Physical processes behind interactions of microplastic particles with natural ice

Abstract

Microplastic particles (MPs, <5 mm) are found in marine ice in larger quantities than in seawater, however, the distribution pattern within the ice cores is not consistent. To get insights into the most general physical processes behind interactions of ice and plastic particles in cool natural environments, information from academic and applied research is integrated and verified against available field observations. Non-polar molecules of common-market plastics are hydrophobic, so MPs are weak ice nucleators, are repelled from water and ice, and concentrate within air bubbles and brine channels. A large difference in thermal properties of ice and plastics favours concentration of MPs at the ice surface during freeze/thaw cycles. Under low environmental temperatures, falling in polar regions below the glass / brittle-ductile transition temperatures of the common-use plastics, they become brittle. This might partially explain the absence of floating macroplastics in polar waters. Freshwater freezes at the temperature well below that of its maximum density, so the water column is stably stratified, and MPs eventually concentrate at the ice surface and in air bubbles. In contrast, below growing sea ice, mechanisms of suspension freezing under conditions of (thermal plus haline) convection should permanently entangle MPs into ice. During further sea ice growth and aging, MPs are repelled from water and ice into air bubbles, brine channels, and to the upper/lower boundaries of the ice column. Sea ice permeability, especially while melting periods, can re-distribute sub-millimeter MPs through the brine channels, thus potentially introducing the variability of contamination with time. In accord with field observations, analysis reveals several competing factors that influence the distribution of MPs in sea ice. A thorough sampling of the upper ice surface, prevention of brine leakage while sampling and handling, considering the ice structure while segmenting the ice core – these steps may be advantageous for further understanding the pattern of plastic contamination in natural ice.

Entrainment and Enrichment of Microplastics in Ice Formation Processes: Implications for the Transport of Microplastics in Cold Regions

“…However, the air bubbles were not observed in the ice formed by saline water. It has been reported in the literature that the entrainment of MPs in ice is associated with the air bubbles in ice because the hydrophobicity and icephobicity of the MPs cause them to be pushed out into the air bubbles . Meanwhile, it has also been noted that air bubbles in the water are more likely to be attracted to more hydrophobic MPs .…”

“…Low temperatures can cause plastics to become more mechanically brittle, and ice formation and motion can pose mechanical impacts on plastics; both of these factors accelerate the fragmentation of plastics . The transport of MPs in cold regions can be a critical pathway affecting their fate.…”

“…As noted in the literature in this regard, MPs in ice tend to be pushed into brine channels due to their hydrophobic nature; in this manner, the brine channels can temporarily trap the particles until further redistribution during freeze/thaw cycles. 31 Brine networks present in sea ice facilitate the exchange of MPs between ice and water. In this regard, batch experiments conducted in calm seawater as part of a recent study have suggested that the MPs in sea ice can be gradually expulsed from ice and transferred to water.…”

“…It has been reported in the literature that the entrainment of MPs in ice is associated with the air bubbles in ice because the hydrophobicity and icephobicity of the MPs cause them to be pushed out into the air bubbles. 31 Meanwhile, it has also been noted that air bubbles in the water are more likely to be attracted to more hydrophobic MPs. 36 The higher enrichment of PE-2 and PTFE-2 in calm water compared to under low rotation speeds (250 and 500 rpm) observed in our study could be attributable to this phenomenon.…”

Evaluation of microplastics sediment sampling techniques—efficiency of common methods and new approaches

“…Additional fragmentation caused by rapid freezing was not observed in this study. Weathered and porous MP in environmental samples however may break apart during sample processing and particle identification processes [4]. Thus, this sampling method might be not suitable for approaches which determine particle numbers and shape.…”