Adding depth to microplastics for particle characterization and assessing settling behavior

Knowledge of the shape and volume of microplastics (MPs), i.e., in 3D, is crucial for quantifying transport, exposure, and toxicological effects of MPs. For instance, settling of MPs is a process that plays an important role in retention in water systems. However, the available information about environmental MPs is usually in 2D, based on image analysis or µ-FTIR data. Here, we present an innovative approach for estimating 3D data from 2D data ('2D-3D') and illustrate how this can be applied to determine settling velocities and 3D trajectories of MPs in water with different algal densities. The reliability of these approaches has been tested and showed consistent and satisfactory results. A new feature of the 2D-3D approach is that the translation is not done per individual particle, but for all MP particles in an environmentally relevant mixture at once. This is important, for example, when studying the food dilution effect of MPs. It is also innovative because it does not require calibration but is only based on information obtained from the image analysis. The 2D-3D translation can also be applied for the validation of theoretical settling models. We demonstrate such a validation application for the settling velocities of MPs in water with different algal densities. 3D velocity vectors of MP particles were reconstructed from images taken by cameras placed outside the algae-containing water column. By using two cameras, the actual settling trajectories, including the lateral component, could be accurately determined. Algal density was found to have no effect on settling. The results were consistent with the theory for some of the tested theoretical models. These findings are highly relevant for the development of fate models for MP transport in aquatic systems. Also see: https://micro2024.sciencesconf.org/559487/document