Weathering alters the settling dynamics of polyethylene microplastics: Integrating morphology-dependent drag coefficient with numerical simulations.

Weathering alters the physical properties of microplastics and can modify their transport behavior in aquatic systems, but its impact on settlement dynamics remains under-quantified. This study investigates how physico-chemical weathering affects the morphology and settling behavior of polyethylene microplastics by integrating laboratory experiments with numerical simulations. Particles underwent four laboratory-simulated weathering scenarios combining mechanical stirring, ultraviolet irradiation, and hydrogen peroxide treatment. Weathering resulted in substantial mass loss (∼25%) and systematic geometric changes, indicating size reduction accompanied by partial shape regularization. A numerical model based on an ad hoc Maxey-Riley formulation with a morphology-dependent drag coefficient was validated against independent experimental data (R = 0.90). Modeling results show that weathering reduced the terminal settling velocity by 5.2-7.1% relative to pristine particles. Mechanical stirring produced the largest reduction (6.7%) through particle size reduction and the associated mass loss, whereas hydrogen peroxide partially offset this reduction (+1.6%) through shape regularization. Ultraviolet irradiation had a negligible effect (<0.5%). These secondary effects were associated with slight shape regularization, partially compensating for the velocity reduction. When particle shape was explicitly incorporated into the corrected Reynolds number, settling velocities collapsed into a single scaling relationship (R > 0.99). These findings demonstrate that weathering causes slower settling of particles through size reduction, while shape-induced drag acts as a secondary but important factor in improving predictions of microplastic transport.