Settling Velocities of Environmentally Weathered Plastic Fibers from the Mekong River in Southeast Asia

The
influences that weathering processes may have on
plastic settling
have not been well documented or quantified. In this study, the physical
characteristics and settling velocities of environmentally weathered
micro- and macroplastic fibers collected in the Mekong River in Southeast
Asia were measured in a laboratory setting for the first time. Size
and shape ranges were larger than previous studies in which lab-generated
particles were employed for experimentation. Measurements were used
to evaluate several current drag equations. Out of four tested drag
equation candidates, the Zhang & Choi (2022) equation was best
at predicting the behaviors of weathered particles with an average
error of 18% and root-mean-square error of 2.4%. The impacts of settling
on estimating plastic loads were demonstrated using a plastic transport
model parametrized for the Mekong directly upstream of Phnom Penh.
Results suggest that a much larger fraction of plastic particles move
toward the bottom of the water column than estimated with the conventional
settling velocity method (97% vs 1%). These findings provide essential
information on the settling characteristics of environmentally weathered
microplastics, which will subsequently allow transport models to better
represent the behaviors of plastics found in the aquatic environment.