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Settling behaviors of microplastic disks in water
Ge Yang1, Zijian Yu1, Abul B M Baki2
1Department of Civil and Environmental Engineering, University of Alberta, Edmonton, AB T6G 1H9, Canada.
Abstract:
Microplastic (MP) disks have not been studied for settling behaviors in aquatic environments, which affects the transport and fate of MPs. Therefore, settling experiments were conducted on MP disks of three shapes and four common-seen materials. Lighter MP disks (with density ρs = 1.038 g/cm3 and length l ≤ 5 mm) followed rectilinear vertical trajectories, while heavier MP disks (ρs = 1.161-1.343 g/cm3 and l = 5 mm) followed zigzag trajectories with oscillations and rotations. The mean terminal settling velocities of MP disks were 19.6-48.8 mm/s. Instantaneous settling velocities of heavier MP disks fluctuated. Existing formulas could not accurately predict the settling velocity of MP disks; thus, a new model was proposed with an error of 15.5 %. Finally, the Red - I* diagram (Red is the disk Reynolds number and I* is the dimensionless moment of inertia) was extended for MP disks to predict settling trajectories.
Insights
Microplastic disk settling behavior was unstudied, impacting transport. This research characterized MP disk settling, developing a new predictive model for their aquatic fate.
Area of Science:
- Environmental Science
- Fluid Dynamics
- Materials Science
Background:
- Microplastic (MP) pollution is a global concern.
- The settling behavior of microplastic disks in aquatic environments is poorly understood.
- This knowledge gap hinders accurate assessment of MP transport and fate.
Purpose of the Study:
- To investigate the settling behavior of microplastic disks.
- To determine the influence of shape, material, and density on settling dynamics.
- To develop a predictive model for microplastic disk settling velocity and trajectory.
Main Methods:
- Conducted settling experiments using microplastic disks of varying shapes and materials.
- Measured terminal settling velocities and observed trajectories.
- Analyzed data to compare with existing formulas and develop a new model.
Main Results:
- Lighter MP disks exhibited rectilinear vertical settling.
- Heavier MP disks showed zigzag trajectories with oscillations and rotations.
- Mean terminal settling velocities ranged from 19.6-48.8 mm/s.
- A new model was developed with a 15.5% error rate for predicting settling velocity.
Conclusions:
- Microplastic disk settling behavior is complex and depends on density and shape.
- Existing models are inadequate for predicting MP disk settling.
- The proposed model and extended Red - I* diagram offer improved prediction of MP disk settling trajectories.
