Morphology and mineral encrustations of density-separated tire- and road-wear particles collected in Charleston, South Carolina

Abstract Tire- and road-wear particles (TRWPs) are one of the main types of primary microplastics in the environment. Generated through driving a vehicle, they accumulate on roads, where they are often found encrusted with varying amounts of road dust. Here, we report physical and chemical data for individual TRWPs (> 150 µm across), which were collected in Charleston, South Carolina, and separated via density fractionation. For this study, image and elemental analysis of individual TRWPs was conducted on only the lowest (≤ 1.179 g/cm 3 ) and highest (≥ 1.43 g/cm 3 ) density categories. Images of TRWPs, captured through scanning electron microscopy, were processed to determine axial ratios, volumes, and degree of encrustations. The high-density TRWPs were overall more elongated than their low-density counterparts but the TRWP volumes were similar in both density categories. The particles were mapped using energy-dispersive X-ray spectroscopy to visualize the distribution of Na, Mg, Al, Si, K, Ca, Ti, and Fe on the surface of the TRWPs. Subsequently, these element distribution maps were used to identify the mineral phases present in the TRWP encrustations. The obtained data revealed the main types of minerals occurring in the encrustations were quartz, feldspar, and Fe-oxide/hydroxides. The high-density TRWPs exhibited a greater extent of mineral encrustation than their lower-density counterparts. Iron and Ti were more prominent in the encrustations of high-density particles than in those of low-density TRWPs. These results demonstrate that the density of TRWPs, and consequently their transport and fate, are influenced by the extent and mineralogical composition of their mineral encrustations.