Compound Class-Specific\nTemporal Trends (2021–2023)\nof Tire Wear Compounds in Suspended Solids from Toronto Wastewater\nTreatment Plants

Tire wear compounds (TWCs) have received increasing attention\ndue\nto their ubiquitous environmental occurrence and toxicity. In this\nstudy, the temporal trends of 23 TWCs in two Toronto wastewater treatment\nplants were systematically investigated through a two-year wastewater\nsurveillance. Through an optimized analytical method, 20 TWCs were\ndetected across 161 weekly influent suspended solid samples at a total\nconcentration of 273–52,500 ng/g dw. Phenyl-<i>p</i>-phenylenediamines (PPDs), <i>N</i>-(1,3-dimethylbutyl)-<i>N</i>′-phenyl-<i>p</i>-phenylenediamine-quinone\n(6PPD-Q), and 1,3-diphenylguanidine (DPG) showed a strong co-occurrence,\nand their concentration spikes were coincident with both flow rates\nof influents and precipitation, which was not observed for other TWCs.\nTherefore, stormwater runoff is a major source of PPDs, PPD-Qs, and\nDPG, but not other TWCs. The temporal trends of <i>N</i>-1,3-dimethylbutyl-<i>N′</i>-phenyl-<i>p</i>-phenylenediamine (6PPD) transformation products were further determined.\nAmong six detected transformation products, four compounds including <i>N</i>-(1,3-dimethylbutyl)-<i>N′</i>-phenyl-<i>p</i>-quinonediimine (6QDI) showed a strong co-occurrence with\n6PPD but not with 4-hydroxydiphenylamine (4-HDPA) and <i>N</i>-phenyl-<i>p</i>-phenylenediamine (4-ADPA). Rapid hydrolysis\nof 4-ADPA to 4-HDPA was observed (<i>t</i><sub>1/2</sub> = 35.7 h), suggesting that 4-ADPA, rather than 6PPD, is the major\nprecursor leading to the formation of 4-HDPA in wastewater. The compound\nclass-specific temporal trends of TWCs in wastewater suggest the existence\nof distinct emission sources of TWCs in addition to traffic-related\nstormwater runoff.