Competing effects of bed sediments on turbulence-driven microfiber infiltration

Microfibers (MFs) are the most common microplastics found in most terrestrial, freshwater, and marine ecosystems. Little is known about the mechanisms by which MFs are sequestered in sediment beds. Specifically, we aim to determine how dynamic infiltration, fueled by natural turbulence, is mediated by both sediment consolidation times and sediment bed granulometry. To ascertain the dynamic infiltration of MFs in sediment beds, a series of tests with turbulence dissipations ranging from 1.01 to 4.15 cm² s^-3were carried out. An oscillating grid was used to create shear-free turbulence in a system where eight distinct synthetic types of MFs (varying in length, diameter, and polymer type) were previously deposited over a sediment bed. Four sediment consolidation times and six granulometries were considered to get more knowledge on the MF infiltration into the sediment caused by turbulence on the bottom floors of fresh and marine environments. It was discovered that during the applied turbulence, the finer part of the sediment bed with the 10th-percentile of particle diameter d₁₀, the MF diameter, the shape of the MFs, their flexibility, the shear velocity and the MF settling velocity affected how deeply the MFs infiltrated the sediment floor. Interestingly, the sequestration of MFs deposited in sediment floors was independent of the sediments' time of consolidation. Unlike previous experimental observations, this study demonstrated that infiltration occurs even when the microfiber-to-sediment diameter ratio exceeds 1 indicating the high capacity of sediment layers to allow MF infiltration. The MF infiltration was maximized for beds composed of fine and medium sands.