Beyond Hindered Phenolic Antioxidants: Aromatic Amine Antioxidants Dominate Hazard Profiles in Municipal Sludge across China

Synthetic antioxidants (AOs) can accumulate in municipal sludge during wastewater treatment, yet current occurrence-based assessments remain largely focused on hindered phenolic antioxidants (HPAs), potentially overlooking other AO classes and transformation products (TPs) with distinct hazard characteristics. Here, we conducted a nationwide assessment of four major AO classes comprising 60 AOs and TPs in municipal sludge across China by integrating occurrence and hazard information. Forty-seven compounds were detected nationwide, including 17 compounds reported in sludge for the first time (e.g., AO 3052, PANA/PBNA, AO 168═O, and DLTP). HPAs overwhelmingly dominated AO occurrence, accounting for 94.0% of total AO concentrations nationwide, with BHT and DBP as the major contributors. In contrast, hazard-informed prioritization revealed that aromatic amine antioxidants (AAs) and their TPs, particularly 6PPD, DPA, and 6PPD-quinone, disproportionately dominated hazard profiles despite their substantially lower abundance. The high rankings of TPs further suggest an important role of environmental transformation in shaping AO-related hazards in sludge. These findings reveal a pronounced mismatch between abundance and hazard profiles across AO classes, suggesting that concentration-driven monitoring strategies may overlook lower-abundance but highly hazardous contaminants in municipal sludge.