Rapid generation of aged tire-wear particles using dry-, wet-, and cryo-milling for ecotoxicity testing

Strict environmental laws have been enacted to regulate the emission of exhaust particulate matter (PM), which is one of the most hazardous pollutants that reduce air quality and pose a serious risk to the human health. In addition, non-exhaust PM, such as road wear, tire wear, and brake wear debris, is a significant source of airborne pollutants. Road dust less than 100 μm in size may include tire wear particles (TWPs), which are broken down into finer particles with sizes on the order of tens of micrometers because of weathering. TWPs can be transported to water bodies via runoff, potentially contaminating water systems and negatively affecting aquatic ecosystems. Therefore, ecotoxicity tests using reference TWPs are required to investigate the impact of TWPs on the human health and environment. In this study, aged TWPs were produced using dry-, wet-, and cryo-milling methods, and the dispersion stability of TWPs in dechlorinated water was evaluated. Aged TWPs prepared by dry- and wet-milling had an average particle size of 20 μm, whereas pristine TWPs had an irregular shape and average particle size of 100 μm. The capacity of the ball-milling cylinder and excessively long 28-d generation time constrain the amount of aged TWPs that can be produced through conventional milling. In contrast, cryo-milling reduces the particle size of TWPs at the rate of -275.0 μm/d, which is nine times higher than that upon dry- and wet-milling. Dispersed cryo-milled TWPs had a hydrodiameter of 2.02 μm and were more stable in the aqueous phase in relation to the other aged TWPs. The results of this study suggest that cryo-milled TWPs can be used for aquatic exposure assessments as controls for real-world TWPs.

Overall distribution of tire-wear particles, nano‑carbon black, and heavy metals in size-fractionated road dust collected from steel industrial complexes

Tire-wear particles (TWP) from vehicles serves as a non-exhaust emission source. The mass content of metallic species in road dust may increase owing to the traffic of heavy vehicles and industrial activity; consequently, metallic particles are also present in road dust. Herein, road dust collected from steel industrial complexes with high traffic of high-weight vehicles and the composition distribution of five size-fractioned particle sizes were analyzed. Road dust samples were collected from three areas near steelmaking complexes. The mass distribution of TWP, carbon black (CB), bituminous coal, and heavy metals (Fe, Zn, Mn, Pb, Ni, As, Cu, Cd, and Hg) in different size fractions of road dust was quantified by combining four different analytical techniques. In the magnetic separation for <45 μm fraction, 34.4 wt% and 50.9 wt% was removed for steelmaking and steel-related industrial complexes, respectively. As the particle size decreased, the mass content of Fe, Mn, and TWP increased. The enrichment factors of Mn, Zn, and Ni were higher than two, indicating that they were related to industrial activities in steel complexes. The maximum concentrations of TWP and CB originating from the vehicle varied depending on the region and particle size range: TWP 2.066 wt% at 45-75 μm (industrial complex) and CB 5.559 wt% at 75-160 μm (steel complex). Coal was only found in the steel complex. Finally, to reduce the exposure of the finest particles to road dust, three methods were suggested. Magnetic fraction must be removed from road dust using magnetic separation; the fly dust of coal during transportation must be suppressed, and covers must be used in coal yards; the mass contents of TWP and CB in road dust should be removed by vacuum cleaning instead of water flushing.

Quantitative analysis of the concentration of nano‑carbon black originating from tire-wear particles in the road dust

Exhaust and non-exhaust particulate matter (PM) is regarded as the most significant airborne during driving. Among the source of non-exhaust PM, the tire-wear particles (TWP) can be quantified using pyrolysis-gas chromatography/mass spectrometry (Py-GC-MS). TWPs are fragmented by continuous weathering once exposed to the road. Approximately 5 wt% of carbon black (CB) bound in the rubber matrix of TWPs tends to detach from it, and thus some portion of free-bound CB could be co-existed in the road dust. Although there are existing methods for analyzing pure CB and TWPs, only few analysis techniques on the amount of free-bound CB in contaminant samples have been discovered. Herein, we propose a method for quantifying the total and free-bound CB in road dust using a combination of four analytical tools: a semi-continuous carbon analyzer, element analyzer, thermogravimetric analyzer, and Py-GC-MS. This study is the first attempt in quantifying the concentration of nano-CB derived from TWPs in road dust. The proposed methodology was applied to the samples collected from five open sites, three closed sites, and four types of air conditioner (AC) filters in passenger vehicles. Compared to the samples obtained in open sites, the road dust in the closed sites exhibited 21.5 times higher TWP content (59,747 mg/kg) and 5.1 times higher free-bound CB content (14,632 mg/kg). In addition, unintentional driver respiratory exposure to PM fixed in the vehicle filters was discovered owing to the increase in CB and TWP contents in aged AC filters.

Estimation of the concentration of nano-carbon black in tire-wear particles using emission factors of PM10, PM2.5, and black carbon

Particulate matter (PM) from automobile exhaust has drastic effects on human health. The enforcement of environmental laws has controlled vehicle emissions and reduced the total PM. However, another significant source of PM is debris from tire wear, break wear, and road wear. In particular, tire-wear particles (TWPs) are further fragmented into nanoparticles, similar to the PM_x or black carbon (BC) sources. As approximately 30 wt% of carbon black (CB) is used as filler in tires, TWPs can fragment into free-bound nano-CB. This study evaluates the emission factors of BC from the ternary plots of PM_x and BC to estimate the concentration of nano-CB in TWPs. Based on the emission factors of BC for TWP, approximately 500 monitoring data points were acquired at four different sites. Semi-closed sites in a field measurement test have 2.9-4.0 times larger BC concentration than open sites. The mass concentration of nano-CB evaluated with the BC data and emission factors at the open sites is 22.47-23.96 ng/m³, whereas that at the semi-closed sites is 66.32-90.33 ng/m³. Transmission electron microscopy analysis with scanning mobility particle sizer and selected-area electron diffraction reveals grape-like aggregated nanoparticles, which is considered as CB. To compare the effect of the washing out of airborne particulates by rain, further analysis is conducted on the interior and exterior of the tunnel on a rainy day. While the concentration of PMs was effectively reduced by rainfall, the amount of BC and CB in the interior of a tunnel was not changed. Namely, even under rainfall, nano-CB still exists in the tunnels and thus free-bound CB and nanoparticles released from TWP will be effected on the human health.