Synthetic phenolic antioxidants and their transformation products in Chinese municipal landfill leachates: Occurrence, fate, and ecological risk.

Synthetic phenolic antioxidants (SPAs) are widely used additives that readily undergo environmental transformation. However, current research has paid limited attention to the environmental occurrence, fate, and potential risks of high-molecular-weight (HMW, molecular weights ≥ 450 Da) SPAs. In particular, few studies have simultaneously investigated the relationships among low-molecular-weight (LMW, molecular weights < 450 Da) and HMW SPAs and their typical transformation products (TPs) in landfill leachate. In this study, we measured 12 LMW and 9 HMW SPAs, as well as 5 TPs in raw leachates from 47 municipal landfills across 36 Chinese cities. The total concentrations of SPAs ranged from 12.7 to 1710 μg/L (median: 205 μg/L). HMW SPAs generally showed low detection frequencies and concentrations, whereas the LMW SPAs profile was dominated by 2,4-di-tert-butylphenol (DBP, 48.3%), 2,6-di-tert-butyl-4-ethylphenol (BHEB, 40.1%), and 2,6-di-tert-butyl-4-methylphenol (BHT, 7.41%). Meanwhile, TPs were widely detected, primarily consisting of 3,5-di-tert-butyl-4-hydroxybenzoic acid (BHT-COOH, median: 68.1 μg/L) and 3,5-di-tert-butyl-4-hydroxybenzaldehyde (BHT-CHO, median: 15.4 μg/L). When categorized into three time periods based on landfill age, TPs generally exhibited significantly higher concentrations during the second stage (5-10 years) compared to landfills aged < 5 years and > 10 years. BHT concentrations decreased across the age groups, whereas DBP and BHEB remained consistently high in all groups. Furthermore, the physicochemical properties of the leachates were correlated with the predominant LMW SPAs and their TPs, suggesting that the occurrence of these compounds in landfill leachate may be influenced by both direct leaching and in situ transformation. Based on conservative hazard screening, DBP and BHEB were identified as the primary contributors to medium-to-high ecological risks in untreated leachate. These findings highlight the need for targeted removal of these compounds during leachate treatment.