Global multi-media distribution and transport of 6PPD and 6PPD-Q: a source-to-sink continuum

The tire antioxidant N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine (6PPD) and its toxic oxidation product 6PPD-quinone (6PPD-Q) are widely detected, yet their global cross-media distribution, transport patterns, and macroscopic anthropogenic indicators remain poorly understood. Here, we compiled a global dataset across air, indoor dust, outdoor dust, soil, snow, road runoff, wastewater treatment plants (WWTP), receiving waters and sediment to support an integrated cross-media analysis. Generally, the detection frequencies of 6PPD-Q (50-100%) were higher than those of 6PPD (31-95%). The fractional contribution of 6PPD-Q (F_Q) increased from air (0.40) to soil (0.57), outdoor dust (0.61), and further to snow (0.73), road runoff (0.87), WWTP influent (0.84), WWTP effluent (0.89), and receiving waters (0.92), suggesting a source-to-sink transport continuum from fresh road-proximal emissions to environmentally mixed aquatic systems. Along this continuum, direct source signatures weakened progressively. Aqueous concentrations declined from snow (6PPD: 84.01; 6PPD-Q: 219.31 ng L^-1) and road runoff (3.05; 87.84 ng L^-1) to WWTP influent, WWTP effluent, and receiving waters (0.41; 1.70 ng L^-1). Parent-product coupling weakened from air (r = 0.92) to outdoor dust (r = 0.65) and receiving waters (r = 0.48). Gross domestic product, population density, and road-transport CO₂ emissions were significantly associated with 6PPD-Q concentrations in most media, although these associations weakened toward environmentally mixed sinks. These findings provide a global framework for understanding the environmental fate of tire-derived antioxidants and support pathway-oriented monitoring and management.