Wastewater as a dual indicator of human and environmental exposure to synthetic antioxidants: Occurrence and fate in biological and advanced wastewater treatment

Synthetic antioxidants (SAOs) are widely used additives in industrial and consumer products, yet their human exposure and fate throughout wastewater treatment remain poorly understood. This study investigates the occurrence of SAOs and their human metabolites in wastewater influent as well as their abatement in three wastewater treatment plants (WWTPs) employing both conventional and advanced treatment technologies. In vitro human liver S9 assays were performed to generate a SAO metabolite MS2 library containing over 2500 potential metabolites, which was matched against wastewater influent data. This approach revealed fenozan, which was detected in the influent of all three WWTPs, as a potential biomarker for five of the seven analyzed 3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate antioxidants. Detection of the glucuronide conjugate in urinary samples together with the tested high hydrolytic stability of the parent SAOs further qualify fenozan as a potential biomarker. Biological wastewater treatment led to high abatement for most SAOs with a mean value of approximately 70% across all 18 detected SAOs in the three investigated WWTPs. Subsequent ozonation led to further abatement (≥68%), particularly for the amine antioxidants. This can be attributed to their aniline-type structures with low pK_a values, which render their second-order rate constants with ozone pH-independent at pH values typical for wastewater. For phenolic antioxidants in contrast, the second-order rate constants with ozone are pH-dependent, leading to varying abatements. These results emphasize the important role of wastewater treatment in mitigating SAO pollution. Overall, this study demonstrates the value of wastewater monitoring as a unified approach to assess both human and environmental exposure to SAOs.