Influence of Overlooked Chlorinating Agents on the Kinetics of Organic Compound Halogenation

Free available chlorine (FAC) is the most commonly used chemical disinfectant for drinking water treatment. While FAC protects humans from waterborne pathogens, it can react with a wide variety of organic compounds to produce disinfection byproducts (DBPs) that are known or anticipated to be toxic. Determining the kinetics of organic compound chlorination could improve our understanding of the health risks associated with DBPs. Most researchers assume HOCl to be the predominant active chlorinating agent in FAC, but a few studies have shown that Cl2 and Cl2O, which are minor constituents of FAC under typical drinking water treatment conditions, can play important roles in the chlorination of moderately-reactive organic compounds. Are those compounds anomalies? Or is the influence of Cl2 and Cl2O on chlorination kinetics more prevalent than is presently recognized? We aim to address these questions by examining the kinetics of phenol and alkene chlorination. To investigate the importance of Cl2, Cl2O, and HOCl for six (chloro)phenols and three ionones, we conducted kinetic experiments in which solution pH, chloride concentration, and chlorine dose were systematically varied. For all the (chloro)phenols and ionones examined, addition of chloride at millimolar levels enhanced chlorination rates at pH < 7. As [Cl2] is proportional to [Cl–] and [H+], our results are consistent with Cl2 serving as a chlorinating agent. For some of the less reactive (chloro)phenols and ionones, Cl2O is also important at high chlorine doses. The second-order rate constants we computed for different chlorinating agents indicate that Cl2 and Cl2O are intrinsically more reactive than is HOCl. Furthermore, in accordance with the reactivity-selectivity principle, selectivity for Cl2 and Cl2O increases as the reactivity of an organic compound decreases. Therefore, despite the low concentrations of Cl2 and Cl2O in drinking water treatment settings, the potential roles of these chlorine species in organic compound chlorination should not be underestimated. Another focus of our work is the development of a novel method for quenching and quantifying free chlorine and free bromine. Many traditional quenchers for free halogens (e.g., sodium sulfite, sodium thiosulfate, and ascorbic acid) are reducing agents that can interact with redox-labile analytes. We propose to use 1,3,5-trimethoxybenzene (TMB) for quenching free halogens instead. TMB rapidly reacts with excess free chlorine and free bromine to form monohalogenated products. We found that TMB did not interact with redox-labile DBPs that otherwise degraded in the presence of traditional quenchers. Moreover, TMB was as effective as sodium thiosulfate when used as a quencher in kinetic experiments involving the chlorination of 2,4-dichlorophenol and bromination of anisole. By quantifying TMB and its halogenated products in the quenched samples, we were also able to determine the concentrations of free halogens that were present at the time of quenching. Findings from our work show that TMB can serve as an effective quencher in aqueous halogenation experiments that involve redox-labile analytes and/or that require selective quantification of residual free halogens