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Influence of suspended sediment concentration on the settling velocity of irregularly shaped microplastic particles
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Researchers conducted settling column experiments to investigate how suspended sediment concentration affects the settling velocity of irregularly shaped, negatively buoyant polyethylene terephthalate (PET) microplastic particles, testing cylindrical and flaky particle shapes with densities of 1,290 to 1,470 kg/m3 under clear water, low, and high suspended sediment conditions.
Settling velocity is a fundamental property of plastic particles that helps in understanding transport processes by evaluating hydraulic threshold conditions for different transport modes, such as incipient motion, bed load, and suspended load. It is essential for predicting the pathways and accumulation zones of plastic particles in aquatic environments. In this study, settling column experiments were conducted to investigate the influence of suspended sediment concentration (SSC) on the settling behavior of irregularly shaped, negatively buoyant microplastic particles (MPs). Polyethylene terephthalate (PET) particles of cylindrical and flaky shapes, with a density ranging from 1290 to 1470 kg m-3, were selected for the experiments. The terminal settling velocity (Ws) of these 13 irregularly shaped MPs was measured under three distinct treatments: clear water (without sediments), with an SSC of 50mg/L, and 100mg/L. Each experiment performed three times and a total of 117 tests were conducted. The results showed that the terminal settling velocity (Ws) of cylindrical particles is higher than that of flaky-shaped particles, highlighting shape as a main influencing parameter. As SSC increases from 50mg/L to 100mg/L, Ws decreases for both the particles. Compared to clear water results, at 50mg/l and 100 mg/l SSC, the Ws decreased by 3.56% and 6.28% for cylindrical and 1.84% and 3.60% for flakes particles, respectively. The drag coefficient of microplastic particles rises with increasing SSC due to the presence of suspended sediment particles, which provide additional resistance to the settling process. Furthermore, the larger surface area relative to their volume leads to slower settling velocities. An equation was developed to predict the terminal settling velocity as a function of particle density, shape, size, and sediment concentration. This study provides valuable insights into the settling behavior of irregularly shaped microplastic particles in sediment-rich environments.Keywords: Terminal settling velocity, suspended sediment concentration, irregularly shaped microplastics, settling column experiments.
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