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Settling velocities of microplastics and tire and road wear particles
Summary
Researchers developed a high-precision optical imaging method to measure how fast small microplastics (10–400 micrometers) and tire-and-road wear particles sink through water, filling a critical data gap needed to predict where these pollutants accumulate in aquatic environments.
Microplastics (MPs) were found to be ubiquitous in the environment, yet much uncertainty remains to be resolved with respect to their ecological impact and fate.Within aquatic environments, the transport of MPs and similar particulates is driven by numerous factors and processes.Gravitational settling plays a major role in the vertical transport of nonbuoyant or ballasted MPs and thus influences their distribution within the water column as well as their possible deposition.So far, terminal settling velocities have been mostly studied for large MPs (> 500 m), while empirical data on small MPs is still sparse.For tire and road wear particles (TRWP), no respective measurements have been reported at all, although variabilities in size and density of TRWP impede reliable settling velocity predictions.TRWP are not consistently defined as MPs, but emitted at large scale and proven to be environmentally relevant.This thesis presents a method for measuring the single particle settling velocities of small MPs (10-400 m).The measurement is based on optical imaging and features an image processing routine for automated particle tracking.The experimental setup was validated extensively according to experiments with spherical particles of different sizes and densities.It was demonstrated that vibrations and thermal convection could be suppressed effectively to allow for high precision and robustness.At elevated particle doses, interactions between particles were observed to induce deviations from single particle settling behavior.An empirical model was proposed and calibrated in order to monitor and thus minimize these effects during measurements by optimizing the particle dose accordingly.
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