Quantifying emissions and environmental risks of cemented carbide (WC) nanoparticles from tire studs

Paper in proceeding, 2016

Tire studs, with pins typically made out of cemented carbide with cobalt (WC-Co), are used in several countries to improve the gripping power during winter. Throughout their use, the tire studs are worn. This leads to emissions of particles, of which some are nano-sized. Until now, WC-Co nanoparticles have seldom been included in environmental risk assessments of nanomaterials. The aim of this study is to assess the magnitude of emissions of WC-Co (nano)particles from tire studs on a national level, compare WC-Co emissions to those of other nanomaterials, and conduct a screening risk assessment of the WC-Co emissions. The country in focus was Sweden, where 66% of the passenger cars use studded tires during winter. Substance flow analysis (SFA) was used to quantify the mass flows related to tire studs from the raw material extraction phase to the waste management phase. Furthermore, particle flow analysis (PFA) was used to quantify particle number flows in the use phase, where emissions of W-containing nanoparticles have been observed. As part of these quantifications, use phase emissions of WC-Co nanoparticles were calculated using a leaching-type model based on the number of km driven by the Swedish car fleet and experimental measurements of W-emission rates. Two scenarios – a low-emission and a high-emission scenario – were employed. Preliminary results show annual WC-Co emissions of 10-10^3 kg/year and 10^15-10^20 particles/year. These estimated emissions were then compared to estimated emissions of some other nanomaterials in Sweden, including silver, titanium dioxide, cerium oxide, fullerenes and carbon nanotubes. This comparison indicated that the emissions of WC-Co nanoparticles from tire studs are in the same range as estimated emissions of some of these other nanomaterials. Our estimation of the predicted environmental concentration (PEC) of WC-Co nanoparticles in road runoff was in the same order of magnitude as measured environmental concentrations (MEC) of W-containing particles. The limited existing ecotoxicity data indicate that much higher concentrations are required in order to reach toxic levels, but further studies are needed to confirm this. Overall, the notable emissions of WC-Co nanoparticles during the use of tire studs support the further study and inclusion of this material in risk-related studies of nanoparticles.