Separation and quantification of tire and road wear particles in road dust samples: Bonded-sulfur as a novel marker

Tire road wear particles (TRWPs) are a large source of microplastics in the environment, while the quantification of TRWPs is still challenging due to the complex interferences and the uncertainties and inconsistencies among different methods. This study developed a TRWPs quantification method using optimized pretreatments and bonded-sulfur as marker. Road dust samples (n = 48) were collected, pretreatments including density separation, digestion and extraction were optimized to remove interferences of the bonded-sulfur (minerals, sulfur containing proteins, hydrosoluble/hydrophobic sulfur-containing substances). Presence of TRWPs in the samples was confirmed by microscopy and scanning electron microscopy-energy dispersive spectrometry. Bonded sulfur in the samples were quantified by inductively coupled plasma-mass spectrometry (ICP-MS). Additionally, bonded-sulfur in tire wear particles (TWPs) abraded from tires of top 10 best-selling brands were measured to calculate conversion factor (1.1 x104 mu g/g) for the quantification of TRWPs in real samples. TRWPs contents were 5.40 x 104 mu g/g-11.02 x 104 mu g/g and 2.36 x 104 mu g/g-5.30 x 104 mu g/g in samples from heavy and light traffic roads, respectively. The method provided better recoveries (88-107%, n = 18) and repeatability (RSD=2.0-7.9%, n = 3) compared to methods using rubber, benzothiazole and organic zinc as markers. Furthermore, stability of the bonded-sulfur was validated by Raman and ICP-MS. Thus, this accurate and stable quantification method could promote research on TRWPs.