water faster due to the increased buoyancy force on the particles based on calculations of the balance between gravitational, buoyant, and drag forces on the particles.
Although freezing can last several months or years in natural environments, the researchers admit that the freezing period used in this study is shorter. This study fundamentally analyzes the fate of different polymer particles released from ice in cold climates. It suggests that even rapid freezing may hasten the amount of microplastics that settle in sediments.
The Natural Resources Canada and the Natural Sciences and Engineering Research Council of Canada provided funding, which the authors acknowledge.
Journal Reference:
Chen, Z., et al. (2024) Revealing the Freezing-Induced Alteration in Microplastic Behavior and Its Implication for the Microplastics Released from Seasonal Ice. Environmental Science & Technology. doi.org/10.1021/acs.est.4c05322