Identification, Precursors, and Formation Mechanisms of Chloroguaiacols in Drinking Water

As a major component of natural organic matter (NOM), lignin represents a significant source of potential disinfection byproducts (DBPs) in drinking water. However, lignin-derived DBPs have remained largely uncharacterized until now. Our study identified a class of aromatic DBPs - chloroguaiacols - in chlor(am)inated drinking water using non-target analytical approaches, with concentrations of 17.4-153 ng/L in 11 tap water samples. Controlled chlorination experiments demonstrated that lignin-derived phenols serve as important precursors, with vanillin showing particularly high reactivity (yield: 7705 ng/mg-C), followed by guaiacol (378 ng/mg-C) and vanillic acid (168 ng/mg-C). These yields were 27- to 272-fold higher than that observed for Suwannee River NOM, clearly establishing the significance of these specific phenolic precursors for chloroguaiacol formations. Through combined experimental and computational approaches, we elucidated potential formation pathways that provide a foundation for future mechanistic studies. While chloroguaiacols exhibit substantial cytotoxicity (28× greater than trihalomethanes and comparable to other aromatic DBPs), their low concentrations in drinking water suggest minimal contribution to overall toxicity compared to priority aliphatic DBPs. By elucidating both the precursor structures and their transformation pathways, this study provides important mechanistic insights into the transformation of lignin-derived phenols during chlorination.