Infiltration of Microfibers in Sediment Beds in Moderate- to High-Turbulent Aquatic Environments

Sediment beds of aquatic ecosystems are known to present high microplastic contamination levels, where the infiltration of microplastic particles can extend deep into the sediment. In particular, turbulence can fuel the infiltration of microplastics in static beds. A set of experiments varying the turbulence intensity (shear rates from 10.03 to 20.36 s^-1) were conducted to determine the dynamic infiltration of microfibers in sediment beds. Eight types of microfibers (with lengths ranging from 3 to 5 mm, diameters from 0.045 to 1 mm, and two polymer types, here, polyethylene terephthalate and polyamide), previously deposited over a sediment floor, were submitted to vertical turbulence generated by an oscillating grid. The turbulence-induced infiltration of microfibers into sediment mimicked the sediment reworking in the bottom floors by episodes of moderate- to high-turbulent activity. The infiltration depth of the microfibers into the sediment floor increased with the shear stress on the bottom and was modulated with the fiber properties. Mainly, the infiltration of microfibers was greater as the diameter of the fibers decreased. In addition, the infiltration was from top to bottom, following an exponential decay in the number of microfibers, in accordance with the fine bridging infiltration mode.