Optimized and Validated Settling Velocity Measurement for Small Microplastic Particles (10–400 μm)

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.