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

Chlorine dioxide (ClO 2 ) is increasingly used as an alternative to chlorine in oxidative water treatment to mitigate the formation of halogenated disinfection byproducts. However, ClO 2 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 ClO 2 -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 ClO 2 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 ClO 2 -based microbial inactivation. Additionally, disinfection efficiency decreased with increasing pH, consistent with FAC speciation (HOCl/OCl –; p K a = 7.54), as OCl – is less effective than HOCl. Moreover, pH-dependent speciation of NOM (phenol/phenolate; p K a ≈ 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 ClO 2 -based disinfection.