Environmental aging and biodegradation of tire wear microplastics in the aquatic environment

Tire wear microplastics (TWM) are formed by friction between tires and road surfaces during driving and they belong among the most abundant microplastics in the environment. However, the information about their fate in the environment is still unknown. The aim of this study was to investigate the aging of TWM in freshwater under controlled laboratory conditions over 12 weeks. The development of biofilm, changes in physical properties and chemical composition, leaching and biodegradation of TWM were followed. The results showed colonization of the TWM surface by microorganisms (up to 45 mg/g), which, however, began to detach from the particles after eight weeks, reducing the amount of biofilm. TWM initially leached zinc and organic compounds (expressed as dissolved organic carbon - DOC), but their concentrations were low and decreased with time. The increase in DOC was observed after 10 weeks, possibly due to the decomposition of the biofilm and the release of organic matter. Aging resulted in changes of density of TWM, but the morphology and chemical composition of the TWM surface did not change. This confirms the results of the biodegradability tests, which showed no biodegradation within 12 weeks. Overall, the results indicated that TWM are not readily biodegradable and therefore may accumulate and persist in the aquatic environment.