Adapting Methods for Isolation and Enumeration of Microplastics to Quantify Tire Road Wear Particles with Confirmation by Pyrolysis GC–MS

The complex, varied composition (i.e., rubbers/elastomers, carbon black, fillers, additives, and embedded road materials) and wide density range of tire road wear particles (TRWPs) present challenges for their isolation and identification from environmental matrices. Reliable quantification is important for understanding the environmental fate and potential adverse effects of TRWPs. To address environmental monitoring needs, the present work adapts a series of isolation and identification steps from methods commonly applied for microplastic analysis for single-particle-level enumeration of TRWPs from environmental samples. We present the method performance of a two-stage density separation with saturated NaCl and sodium polytungstate to isolate TRWPs from sediment matrices, the compatibility of tire microrubber with reagents used for digestion of environmental matrices, and the use of elasticity and heat resistance observations to differentiate TRWPs from bitumen particulates, which are potential visual interference. We found that alkaline digestions (NaOH and KOH) are compatible with tire microrubbers, but hydrogen peroxide and sodium hypochlorite can cause TRWPs to lose elasticity and flatten or break when probed. The adapted methods were applied to road dust samples, and a subset of identified TRWPs was qualitatively confirmed by both scanning electron microscopy and pyrolysis-gas chromatography-mass spectrometry. Further, a compilation of aspect ratio measurements of TRWPs between 63 and 500 μm (N = 780) that were isolated from urban sediments shows the potential diversity of TRWP shapes in the environment.