The reaction of superoxide radical (O2●–) with hypochlorous acid (HOCl) has long been recognized as a non-photochemical source of hydroxyl radical (HO●) in aqueous systems, in particular within phagocytes during immune system responses to infection by foreign cells. Increasingly, this reaction has also been recognized as a driver of radical chemistry within advanced and even conventional water treatment processes employing free available chlorine, or FAC (primarily comprising HOCl and hypochlorite, OCl–); for instance in reactions between FAC and hydroxyphenols in natural organic matter, as recently reported by Rodríguez and von Gunten. The present work has been undertaken with the aim of harnessing this reaction as a controlled source of HO● for use in advanced oxidative water treatment, with potential applications for trace organic contaminant (TrOC) degradation and microbial inactivation. We have found that moderately-stable (t1/2 ~ minutes), alkaline (pH&gt;12) O2●– stocks generated via several approaches (including from KO2 solids and from ozone or UV treatment of alkaline hydrogen peroxide/hydroperoxide), can be added to FAC-containing receiving waters at circumneutral pH to generate nearly instantaneous “pulses” of HO● (at ~0.8 mol HO● formed/mol FAC consumed). The resulting HO● can in turn react with FAC or Cl– to generate various reactive chlorine species (RCS), including ClO●, Cl●, or Cl2●–, enabling treatment at HO● and RCS exposures approaching ~10–9 M×s – comparable to levels characteristic of advanced oxidation processes such as UV/H2O2, O3/H2O2, and UV/FAC. This approach can in turn be applied to achieve extremely rapid (t1/2 &lt; 1 s) TrOC degradation under conditions representative of drinking or reuse waters (as demonstrated in several natural and synthetic waters using nitrobenzene, benzoic acid, and 1,4-dimethoxybenzene as model compounds), with negligible direct formation of regulated (in)organic disinfection byproduct (DBPs) and only moderate enhancement of organic DBP formation during post-chlorination. In addition to providing an overview of these findings, this talk will emphasize opportunities for process optimization, potential extensions to improving disinfection of microorganisms, implications for practical applications, and inspirations drawn from the work and mentorship of Professor Urs von Gunten - to whom this presentation is dedicated in honor of his receipt of the 2022 ACS Award for Creative Advances in Environmental Chemistry.

Harnessing the superoxide-hypochlorous acid reaction as a source of hydroxyl radical and reactive chlorine species for advanced oxidative treatment

ACS Spring 2022

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