Microplastic Transport Within and Downstream of Circular Porous Vegetation: A Numerical Study in Open-Channel Flow

This study numerically investigates how a finite, circular patch of emergent vegetation alters microplastic (MP) transport, concentration, and retention in open-channel flow. A validated numerical model was developed to represent the vegetation patch as a porous zone and simulate MP transport. The framework was validated against laboratory data for two configurations: a low-blockage case and a high-blockage case. After validation, 36 MP cases, comprising four polymer densities, three particle diameters ranging from 0.1 to 0.5 mm, and two categories of shape factors (elongated and spherical), were released upstream and tracked over 180–420 s. Results show that vegetation density, represented by the blockage parameter and solid volume fraction, primarily controls the interception of microplastics. Dense patches create persistent recirculation and low-velocity zones that increase residence time and trapping, whereas sparse patches induce only transient disturbances, allowing rapid downstream advection. Quantitatively, retention in the dense configuration was ≈62% for the smaller MP sizes (0.1–0.2 mm) versus ≈35% in the sparse configuration at 300 s. Polymer density, particle shape, and particle size had only secondary effects under the tested moderate flow conditions. Smaller microplastics and elongated particles showed slightly higher retention. The findings identify dense vegetation as a selective hydrodynamic filter, demonstrating that vegetation-induced flow restructuring is the dominant control on MP fate. These effects should be considered in river restoration and pollution mitigation strategies.