[Aging and Small-sized Particles Release Characteristics of Tire Microplastics in Various Environmental Media].

In recent years, research on microplastics has mostly focused on thermoplastic materials, and there is a lack of research on the pollution status and environmental behavior of tire microplastics, a type of rubber elastomers. In order to investigate the aging and small-sized particles release characteristics of tire microplastics in various environmental media, the aging process of two different tire microplastics, one for cars and the other for electric bicycles, was simulated in dry and aquatic environments under laboratory conditions. The results showed that the tire microplastics would be aged after 30 d of UV illumination, which was manifested by the roughness of the surface and the appearance of cracks and flaking. The Fourier infrared spectra showed that the carbonyl index of the surface also increased. In addition, tire microplastics released a large number of small sub-micron particles under the influence of UV illumination and hydrodynamic action, and the number of particles released from car tire microplastics in aquatic environments reached 694.8 million particles per milliliter of solution at 30 d of the UV light condition, among which 694.6 million particles with a particle size of less than 1 µm were released, which was approximately 100 times of that in the dark condition. The study showed that tire microplastics in aquatic environments were more susceptible to aging and released more small particles under light conditions and that car tire microplastics released more small particles than electric bicycle tire microplastics, posing ecological and environmental risks.

Particle sizes crucially affected the release of additives from tire wear particles during UV irradiation and mechanical abrasion.

In the environment, tire wear particles (TWPs) could release various additives to induce potential risk, while the effects of particle size on the additive release behavior and ecological risk from TWPs remain unknown. This study investigated the effects and mechanisms of particle sizes (>2 mm, 0.71-1 mm, and <0.1 mm) on the release behavior of TWPs additives under mechanical abrasion and UV irradiation in water. Compared to mechanical abrasion, UV irradiation significantly increased the level of additives released from TWPs. Especially, the additive releasing characteristics were critically affected by the particle sizes of TWPs, manifested as the higher release in the smaller-size ones. After 60 d of UV irradiation, the concentration of antioxidant N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine (6PPD) reached 10.79 mg/L in the leachate of small-sized TWPs, 2.78 and 5.36 times higher than that of medium-sized and large-sized TWPs. The leachate of the small-sized TWPs also showed higher cytotoxicity. •OH and O2•- were identified as the main reactive oxygen species (ROS), which exhibited higher concentrations and dramatic attack on small-sized TWPs to cause pronounced fragmentation and oxidation, finally inducing the higher release of additives. This paper sheds light on the crucial effects and mechanism of particle sizes in the release behavior of TWPs additives, provides useful information to assess the ecological risk of TWPs.

Irrigation-facilitated low-density polyethylene microplastic vertical transport along soil profile: An empirical model developed by column experiment.

The emerging issue of microplastic pollution of agricultural soils derives from the intensive utilization of plastic mulching film. Although surface runoff may transport microplastic off-site, infiltration may also facilitate microplastic transport from surface soil to deeper depths. Microplastic comprises a relatively new category of soil contaminants, whose transport in the soil has not yet been widely studied. In this study, we investigated microplastic transport from contaminated surface soil (50 g kg-1) driven by irrigation, from permanent wilting point to saturation, and developed an empirical model to characterize the resulting accumulation of microplastic along soil profile. A soil column experiment was conducted under various treatments: the control, 1, 2 and 4 runs of irrigation. Soil samples were collected from inside and outside of soil cracks (if present) in each soil layer (0-2 cm (source layer), 2-5 cm, 5-10 cm, 10-20 cm, 20-30 cm, 30-40 cm, 40-50 cm). The results showed that with increasing irrigation runs, microplastic in the source soil layer decreased, while microplastic contents in deeper soil depths increased significantly (p < 0.05), varying from 7.03 g kg-1 in 2-5 cm to 0.29 g kg-1 in 40-50 cm soil. The microplastic content detected in soil cracks was 1.3-17.8 times higher than that detected in the soil matrix at similar depths, indicating that the transported microplastic is prone to be enriched in soil cracks. In addition, the total amount of transported microplastic increased 1.5 times after four irrigation runs, and the variations were significantly observed especially at deeper soil depths. Based on correlation analyses, data-fitted empirical models that relate cumulative microplastic to the depth of soil layer and irrigation runs indicate that irrigation-facilitated microplastic transport could be well-characterized (R2 >0.92). Further research is needed to develop an physical-based model in order to assess microplastic migration risks driven by irrigation and other agricultural management practices.