Development of analytical methods for microplastics from road markings

Summary

Microplastic pollution is an increasing environmental challenge, and road-related sources are considered major contributors. Road markings, however, are an understudied source, and standardized methods for identification are lacking. Therefore, the aim of this master’s thesis is to investigate road markings as a potential source of microplastic pollution. Six analytical techniques were applied: stereomicroscopy, SEM-EDX, µ-XRF, Videometer, TOF-SIMS, and ICP-MS. Methods for each technique were developed to identify road marking particles in environmental samples from gully pots, collected during fieldwork in spring 2025. Road marking materials provided by the municipalities of Oslo and Ås were also analyzed to characterize particle signatures. White road marking material consists of approximately 40% glass beads, 5% pigment, 10% filler (organic compounds and polymers), and 35% binders (sand and dolomite). Based on this composition, the following were tested: 1) whether glass beads are present in both used and unused road marking material, 2) whether road markings from Oslo and Ås municipalities differ significantly, 3) whether Si, Ti, and Ca can serve as signatures for the identification of road marking particles (RMPs), and 4) whether RMPs can be identified in roadside environmental samples. Glass beads were identified in both used and unused road marking material using SEM-EDX and µ-XRF. ICP-MS revealed significant differences (p < 0.05, t-test) in 17 of 39 analyzed elements between road marking materials from Oslo and Ås. These differences were further confirmed by principal component analysis (PCA) based on TOF-SIMS data. In roadside environmental samples, RMPs were analyzed as both isolated single particles and bulk samples. Si was poorly suited as a signature for single particles analyzed by SEM-EDX but performed well in bulk samples analyzed by µ-XRF. Elevated levels of Ti and Ca using both techniques reduced their suitability as unique signatures. TOF-SIMS did not detect any of the signatures, but the total mass-to-charge signal allowed the individual particles to be identified. The bulk samples were not recognized as RMPs. Videometer demonstrated potential for quantification of road marking particles in bulk samples based on spectral information, particularly in the near-infrared region (970 nm), where polymer components produced distinct signals. The results highlight the complementary strengths of the applied techniques and emphasize the need for an international protocol. By demonstrating the ability to identify chemical variation and environmental relevance of RMPs, this study contributes to addressing a critical knowledge gap concerning road-related microplastic pollution.