Microbial Transformation of 6-PPDQ in Surface Water, Water–Sediment, and Soil─Results from Simulation Tests with Radiolabeled Substrate

Abstract

6-PPDQ, an oxidation product of the antioxidant 6-PPD added to tire rubber, reaches the environment with tire abrasion on roads. Firm data on the environmental persistence of 6-PPDQ in different compartments is lacking. The microbial degradation of 6-PPDQ was studied in simulation tests under standardized conditions in the laboratory. Biodegradation half-life of 6-PPDQ (DT50) increased from surface water (12–17 d) over water-sediment systems including nonextractable residue (NER I) under aerobic conditions (46–55 d), soils including NER I under aerobic conditions (27–141 d, median 61 d), water–sediment systems under anaerobic conditions (140–202 d) to soil under anaerobic conditions (214–887 d, >10 000 d in one model). On this basis, 6-PPDQ would be considered nonpersistent according to ECHA’s definition under aerobic conditions in water, in water–sediment systems, and in soil (median value and three of the four tested soils). 34% of the applied radioactivity (AR) of [14C]-6-PPDQ was mineralized under aerobic conditions in 60 d in surface water, 27% in the water–sediment system in 100 d and 34% in soil in 120 d. Under anaerobic conditions, mineralization was 4%–5% AR, only, both in water–sediment and in soil. Hydroxylated and carboxylated 6-PPDQ were identified as transformation products, of which 11% AR remained after 60 d. In sediment and soil no single transformation product exceeded 1% AR at the end of the experiment; instead, NERs were formed. Under aerobic conditions, 55% (water–sediment) and 49% (soil) of the AR of degraded 6-PPDQ occurred as NER; it increased to 67% (water–sediment) and 84% (soil) under anaerobic conditions. These first quantitative data on the extent and the rate of biodegradation and mineralization of 6-PPDQ help to understand the occurrence of 6-PPDQ in the environment and support environmental exposure modeling and risk assessment.