Enhanced photodegradation of 6PPD and formation of 6PPD-quinone in ice

The widespread occurrence of 6PPD-quinone (6PPD-Q), a highly toxic transformation product of the tire-derived antioxidant 6PPD, poses serious threat to salmon fisheries. Here, we provide the first quantitative evaluation of 6PPD phototransformation in ice under UV irradiation, a condition directly relevant to salmon habitats. Freezing markedly accelerated both 6PPD photodegradation and its conversion to 6PPD-Q, with a molar yield of 4% in ice versus 1.7% in liquid water. This enhancement arises from the freeze-concentration effect, which enriches 6PPD and oxygen molecules within the quasi-liquid boundary region, selectively amplifying singlet oxygen (¹O₂) production. In ice, ¹O₂ accounted for 45.9% of 6PPD degradation and 70.2% of 6PPD-Q formation, compared to 4.1% and 35.0%, respectively, in liquid water, where ^•OH dominated 6PPD-Q formation. These findings highlight ice as an active photochemical medium and identify ice-covered urban waters as potentially overlooked hotspots for 6PPD-Q formation, with direct implications for salmon-bearing ecosystems in cold regions.