Settling and rising velocities of environmentally weathered micro- and macroplastic particles.

Microplastic is exposed to numerous weathering processes in the environment, which change particle properties and thus influence transport behaviors, including settling and rising velocities in aquatic environments. However, the extent to which particles in the environment differ from virgin particles in their transport behaviors has not yet been investigated. The settling and rising velocities of weathered fluvial microplastic and macroplastic particles collected from environmental samples are determined in this study and the transferability of theoretical approaches to predicting their transport behaviors is examined. The settling velocities of the environmental particles were between 0.16 and 3.52 cm/s and the rising velocities between 0.18 and 19.85 cm/s. Formulas were applied that were developed using experiments with virgin microplastic, but do not account for the effects of environmental impacts such as degradation, fragmentation and biofouling on the velocities. Accordingly, the transferability of the formulas to environmental particles must be verified. The formulas proved to be suitable for describing the settling and rising velocities of environmental microplastic particles in the shapes of pellets, fragments and foams, and were less suitable for describing the velocities of films and macroplastic particles. Further experiments will be necessary in the future to integrate effects from biofilm and particle deformation on the transport behaviors to adequately model the behavior of the highly diverse micro- and macroplastic particles in the aquatic environment.