Microplastic particles in road de-icing salt

, Rødland Elisabeth, Okoffo Elvis, Rauert Cassandra, Heier Lene Sørlie, Lind Ole Christian, Reid Malcom, Thomas Kevin, Meland Sondre.

The largest source of microplastic particles to environment is estimated to be roads, through the wear and tear of road surface and release of tire particles, road particles and road markings. During the winter in cold climates, the road surface may freeze and cause icing on the roads. For traffic safety reasons, de-icing of the road surface is important and road salt is applied for this purpose. Knowledge of microplastic (MP) contamination in road salt has, until now, been lacking. This is contrary to the increasing number of studies of microplastics in food-grade salt. In this study, road salt was investigated as a potential new source of road-related microplastic particles to the environment. Two analytical techniques were employed in this study; Fourier-Transform Infrared spectroscopy (FT-IR) and Pyrolysis gas chromatography mass spectrometry (GC-MS). Polymer content in four types of road salts, three sea salts and one rock salt, was investigated. The particle number of MP in sea salts (range 4-240 MP/kg, mean ± s.d. = 35 ± 60 MP/kg and rock salt (range 4-192 MP/kg, 424 ± 61 MP/kg, respectively) were similar, whereas, MP mass concentrations were higher in sea salts (range 0.1-7650 µg/kg, 442 ± 1466 µg/kg) than in rock salts (1-1100 µg/kg, 322 ± 481 µg/kg). A total of 96% of the concentration (µg/kg) and 86 % of number of particles/kg came from black rubber-like particles, which appeared to be attributable to wear of conveyer belts used in the salt production. The contribution of MP from road salt in Norway was estimated to 0.15 tonnes/year (0.003% of total road MP release), 0.07 tonnes/year in Sweden (0.008%) and 0.03 tonnes/year in Denmark (0.0004-0.0008%). The study shows that road salt is a source of MP, however, it is a negligible source compared to other road-related sources.

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