Potential release of nano-carbon black from tire-wear particles through the weathering effect
Graphical abstract
Introduction
With increasing concern about air quality in the government, particulate matter (PM) is considered one of the most problematic pollutants for human health. The major source of PM in urban areas is particles from the exhaust emission of vehicles [1]. As exhaust PM emissions, containing a variety of hydrocarbons, could lead to respiratory disease [2], the governments have restricted the concentration of vehicular emission via the Environmental Act. Additionally, in order to reduce air pollution, users of vehicles with internal combustion engines have been encouraged to switch to electric vehicles (EVs) by incentivizing purchase subsidies. While exhaust PM emissions will approach zero in EVs, the total emission level of PM by switching to EV may not reduce as expected [3]. The emission factor of PM (3.7–7.2 mg/km) for EVs is almost the same as that for diesel engine vehicles (2.9–6.1 mg/km) [4]. Notably, there is another crucial source of PM apart from exhaust derived PM; namely, non-exhaust sources, such as tire-wear, break-wear, road surface wear, and dust resuspension, act as major airborne pollutants [5].
As rapid electrification does not significantly decrease PM emissions due to the importance of non-exhaust emissions [6], raising interest in non-exhaust PM emissions to improve air quality is necessary. Major non-exhaust sources including tire-wear and break-wear were related to carbon black (CB) due to their use as reinforcing agents in tire and break-pads. Thus, CB could be considered as the main component of non-exhaust PM.
Although several toxicological studies have been reported on lung-inflammation by non-exhaust PM [7], CB particles released from tire-wear were of no interest to toxicologists as it is considered to be tightly bound in the rubber tire matrices [8]. Similar arguments can be found in government reports; exposure to free-bound CB does not occur during the use of products wherein CB is bound to other matrices [9], and CB is generally bound within the rubber of a tire [10]. These reports insisted that unbound CB is not generally released into the environment from tire-wear. However, tire-wear particles (TWP) can become fragmented by weathering effects, such as continuous abrasion on the road, resuspension of road dust, and temperature changes. Therefore, additional wear could release the CB tightly bound within the tire rubber into the environment in a free-bound form.
Herein, we investigated the potential release of nano-CB and the formation of unbound CB from TWP through weathering. Ball milling is used as green technology to prepare the free-standing nanoparticles and de-agglomeration of nanoparticles [11], [12], [13]; this methodology was also used to fragment TWP. TWP was obtained through the surface abrasion of commercial passenger car tires using steel graters; the grinding work for the obtained particles was conducted for 14 days using ball milling instruments. Nano-sized particles were obtained from the parent TWP with increasing milling time, and the presence of CB was confirmed by conducting various characterizations of secondary particles. Additionally, the particle size was measured in real time around highways, and the samples collected on the highway were analyzed for the existence of free-bound CB in the environment.
Section snippets
Collection of tire-wear particles
TWP can be collected on the highway, but street dust is a mixture of road wear, aerosols, geogenic materials (soils and rocks), etc. Additionally, the non-exhaust PM could be differentiated from the exhaust PM in the field. Therefore, TWP was obtained by grinding the surface of a passenger car tire (Nexen CP21 145R13C, Nexen tire, Korea) using a steel grater with ca. 100 μm holes. TWP collected were sized using a sieve with mesh sizes of 75, 160, and 1000 μm (Chunggye Co., Korea). To compare
Particle size change of TWP with weathering time
Car tier rubber contains an array of components, including fillers such as CB, clay, silica, and ZnO [16]. Among these components, the weight of CB and rubber in the tire reached ca. 30 and 45 wt.%, respectively. Therefore, TWP exhibits a uniform mixture of rubber and CB particles, and CB is tightly bound in the rubber matrix. When we eat chocolate chip cookies, the cookies break down and the chocolate chips fall off. In a similar way, CB particles in the rubber matrix may be exposed to the
Conclusions
CB particles used as reinforcing agents in tire treads are tightly bound in the rubber matrix. TWP is generated by the tire-wear while driving vehicles, and CB is bound in the TWP. However, the possibility of CB detachment from TWP has been overlooked. In this work, we investigated the potential release of free-bound CB from TWP through the weathering effect. The TWP sample was obtained by grinding the surface of the tire using a steel grater, and the fragmentation of TWP through abrasion (one
Declaration of Competing Interest
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Acknowledgements
This study was supported by the National Institute of Environmental Research [NIER-2020-01-027], the National Research Foundation of Korea[NRF-2020R1F1A1048192], and the Research Grant of Kwangwoon University (2021).
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