Abiotic transformation of radiolabelled 6-PPD and 6-PPDQ in water and in presence of nitrogen/air/ozone and light

N-(1,3-Dimethylbutyl)-N′-phenyl-p-phenylendiamine (6-PPD) and one of its oxidation products, 6-PPD quinone (6-PPDQ) are environmental contaminants released from tire and road wear particles (TRWP). The abiotic transformation of both compounds by hydrolysis and in contact with atmosphere with and without oxygen species and simulated sunlight was studied, following relevant portions of OECD-guidelines 111 and 316, and using radiolabelled test substances. This allowed for mass balances including transformation products, those identified by liquid chromatography-mass spectrometry and those remaining unidentified. 6-PPD was effectively degraded by hydrolysis at neutral and basic pH (DT50 5 – 12 h at 25°C), with 4-HDPA being the major reaction intermediate. 6-PPDQ remained below 0.2% of the applied radioactivity (AR) of 6-PPD, except at very acidic conditions and low temperature (10°C) with an intermediate yield of 7.5% AR. In contact with atmosphere 6-PPD was transformed by direct photolysis (DT50 14 h). Direct reaction with ozone was slow (DT50 3.5 d) but degradation by OH-radicals (ozone/light) was very fast (DT50 3.7 h). A diverse set of products was formed; 6-PPDQ was determined in some cases, with a maximum intermediate yield of 1.3% AR and decreasing again with ongoing oxidation of 6-PPD (to levels ≤ 0.6% AR with ongoing oxidation). 6-PPDQ is not susceptible to pure hydrolysis (DT50 > 1000 d at pH 7, mean 490 d in 13 natural waters) and very limited to direct photolysis (DT50 56 d). On the contrary, reactivity towards reactive oxygen species (air/ozone and light) is in the same range as for 6-PPD (DT50 2 – 7 h). Oxidative transformation of 6-PPDQ proceeds via opening of the quinone-ring, with 20–50% AR recorded as CO 2 within 2 d. For 6-PPDQ oxidation in the presence of light only 50 – 80% of AR could be recovered. This study with 14 C-labelled test substances provides solid information on the abiotic transformation of both, 6-PPD and 6-PPDQ, and supports environmental risk assessment. • 6-PPD hydrolyses rapidly at neutral pH (DT50 5 – 8 h at 25°C) but 6-PPDQ only slow. • At neutral pH percentage of 6-PPDQ formed from 6-PPD in water remained below 0.2%. • Both compounds react equally fast with air in presence of sunlight (DT50 7 – 12 h). • Oxidation of 6-PPD with oxygen species and light led to 6-PPDQ levels of < 0.2 – 1.3%.