Exposure of nanoplastics to freeze-thaw leads to aggregation and reduced transport in model groundwater environments

, Alimi Olubukola, Farner Jeffrey, Tufenkji Nathalie.

Plastic pollution presents a significant environmental challenge given the potential risks associated with freshwater contamination. Nanoplastics released into aquatic systems are affected by various physical, chemical and biological weathering processes that will influence their mobility in saturated porous media. Currently, however, little is known regarding the impacts that environmental conditions such as temperature and climate instability will have on nanoplastic transport and fate in the environment. To improve our understanding of nanoplastic mobility in groundwater following exposure to temperature cycles of freezing and thawing, controlled laboratory scale columns were used to investigate the transport potential of polystyrene nanoplastics exposed to either constant (10°C) temperatures or freeze-thaw (FT) cycles ( -10°C to 10°C) in water saturated quartz sand. The stability and transport of nanoplastic suspensions were examined both in the presence and absence of 5 mg/L natural organic matter (NOM) over a range of solution ionic strengths (IS) (3 - 100 mM NaCl). Under all conditions, exposure to 10 FT cycles led to significant aggregation and reduced mobility compared to nanoplastics held at 10 °C. This was especially apparent at low IS (3 and 10 mM NaCl) in the absence of NOM. Although the presence of NOM increased nanoplastic mobility, it did not prevent the aggregation of nanoplastics exposed to FT. Even though previous studies show that nanoplastics and nanomaterials are more mobile in cold temperatures, our findings illustrate that nanoplastics are more likely to be associated with soils and less likely to undergo long range transport in groundwater in colder climates following freezing temperatures. This highlights the need to account for climate and temperature changes when assessing the risks associated with nanoplastic release in aquatic systems.

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