Occurrence of Substituted p-Phenylenediamine Antioxidants and Their Quinone Derivatives in the Pearl River Estuary Water System, China

-phenylenediamines (PPDs) and their quinone derivatives (PPD-Qs) are emerging contaminants associated with tire-related antioxidants and antiozonants and have raised increasing concern because of their potential environmental effects. However, information on their occurrence in estuarine systems, particularly their combined distribution in water and sediment, remains limited in the Pearl River Estuary. In this study, 30 water samples and five sediment samples collected from the Pearl River Estuary were analyzed for selected PPDs and PPD-Qs. Four target compounds were detected in water, whereas nine were found in sediments, indicating broader occurrence in the sediment phase. The total concentration of PPDs ranged from below the detection limit to 17.6 ng/L in water and from 0.140 to 1.37 ng/g in sediments. In water, 6PPD and 6PPD-Q showed relatively high detection frequencies, while elevated IPPD concentrations were observed at several sites near urbanized coastal areas. In sediments, the highest ΣPPDs level was recorded in Shenzhen Bay. The observed spatial patterns suggest that mixed anthropogenic inputs may influence the occurrence of these compounds in the estuary, although direct source attribution requires further investigation. Overall, this study provides preliminary baseline information on the occurrence and phase-specific distribution of PPDs and PPD-Qs in the Pearl River Estuary and supports future investigations into their environmental fate and ecological relevance.