Tyre stud derived tungsten carbide particles in urban street dust

In countries where studded winter tyres are used they contribute to the generation of street dust by grinding the road surface and traction sand into finer particles. At the same time the hard metal tips of the studs, made out of tungsten carbide (WC), wear to finer particles dispersed into the environment. Elevated tungsten concentrations in different sampling media, probably caused by the use of studded tyres, have also been reported. In this study three size fractions of street dust sampled in Turku, Finland, were investigated. Tungsten and various other element concentrations were determined with ICP-MS after total dissolution, pseudo total concentration (aqua regia) and a weaker extraction (1M NH4Ac). A visual analysis was made with a SEM-EDX to study the presence and size fraction of WC particles, which has not been studied before. The total concentrations (median values) of tungsten in the fractions were 9.2 µg g–1 (100 µm–2 mm), 21 µg g–1 (45 µm–100 µm) and 39 µg g–1 (< 45 µm). As expected, tungsten showed a tendency to accumulate into the finer size fractions. However, more surprising was the result that out of all elements determined, tungsten had the greatest (median values) relative enrichment in the fine fraction. In the SEM-EDX analysis particles consisting of tungsten were identified and ruled out to be WC abraded from tyre studs. The WC particles occurred either separately or in clusters with size range of 0.1–1.4 µm.