Settling Velocities of Small Microplastic Fragments and Fibers

Abstract

There is only sparse empirical data on the settling velocity of small, nonbuoyant microplastics thus far, although it is an important parameter governing their vertical transport within aquatic environments. This study reports the settling velocities of 4031 exemplary microplastic particles. Focusing on the environmentally most prevalent particle shapes, irregular microplastic fragments of four different polymer types (9-289 μm) and five discrete length fractions (50-600 μm) of common nylon and polyester fibers are investigated, respectively. All settling experiments are carried out in quiescent water by using a specialized optical imaging setup. The method has been previously validated in order to minimize disruptive factors, e.g., thermal convection or particle interactions, and thus enable the precise measurements of the velocities of individual microplastic particles (0.003-9.094 mm/s). Based on the obtained data, ten existing models for predicting a particle's terminal settling velocity are assessed. It is concluded that models, which were specifically deduced from empirical data on larger microplastics, fail to provide accurate predictions for small microplastics. Instead, a different approach is highlighted as a viable option for computing settling velocities across the microplastics continuum in terms of size, density, and shape.

Keywords: fibers; fragments; microplastics; sedimentation; settling velocity; sinking velocity; transport.

Figure 1

Exemplary scanning electron microscopy (SEM) images of all investigated MP fragments and the 100 μm length fraction of PA 6.6 fibers (columns). Three uniform magnifications are displayed (rows, note color codes, and scale bars on the left-hand side), and the magnified regions are indicated, respectively. All samples were Au sputtered before SEM images were acquired in secondary electron mode.

Figure 2

Exemplary trajectories of different MP fragments and one 400 μm long fiber obtained from settling experiments. Note the scale bar and the respective image acquisition frame rate (annotated in brackets).

Figure 3

Measured settling velocities at 15 °C and equivalent spherical diameters of all of the investigated MP fragments. Note the variation of y-axis scaling for the different polymer types. Terminal settling velocities predicted for spherical particles of the respective densities ρ are plotted according to Stokes’ law (valid for laminar flow, Re ≪ 1) and the model of Su et al. Distinct Reynolds numbers are annotated (orange).

Figure 4

Length distributions (a) and boxplots of settling velocities measured at 15 °C (b) of all investigated fractions of PET and PA 6.6 fibers. Mean lengths and standard deviations (cf. a) as well as the number of fibers per fraction measured during settling experiments (cf. b) are annotated next to the respective plots. The best tested terminal velocity models are plotted with coefficients of determination R² given for PA 6.6 and PET in respective colors (cf. Section S7 of the Supporting Information; (*) indicates the use of d₂ instead of d_eq as input diameter). Two regions of interest (ROIs) of raw images are inset to highlight fibers settling with different orientations: The arrows indicate their respective velocities, and colored scale bars denote 200 μm.

Figure 5

Measured settling velocities versus corresponding predictions by each of the tested terminal settling velocity models (note the log–log scale). Performance measures R_log², R² and |AE| are indicated, respectively. If a validity range is specified for a model according to the Reynolds number, outliers are not considered and are depicted pale. Use of d₂ instead of d_eq as input diameter is indicated by * (see Section S7). Models derived specifically for MP are highlighted with a yellow background.