Effect of <i>In Situ-</i> Formed Free Available Chlorine during Chlorine Dioxide-Based Disinfection of <i>Escherichia Coli</i> in the Presence of Natural Organic Matter

Chlorine dioxide (ClO2) is increasingly used as an alternative to chlorine in oxidative water treatment to mitigate the formation of halogenated disinfection byproducts. However, ClO2 can generate in situ free available chlorine (FAC) as a secondary oxidant, whose role in microbial inactivation remains poorly understood. This study investigates the impact of in situ-formed FAC during ClO2-based disinfection of Escherichia coli in the presence of natural organic matter (NOM). Bacterial inactivation was quantified using a novel approach based on lag phase extension in the growth curves. Exposure to 100 μM ClO2 extended the E. coli lag phase by approximately 40% relative to untreated controls. In contrast, the presence of 100 μM methionine, a selective FAC scavenger, markedly reduced disinfection efficacy, indicating that in situ FAC formation is a key driver of ClO2-based microbial inactivation. Additionally, disinfection efficiency decreased with increasing pH, consistent with FAC speciation (HOCl/OCl–; pKa = 7.54), as OCl– is less effective than HOCl. Moreover, pH-dependent speciation of NOM (phenol/phenolate; pKa ≈ 10) influences FAC reactivity, with phenolate exhibiting a higher scavenging capacity. These findings highlight the critical role of both FAC formation and NOM composition in determining the efficacy of ClO2-based disinfection.