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Experimental Study on the Erodability of Microplastics in Muddy Environments
Summary
Researchers experimentally quantified the erodability of microplastics of diverse compositions, densities, shapes, and sizes on smooth and synthetic cohesive sediment beds in a hydraulic flume, measuring critical shear stresses required to mobilise individual particles. They found that particle shape was the dominant factor governing mobilisation (spheres eroding most easily, followed by pellets, fibres, and sheets), with density as a secondary factor for particles of equivalent shape.
This study aims to elucidate the erodability behavior of microplastics in muddy environments like lakes, rivers, estuaries, and deltas, quantifying their critical shear stress on muddy sediment beds. Microplastics of diverse compositions, densities, shapes, and sizes were tested in a hydraulic flume with smooth and synthetic cohesive sediment beds. As flow intensity gradually increased, leading to particle mobilization, friction velocities and critical shear stresses were calculated. Initial results on smooth beds reveal that particle shape was a dominant factor in mobilization (sphere > pellet > fiber > sheet), followed by density: for equivalent shapes, denser particles required higher friction velocities for mobilization. Results from tests with different particle sizes and orientations relative to the flow highlight the influence of the exposed surface area: larger surface areas facilitate easier particle mobilization. Comparative experiments on smooth and muddy surfaces revealed higher shear stresses on cohesive sediment beds, attributed to particles sinking. Particle Image Velocimetry (P.I.V.) analysis showcased roughness-induced turbulence, marked by acceleration peaks and depressions, as the primary mechanism facilitating particle detachment from sediment.
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