Settling and Rising Hydrodynamics of Microplastic Pollutants: A Numerical Study

With ever-growing plastic production, the pollution of microplastics (MPs) has become a threatening environmental problem in the twenty-first century. It is crucial to investigate MPs' hydrodynamics due to their widespread pollution in aquatic environments. MPs are particularly difficult to characterize because they not only have a wide range of sizes and densities and are also found with highly variable shapes. So far, no numerical investigation exists that has sufficiently accounted for MPs' complex shapes. To accurately predict the fate, transport, and mobility of aquatic MPs, formulating settling hydrodynamics of MPs with complex shapes is essential. In the present study, a finite volume-based three-dimensional numerical model is employed to investigate the settling trajectories of MP particles with a wide variety of shapes and densities in a quiescent fluid. Seven test runs are performed utilizing the present model, with negatively buoyant MP particles whose densities range from 1100 to 2000 kg/m(3). The particle Reynolds numbers of these settling particles fall in the nonlinear range of 100-1300. Our initial results have proven the present model's robustness in simulating the dynamics of MP-sized particles in a quasi-static fluid. Preliminary results with MP particles of regular shapes are shown to be consistent with previous empirical formulations derived from particle settling experiments. With the present model, complex shaped MPs, e.g., thin cylinders, resembling MP films, are also tested. The settling dynamics of thin cylindrical MP particles obtained from the present model match well with the settling patterns observed from experiments. Results from the present model will be used to parameterize MP particles with irregular shapes, enabling the prediction of MPs' transport history through large-scale models.