A new model for the terminal settling velocity of microplastics

Microplastic (MP) settling process is important for the transport of microplastic particles (MPs, <5 mm) in water bodies. However, for the control parameter of the drag coefficient (C_d), no generalized formula has been proposed for MPs of different shapes and materials. In this study, a total of 1343 MP settling data were collected from the literature. It was found that the drag law for perfect spheres cannot reasonably predict C_d for MPs with particle Reynolds number of 1-10³. A new formula for C_d was developed by introducing the dimensionless particle diameter (d_⁎) and two shape descriptors. The absolute error of the new formula is 15.2%, smaller than those (42.5-72.8%) of other existing formulas. Moreover, an explicit model was developed for MP settling velocity by correlating C_d, d_⁎, and shape descriptors, with lower absolute error (8.8%) than those (15.4-77.2%) of existing models.