Systematic Evaluation of Physical Parameters Affecting the Terminal Settling Velocity of Microplastic Particles in Lakes Using CFD

Abstract

Microplastic (MP) particles are commonly found in freshwater environments such as rivers and lakes, negatively affecting aquatic organisms and potentially causing water quality issues. Understanding the transport and fate of MP particles in these environments is a key prerequisite to mitigate the problem. For standing water bodies (lakes, ponds) the terminal settling velocity (TSV) is a key parameter, which determines particle residence times and exposure times of organisms to MP in lakes. Here we systematically investigate the effects of the physical parameters density, volume, shape and roundness, surface roughness and hydrophobicity and lake water temperature on the TSV of a large number of particles with regular and irregular shapes (equivalent diameters: 0.5–2.5 mm) and different polymer densities using computational fluid dynamics (CFD) simulations. Simulation results are compared to laboratory settling experiments and used to evaluate existing, semi-empirical relationships to estimate TSV. The semi-empirical relationships were generally found to be in reasonable agreement with the CFD simulations (R2 > 0.92). Deviations were attributed to simplifications in their descriptions of particle shapes. Overall the CFD simulations also matched the TSVs from the experiments quite well, (R2 > 0.82), but experimental TSVs were generally slower than model TSVs with the largest differences for the irregular particles made from biodegradable polymers. The deviations of up to 58% were found to be related to the attachment of air bubbles on irregularities in the particle surfaces caused by the hydrophobicity of the MP particles. Overall, density was the most decisive parameter for TSV with increases in TSV of up to 400% followed by volume (200%), water temperature (47%) and particle roundness (45%). Our simulation results provide a frame of reference for an improved evaluation of the relative effects of different particle characteristics on their TSV in lakes. This will in turn allow a more robust estimation of particle residence times and potential exposure times of organism to MP in the different compartments of a lake.

Modeling of vertical microplastic transport by rising bubbles

“…It is expected that weathered particles in nature stick less to the water surface due to their increased hydropilicity [7,[53][54][55]. Thus we separately investigate the transport of weathered, non-sticky particles, and clean, sticky particles.…”

“…Waves on the ocean surface create myriads of air bubbles [1] that rise to the surface and burst. During rise, bubbles can interact with particles suspended in the water [2][3][4][5][6][7] and enrich particle concentration prior to bubble burst by bubble scavenging [8][9][10][11][12]. At burst, bubbles eject fine water droplets into the air, either in the form of film droplets or jet droplets of various size depending on bubble diameter [13][14][15][16][17][18][19][20][21].…”

Vertical transport of microplastics by rising bubbles

“…It is expected that weathered particles in nature stick less to the water surface due to their increased hydropilicity [7,[49][50][51]. Thus we separately investigate the transport of weathered, non-sticky particles, and clean, sticky particles.…”

“…Waves on the ocean surface create myriads of air bubbles [1] that rise to the surface and burst. During rise, bubbles can interact with particles suspended in the water [2][3][4][5][6][7] and enrich particle concentration prior to bubble burst by bubble scavenging [8][9][10][11][12]. At burst, bubbles eject fine water droplets into the air, either in the form of film droplets or jet droplets [13][14][15][16][17][18][19][20][21].…”

Assessing the Behavior of Microplastics in Fluvial Systems: Infiltration and Retention Dynamics in Streambed Sediments

“…MPs in rivers and streams are subject to a variety of transport processes (Kumar, Sharma, Verma et al., 2021) and exhibit unique transport characteristics (Waldschläger & Schüttrumpf, 2019) and transport modes compared to natural particles such as mineral sediments (Valero et al., 2022). Flow conditions (Haberstroh et al., 2020), channel characteristics (Mostefaoui et al., 2022) and particle properties (Ahmadi et al., 2022; Elagami et al., 2022) are the major controls for the hydrodynamic transport behavior of MPs in fluvial systems. Surface flow velocity has a direct effect on the abundance of MPs in streambed sediments (Eibes & Gabel, 2022): Low flow velocities, such as found in pool structures or near river banks, facilitate gravitational settling, and can lead to a temporal accumulation of MPs in streambed sediments (Hübner et al., 2020) which can be reversed during flooding (Ockelford et al., 2020).…”