Distribution of microplastics between ice and water in aquatic systems: The influence of particle properties, salinity and freshwater characteristics.

Microplastics (MPs) are an anthropogenic emerging pollutant, with global contamination of both marine and freshwater systems extensively documented. The interplay of MP particle properties and environmental conditions needs to be understood in order to assess the environmental fate and evaluate mitigation measures. In cold climate, ice formation has appeared to significantly affect the distribution of MPs, but so far, limited research is available comparing different aquatic systems, especially freshwater. Experiments often rely on artificial water and specific MP model particles. This study used laboratory tests to investigate the ice-water distribution of a variety of environmentally relevant MP particle types (PP, PE, PS and PVC fragments (25-1000 μm), PET fibers (average length 821 μm, diameter 15 μm)) across different water types, including artificial water of high and low salinity, as well as natural water from a lake and a treatment wetland. Overall, ice entrapment of MPs occurred in almost all tests, but the ice-water distribution of MPs differed across the different water types tested. Among the tests with artificial water, salinity clearly increased MP concentrations in the ice, but it cannot be resolved whether this is caused by increased buoyancy, changes in ice structure, or thermohaline convection during freezing. In the natural freshwater tests, the partition of MPs was shifted towards the ice compared to what was seen in the artificial freshwater. The influence of different types of dissolved and particulate substances in the different waters on MPs fate should be considered important and further explored. In this study, the higher content of suspended solids in the lake water might have enhanced MP settling to the bottom and thereby contributed to the absence of MPs in the ice of the lake test, compared to the wetland test with low suspended solids and considerably more MPs in the ice. Furthermore, the higher negative charge in the lake water possibly stabilized the negatively charged MPs in suspension, and reduced ice entrapment. Regarding particle properties, shape had a distinct effect, with fibers being less likely incorporated into ice than fragments. No fibers were found in freshwater ice. However, it became clear that ice entrapment of MPs depends on factors other than the particles' buoyancy based on density differences and particle size and shape alone.