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Reclaimed water in road cleaning: An unrecognized risk for toxic 6PPD-Q formation from tire wear waste
Summary
Researchers found that washing roads with reclaimed water instead of rainwater accelerates the aging of tire wear particles and significantly increases conversion of the tire additive 6PPD into its more toxic transformation product 6PPD-quinone, suggesting that urban water reuse practices may inadvertently amplify the release of a compound acutely lethal to coho salmon.
Tire wear particles (TWPs) are a major source of microplastic pollution, particularly due to toxic additives like N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine (6PPD) and its transformation product, 6PPD-quinone (6PPD-Q). However, the mechanisms governing 6PPD transformation in TWPs are not yet fully understood. This study investigates photo-aging of TWPs as solid waste under simulated road conditions, focusing on how reclaimed water versus rainwater influences aging and 6PPD/6PPD-Q release. Sunlight and road exposure altered TWPs properties, characterized by surface fragmentation, increased hydrophilicity, and oxygen-containing functional groups. Following three weeks of aging in reclaimed water, the carbonyl index (CI) and hydroxyl index (HI) increased by 323.75% and 72.82%, respectively. The surface O/C ratio rose from 0.149 in pristine TWPs to 0.207 in the reclaimed water group and 0.161 in the rainwater group. This oxidation process was also accompanied by an increased abundance of reactive oxygen species. Under reclaimed water aging conditions, the concentrations of O, O, and •OH increased significantly by 82.95%, 33.72%, and 6.69%, respectively. Leaching experiments revealed a gradual decrease in 6PPD concentration during aging, accompanied by a significant increase in its conversion product 6PPD-Q. Pearson analysis revealed strong positive correlations between 6PPD-Q formation and C=O (r = 0.65, p = 0.003), O (r = 0.44, p = 0.031), and •OH (r = 0.65, p = 0.01). These findings indicate that tire wear solid waste management is highly sensitive to external conditions, with reclaimed water in road cleaning promoting 6PPD-Q formation. This underscores the need to reassess water reuse practices in urban maintenance to mitigate hazardous emissions.