Laboratory Experiments on the Transport of Microplastic Particles in Gravel and Sand Sediments

Abstract

Column experiments are used to study infiltration and transport behavior of microplastics (MP) in aquifers. The transport behavior of various MP polymer types, including PET, POM, PMMA, and PS, which differ in size (0.8 – 3 mm), density (1.03 – 1.42 g/cm3), and shape, has been examined. The MP particles were added to the inflow of columns containing different gravel compositions, having both unimodal (d50 = 12, 6.5, 2.5, 1.2 mm) and bimodal distribution (d50 = 10, 6, 3.5 mm), and also of columns filled with sand (d50 = 0.031 and 0.065 mm). To introduce MP particles into the sediment, a novel approach involving melting frozen MP particles embedded in ice layers was employed. This method naturally replicated the infiltration process while minimizing blockages or particle losses in the circuit's pipes and connectors. After infiltration at defined flow rates for three days, MP particles have been separated from sediment layers of 3 cm thickness manually or using density separation. The depth where they were identified is defined as infiltration depth. Micro computed tomography (Micro-CT) was applied to visualize sediment pores and throats where MP passed through. Results for infiltration in different gravel textures showed as expected that smaller MP are transported to a greater depth. Lighter MP were also found in deeper layers due to its shape. Concerning shape effects, flat circular discs showed a higher potential to be found at greater infiltration depth, compared to spheres, fibers and pellets. Concerning the size ratio between MP and sediment grains bimodal sediment reveals to hinder the infiltration of MP due to its lower pore size, which is consistent with results from the Micro-CT pore measurement. Initial results from sand column experiments will be analyzed to explore the size ratio range between sediments and MP under saturated conditions, highlighting the differences in MP behavior between sand and gravel. The findings enhance our understanding of MP transport mechanisms in aquifer sediments and infiltration basins and offer insights for groundwater and sediment MP contamination mitigation.