Exotic Electrophiles in Chlorinated and Chloraminated Water: When Conventional Kinetic Models and Reaction Pathways Fall Short

Abstract

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.