Leaching of tire particles and simultaneous biodegradation of leachables

The fate of organic compounds released from tire wear particle (TWP) in the aquatic environment is still poorly understood. This is especially true near sources where biotic and abiotic transformation and leaching from TWP are simultaneous and competing processes. To address this knowledge-gap an experiment was performed, allowing for biodegradation (a) during the leaching from a suspension of cryo-milled tire tread (CMTT) and (b) subsequent to leaching. Besides measuring the Dissolved Organic Carbon (DOC) content, 19 tire-related chemicals were quantified, and non-target screening was performed by LC-HRMS. The non-inoculated control experiment exhibited a DOC of up to 4 mg g^-1, with up to 700 µg g^-1 of 1,3-diphenylguanidine (DPG) as the most prominent compound, followed by three benzothiazoles (2-mercaptobenzothiazole (2-MBT), 2-hydroxybenzothiazole (2-OHBT) and benzothiazole-2-sulfonic acid (BTSA); 50 µg g^-1 each) and 4-hydroxydiphenylamine (4-HDPA) (50 µg g^-1). Biodegradation reduced the DOC by 88 % and the concentration of most organic compounds by more than 85 %. At the end of the experiment hexamethoxymethylmelamine (HMMM) was the most prominent single compounds (18 µg g^-1). Non-target screening showed a more complex picture. Another 25 transformation products (TPs) of N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine (6-PPD) and 44 TPs and derivatives related to DPG were detected in solution, of which 11 and 28 were still present after or formed by biodegradation, respectively. Of these 39 TPs and derivatives, 31 could be detected in road runoff samples. This study provides a more comprehensive picture of the leachables of tire particles that are of environmental relevance. It also outlines that derivatives of tire additives formed during tire production and use may deserve more attention as leachables. The large extent of biodegradation of tire leachables suggests that settling ponds may be a useful treatment option for road runoff.