Microplastics in sea ice: A fingerprint of bubble flotation

Sea ice is heavily contaminated with microplastics (MPs), with the repeatedly confirmed increased number of larger-sized particles, deficit of fibers, and prevalence of materials denser than the surrounding water. To get insight into the drivers behind such a specific pattern, sets of laboratory experiments were performed on the formation of ice by cooling from the surface of fresh and salty (NaCl, 34 g/L) water, with particles of different sizes from heavy plastics (HPP) distributed initially over the bottom of the experimental volume. After freezing, about 50-60 % of HPP were trapped in ice in all the runs. Vertical distribution of HPP, plastic mass distribution, ice salinity (in saltwater experiments), and bubble concentration (in freshwater experiments) were recorded. Formation of bubbles on hydrophobic surfaces was the main cause of the entrapment of HPP into ice, with convection playing a secondary role. Supplementary bubble formation experiments with the same particles in water demonstrated that at larger fragments and fibers, several bubbles grow at the same time, so particle rising and residing at the surface is stable. Smaller HPP experience rising/sinking cycles with minimum time spent at the surface: one bubble is enough to cause a particle rising, but it is most often lost when colliding with the water surface. Application of the results to oceanic conditions is discussed. Oversaturation with gases due to various physical/biological/chemical processes and liberation of bubbles from methane seeps and melting permafrost are common in Arctic waters. Convective water motions are able to relocate HPP in vertical. Based on applied research, the bubble nucleation and growth, the hydrophobicity of weathered surfaces, the effectiveness of flotation methods for plastic particles are discussed. Interaction of plastic particles with bubbles is an important feature, still completely overlooked in the context of MPs behavior in marine environment.