Characterization and risk of PPDs and PPD-Qs in an abandoned rubber antioxidant production site

p-Phenylenediamine (PPD) rubber antioxidants and their quinone transformation products (PPD-Qs) are global emerging contaminants that pose severe threats to ecological environments. Abandoned antioxidant sites are key pollution sources, yet limited knowledge regarding their subsurface distribution, transformation drivers, and risks hinders precise site governance. Herein, N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine (6PPD), 6PPD-Q (6PPD's quinone), N-isopropyl-N'-phenyl-p-phenylenediamine (IPPD), IPPD-Q (IPPD's quinone), and p-aminodiphenylamine (PADPA) at an abandoned PPD production site were investigated via field sampling, chemical analysis, correlation analysis and multi-dimensional risk assessment. Results show that the contaminant distribution was dominated by production layout and stratigraphy with weak migration. Quinone/parent compound ratios in groundwater were far higher than in shallow soil, driven by superoxide anions (O₂•^-) in vadose soil and regulated by pH and dissolved oxygen in groundwater. For health risks, soil 6PPD-Q and IPPD-Q showed maximum hazard index (HI) of 19.88 and 28.97, whereas the corresponding maximum acceptable concentration in groundwater was merely 1.61 ng L^-1. Ecologically, 6PPD and 6PPD-Q contaminated soil exhibited maximum risk quotients (RQ) of 540.77 and 19.90, and posed significant aquatic transfer risks. This work fills critical knowledge gaps in subsurface PPD/PPD-Q pollution, and emphasizes the urgency of incorporating PPDs and PPD-Qs into site regulatory frameworks.