Assessing the biodegradability of tire tread particles and influencing factors.

Abrasion of tire tread, caused by friction between vehicle tires and road surfaces, causes release of tire wear particles (TWPs) into various environmental compartments. TWPs contribute to chemical-, microplastic-, and particulate matter pollution. Their fate remains largely unknown, especially regarding the extent and form in which they persist in the environment. This study investigated 1) the biodegradability of tread particles (TPs) in the form of ground tire tread, 2) how accelerated UV-weathering affect their biodegradability, and 3) which TP constituents are likely contributors to TP biodegradability based on their individual biodegradability. A series of closed bottle tests, with aerobic aqueous medium inoculated with activated sludge, were carried out for pristine TPs, UV-weathered TPs, and selected TP constituents; natural rubber (NR), isoprene rubber (IR), butadiene rubber (BR), and treated distillate aromatic extracts (TDAE). Biodegradation was monitored by manometric respirometry quantifying biological oxygen consumption over 28 days. Pristine TP biodegradability was found to be 4.5%. UV-weathered TPs showed higher biodegradability of 6.7% and 8.0% with similar and increased inoculum concentration, respectively. The observed TP biodegradation was mainly attributed to biodegradation of NR and TDAE, with individual biodegradability of 35.4% and 8.0%, respectively. IR and BR showed negligible biodegradability. These findings indicate biodegradability of individual constituents is decreased by a factor of 2 to 5 when compounded into TPs. Through scanning electron microscope (SEM) analysis, biodegradation was found to cause surface erosion. Processes of TP biodegradation are expected to change throughout their lifetime as new constituents are incorporated from the road and others degrade and/or leach out. Tire emissions likely persist as particles with an increased fraction of synthetic rubbers and carbon black.