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.

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<p>Mismanaged waste leads to inputs of microplastics into the environment and the aquatic system affecting rivers and lakes. The physical properties of microplastic (MP) particles affect their terminal settling velocity (TSV) in the water column and in turn their distribution patterns in aquatic systems. To evaluate the settling behavior and the TSV of MP particles we simulated the settling of a large range of MP particles with regular and irregular shapes in the water column using a computational fluid dynamics (CFD) model. To validate the results returned by our model, we compared CFD findings to the corresponding results obtained by semi-empirical relationships as well as the results from experiments for 120 irregularly shaped MP particles with sizes and densities ranging from 500 to 2000 µm and 1.03 to 1.38 grcm<sup>-3</sup>, respectively. The CFD results are in good agreement with the results from the laboratory and semi-empirical relationships with a 0.05 difference in the slopes of their linear regressions. In a next step, we defined scenarios to systematically investigate the influence of different particle characteristics such as roundness, density, and volume as well as water temperature on the TSV of regular and irregular MP particles. Our simulations revealed a dominant effect of particle density on the TSV compared to the effects of the other parameters. For example, doubling particle densities increased the TSVs of the MP particles up to 500%, while, doubling their volumes only led to a maximum increase in their TSV of 200%. Increasing the roundness of the MP particles, letting them evolve towards a perfect sphere, increased their TSVs by up to 15%, while seasonal changes in lake water temperatures typical for lakes in temperate climate regions, caused changes in TSVs by up to 32%.</p>