Recent studies used the sum of the measured concentrations of individual disinfection byproducts (DBPs) weighted by their Chinese hamster ovary (CHO) cell cytotoxicity LC50 values to estimate the DBP-associated cytotoxicity of disinfected waters. This approach assumed that cytotoxicity was additive rather than synergistic or antagonistic. In this study, we evaluated whether this assumption was valid for mixtures containing DBPs at the concentration ratios measured in authentic disinfected waters. We examined the CHO cell cytotoxicity of defined DBP mixtures based on the concentrations of 43 regulated and unregulated DBPs measured in eight drinking and potable reuse waters. The hypothesis for additivity was supported using three experimental approaches. First, we demonstrated that the calculated additive toxicity (CAT) and bioassay-based calculated additive toxicity (BCAT) of the DBP mixtures agree within 12% on a median basis. We also found an additive toxicity response (CAT ≈ BCAT) between the regulated and unregulated DBP classes. Finally, the empirical biological cytotoxicity of the DBP subset mixtures, independent of the calculated toxicity, was additive. These results support the validity of using the sum of cytotoxic potency-weighted DBP concentrations as an estimate of the CHO cell cytotoxicity associated with known DBPs in real disinfected waters.

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The aqueous chlorination of (chloro)phenols is one of the best-studied reactions in the environmental literature. Previous researchers have attributed these reactions to two chlorine species: HOCl (at circum-neutral and high pH) and HOCl (at low pH). In this study, we seek to examine the roles that two largely overlooked chlorine species, Cl and ClO, may play in the chlorination of (chloro)phenols. Solution pH, chloride concentration, and chlorine dose were systematically varied in order to assess the importance of different chlorine species as chlorinating agents. Our findings indicate that chlorination rates at pH < 6 increase substantially when chloride is present, attributed to the formation of Cl. At pH 6.0 and a chlorine dose representative of drinking water treatment, ClO is predicted to have at best a minor impact on chlorination reactions, whereas Cl may contribute more than 80% to the overall chlorination rate depending on the (chloro)phenol identity and chloride concentration. While it is not possible to preclude HOCl as a chlorinating agent, we were able to model our low-pH data by considering Cl only. Even traces of chloride can generate sufficient Cl to influence chlorination kinetics, highlighting the role of chloride as a catalyst in chlorination reactions.

Halogenation and oxidation of organic matter in chlorinated and chloraminated water are typically attributed to the most abundant electrophiles present. This interpretation sometimes fails to explain laboratory observations, including halogenation kinetics and product distributions. Exotic electrophiles, species commonly overlooked in the environmental literature, can help to resolve these discrepancies. Herein, we review evidence demonstrating the significance of lesser-studied electrophilic chlorinating (Cl2 and Cl2O), brominating (BrCl, BrOCl, and Br2O), and iodinating (H2OI+ and ICl) agents in chlor­(am)­inated water. The evidence includes reaction rate dependencies on [Cl–], [H+], and [HOCl] that cannot be attributed to the reactivity of hypohalous acids or hypohalites alone. For example, enhancement of chlorination and bromination rates by Cl– implicates Cl2 and BrCl, respectively, as active halogenating agents. Herein, we discuss a new method for quantifying the sensitivity of halogenation to rate enhancement by Cl–. We also discuss complexities that Cl– can impart on iodination kinetics. In addition, we highlight recent insights into radical-mediated reaction pathways and unexpected organic electrophiles in chlorinated water. Finally, we discuss practical implications, identify research needs, and offer recommendations to improve the design of future halogenation experiments. Overall, this review aims to spur new research into underappreciated electrophiles in chlor­(am)­inated water.

Although Cl 2 and Cl 2 O have been recognized as highly reactive constituents of free available chlorine (FAC), robust rate constants for Cl 2 and Cl 2 O remain scarce in the environmental literature. In this work, we explored the chlorination kinetics of three structurally related alkenes (α-ionone, β-ionone, and dehydro-β-ionone), a class of compounds whose reactivities with Cl 2 and Cl 2 O have not been previously investigated. Second-order rate constants for Cl 2 , Cl 2 O, and HOCl were computed from experimental rate constants obtained at various pH values, [Cl − ], and [FAC]. Our results show that while HOCl is the predominant chlorinating agent for the most reactive alkene, Cl 2 and Cl 2 O can dominate the chlorination kinetics of the less reactive alkenes at high [Cl − ] and high [FAC], respectively. The tradeoff between overall reactivity with FAC and selectivity for Cl 2 and Cl 2 O previously observed for aromatic compounds also applies to the alkenes examined. In laboratory experiments in which high [FAC] may be used, omission of Cl 2 O in data modeling could yield second-order rate constants of dubious validity. In chlorinating real waters with elevated [Cl − ], formation of Cl 2 may enhance the formation kinetics of chlorinated disinfection byproducts (DBPs) and exacerbate the burden of DBP control for water utilities.