Transport of Tyre Wear Particles through Porous Media: Effect of Particle Density and Sediment Grain Size

Tyre wear particles (TWP), a substantial fraction of microplastics (MPs), have drawn a lot of attention recently because of their pervasiveness in aquatic environments; nevertheless, little is known about the transport behaviour through porous media. Therefore, in this study, column experiments were carried out to investigate the transport behaviour of TWP through porous media with varied sediment grain sizes and particle density. Soda lime glass beads of 10mm and 2mm diameters were used as a porous media material to represent natural sediments, such as fine gravel and coarse sand. Transport of TWPs (1.15 g/cm³) is compared with polypropylene (PP, 0.98 g/cm³) to ascertain the impact of density on MPs transport in porous media. The results show that PP, which is of lower density, was more mobile within the fine gravel media. Despite the buoyant nature of PP and expected less gravitational settling, their movement through porous media was faster and more complete than TWP. In contrast, for the denser TWP, retention in the porous media was higher, perhaps also due to greater surface interaction with the media. However, within the coarse sand, both PP and TWP breakthrough was reduced, with lower peaks and enhanced particle retention in sediments. In this case, mechanical straining and greater surface contact likely dominated over density effects, causing increased retention for both particle types.

This suggests that both particle density and sediment grain size as well as slight differences in surface properties and shapes of TWP and PP have a considerable impact on MP transport in porous media. This research is essential for comprehending the transport dynamics of TWPs in sub-surface environments, hence emphasising the environmental repercussions linked to their extensive distribution.

Keywords: Tyre wear particle, density, glass beads, transport, PP