Road dust-associated microplastics as a carrier of plastic additives in urban small-scale river sediment

Microplastics (MPs) and plastic additives have attracted global attention as emerging environmental pollutants. Herein, we analysed MPs and 56 organic plastic-derived chemicals in road dust (n=3), stormwater (n=4) and urban small-scale river sediment (n=8) samples collected from a downtown area of Kumamoto, Japan. The MP levels were 57,500–160,000 items/kg dw, 2.2–42 items/L and 1,600–15,800 items/kg dw in road dusts, stormwater and river sediments, respectively. The compositions of the polymers (tyre road wear particles [TRWPs], polymethyl methacrylate [PMMA] and polyethylene terephthalate [PET]) were consistent among the analysed samples; this result indicates that road dust–associated MPs were transferred to the river sediment via stormwater. A large amount of glass beads, an indicator of traffic-related particulate contaminants, was found in the road dust and sediments, suggesting that the urban small-scale river was strongly impacted by traffic activities. In total, 32 plastic-derived chemicals were identified in road dust and sediments with 6,900–32,000 and 620–13,000 ng/g dw concentrations, respectively. Notably, these concentrations were positively correlated with the abundance of MPs, indicating that MPs act as carriers of plastic-derived chemicals. We determined the concentrations of plastic additives in 28 traffic-related plastic products to identify the potential source of additive-derived chemicals in the road surface environment. Several road marking sheets and road paints contained large quantities (up to 1.7% w/w) of high-concerned substances, including phthalate esters, benzotriazole ultra-violet (UV) stabilisers and benzophenones. The additive profiles of the MPs and traffic-related products were similar to those of some PMMA-based MPs detected in the samples and PMMA-based road paints. This indicates that PMMA-based road paint was identified as an original source of MPs, contributing to 24%–54% of the total number of MPs in the road dust, stormwater and river sediment. In conclusion, an urban small-scale river was highly contaminated with road dust–associated MPs and plastic additives, and high-priority plastic products were identified as an important source of those contaminants in urban road areas.