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Optimized and Validated Settling Velocity Measurement for Small Microplastic Particles (10–400 μm)
Summary
This study developed and validated a precise laboratory method for measuring how fast small microplastic particles (10–400 µm) sink in water — a key parameter for predicting where microplastics accumulate in aquatic environments. The setup uses a temperature-controlled settling column with optical particle tracking and achieves high accuracy across a range of particle sizes and densities. Accurate settling velocity data for small microplastics is essential for modeling their transport and fate in rivers, lakes, and oceans, which informs risk assessments for aquatic organisms that live at different depths.
The settling velocity of nonbuoyant microplastics is one of the key parameters to describe their vertical transport in water, yet it has rarely been studied for small microplastics (<500 μm) thus far. Respective measurements are challenging as they are prone to disruptive factors such as thermal convection. With decreasing size, it also becomes more difficult to handle target particles separately. Instead, it is favorable to work with suspensions─especially when characterizing particle populations based on sufficient individual measurements. This study establishes and validates a suitable measuring setup, which mainly consists of a precisely tempered settling column that is monitored via optical imaging with subsequent particle tracking. Comprehensive validation experiments with different spherical particles covering the desired size (10–388 μm) and density range (1.05–2.46 g/cm3) verify exceptionally high measurement accuracy and precision. Different investigation schemes were proposed and successfully tested for polydisperse and monodisperse particle samples, respectively. At elevated particle doses, measured settling velocities increased due to swarm effects and interactions between particles. A novel empirical model was fitted to represent those effects. The model can aid in limiting the particle dosage and thus prevent overestimations of single particle velocities.
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