Microplastic Deposition in Bio-cohesive Sediment Beds within Vegetated Channels: Roles of Turbulence, Biofilm, and Particle Size

Microplastic accumulation has emerged as a major concern threatening aquatic ecosystem. Microorganisms in natural streams can affect microplastic deposition by transforming the sediment bed into biologically-cohesive material and altering its porosity. This study investigated how the presence of biofilm influenced deposition in both bare and vegetated channels, and how this effect varied with microplastic size. Laboratory experiments were conducted using cylinder dowels to represent mangrove pneumatophores. Biologically-cohesive sediment beds were generated by mixing sand grains with Extracellular Polymeric Substances (EPS). The EPS concentration, defined as the ratio of dry EPS mass to dry sediment mass, was varied from 0 to 0.24 %. For the deposition experiments, three types of fluorescent spherical particles with different diameters were introduced into the flow, and the deposition rate was measured by counting the number of deposited particles on the bed after a 3 hours-period. The particle deposition decreased as three factors increased: turbulence intensity, the ratio of depositing particle to bed grain size, and the EPS concentration. Specifically, as the size of depositing particles increased, their exposure on the bed surface increased, enhancing resuspension and reducing deposition. Further, EPS filled the pore spaces between the bed sediment grains, which increased the exposure of depositing particles on the bed surface, making resuspension easier and reducing deposition rate. The effect of EPS was more pronounced for smaller particles, which, in the absence of EPS, could settle into the pore spaces and be protected from resuspension. A new model predicting deposition probability produced a good agreement with observed values under both abiotic and biotic sediment conditions.