Unveiling p-phenylenediamine rubber antioxidants as precursors of disinfection byproducts: Kinetics, formation, and toxicity.

Disinfection by-products (DBPs) in drinking water represent a significant public health concern. The contribution of non-natural organic matter compounds, such as rubber antioxidants, as DBP precursors has not been unveiled. This study investigated the degradation of four commonly consumed p-phenylenediamine (PPD) rubber antioxidants under chlorination and chloramination, evaluating their roles as DBP precursors. Results showed that PPDs degraded rapidly in both disinfection processes, with chlorine exhibiting a higher reactivity (59.38 - 139.42 M-1s-1) than chloramine (0.69 - 1.36 M-1s-1). The degradation of PPDs lead to the formation of known chlorinated DBPs (Cl-DBPs), constituting 32 % - 44 % of total organic chlorine. These known Cl-DBPs demonstrated significantly lower acute toxicity than unknown DBPs. Taking N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine as an example, nontarget high-resolution mass spectrometry identified eight chlorination-derived and six chloramination-derived new DBPs. Proposed transformation pathways mainly included chlorine substitution, oxidation, amine hydrolysis and hydroxylation. Quantitative structure-activity relationship modeling predicted acute toxicity (LC₅₀) values as low as 0.15 mg L-1 for Fathead minnow and 0.91 mg L-1 for Daphnia magna, suggesting that these newly identified DBPs could substantially contribute to the overall toxicity of disinfected waters. These findings highlight the importance of considering rubber antioxidants as DBP precursors in drinking water systems, emphasizing the need for more effective control strategies to mitigate the risks posed by these emerging contaminants.