Abstract
Roads are estimated to be the largest source of microplastic particles in the environment, through release of particles from tires, road markings and polymer-modified bitumen. These are all released through the wear and tear of tires and the road surface. During the winter in cold climates, the road surface may freeze and cause icing on the roads. To improve traffic safety during winter, road salt is used for de-icing. 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. The objective of this study was to investigate if road salt could be an additional source of microplastics to the environment. Fourier-Transform Infrared spectroscopy (FT-IR) and Pyrolysis gas chromatography mass spectrometry (GC–MS) were employed to identify and quantify the polymer content in four types of road salts, three sea salts and one rock salt. 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). Black rubber-like particles constituted 96% of the total concentration of microplastics and 86% of all particles in terms of number of particles/kg. Black rubber-like particles appeared to be attributable to wear of conveyer belts used in the salt production. Road salt contribution to MP on state and county roads in Norway was estimated to 0.15 t/year (0.003% of total road MP release), 0.07 t/year in Sweden (0.008%) and 0.03 t/year in Denmark (0.0004–0.0008%) Thus, microplastics in road salt are a negligible source of microplastics from roads compared to other sources.
Highlights
IntroductionSeveral studies have identified microplastic particles (MPs) in both sea salt and rock salt used for food consumption (Gündoğdu, 2018; Iniguez et al, 2017; Karami et al, 2017; Kim et al, 2018; Lee et al, 2019; Seth and Shriwastav, 2018; Yang et al, 2015)
All samples were filtered on the same day as the control sample with the Polyethylene terephthalate (PET) fibre is corrected for the mean PET value, which included all the samples from Torrevieja
Since we found a presence of Styrene butadiene rubber (SBR) in the Ben Gardene sample, it is a possibility that some of the BRP could come from tire wear, as well as both trucks, lorries, tractors and other vehicles may be used at production sites of both sea salt and rock salt
Summary
Several studies have identified MPs in both sea salt and rock salt used for food consumption (Gündoğdu, 2018; Iniguez et al, 2017; Karami et al, 2017; Kim et al, 2018; Lee et al, 2019; Seth and Shriwastav, 2018; Yang et al, 2015). The present study's main objective was to investigate if MP particles are present in road salt, and to estimate the potential emission of these MPs in Norway, Sweden and Denmark
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