Mapping Microplastic Movement: A Phase Diagram to Predict Nonbuoyant Microplastic Modes of Transport at the Particle Scale

Microplastics pose numerous threats to aquatic environments,
yet
understanding their transport mechanisms remains limited. Drawing
from natural sediment research provides valuable insights to address
this knowledge gap. One key dimensionless number used to describe
sediment transport is the transport stage, referring to the ratio
between the flow shear velocity and the particle settling velocity.
However, variations in physical properties, such as shape and density,
raise concerns about the applicability of existing sediment transport
theories to microplastics. To address this challenge, we employed
a physical modeling approach, examining 24 different nonbuoyant microplastic
particles in a turbulent open channel flow. Utilizing 3D particle
tracking, a total of 720 trajectories were recorded and analyzed.
Microplastic particles exhibited transport modes akin to natural sediments,
including rolling/sliding, saltation, and suspension. The transport
stage strongly correlated with these modes, as well as with the mean
forward velocity and mean position in the water column. Notably, particle
shape emerged as a critical factor influencing transport dynamics.
Due to their lower settling velocity, fibers tended to stay closer
to the water surface with lower forward velocities compared to spheres.
Based on the laboratory results, a new phase diagram for microplastics
is introduced analogous to an existing diagram for sediments.