Evaluation of hydroxyl radical and reactive chlorine species generation from the superoxide/hypochlorous acid reaction as the basis for a novel advanced oxidation process

The reaction of hypochlorous acid (HOCl) with superoxide radical (O-2(center dot-)) - a source of hydroxyl radical (HO center dot) and various reactive chlorine species (RCS) - was investigated as the basis for a novel nonphotochemical advanced oxidation process (AOP). Moderately stable (t(1/2) similar to minutes) aqueous O(2)(center dot- )stocks were prepared by several approaches at pH>12 and either (a) added directly to aqueous free available chlorine (FAC; i.e., HOCl/OCl-) at circumneutral pH, or (b) premixed with alkaline FAC and then acidified to pH 7, to degrade various organic probe compounds via in situ generated HO and RCS. Radical production was optimal at [HO2 center dot/O-2 center dot(-)](0)/[FAC](0 )similar to 2 with similar to 0.8 mol HO formed/mol FAC consumed, and HO and RCS exposures reaching similar to 5x10(-19) and similar to 10(-9) Mx s, respectively. Similar trends were observed in natural waters and organic matter-amended phosphate buffer containing up to 5 mgC/L of dissolved organic carbon. Direct formation of oxyhalides, trihalomethanes (THMs), and haloacetic acids (HAAs), was minimal, though THM and HAA formation was moderately enhanced during post-chlorination of O-2(center dot-) /FAC-treated solutions. This process could provide a beneficial addition to the range of available AOPs due to its high radical exposures, simplicity, rapid time-scales, potential for on-site O-2(center dot-) generation, and widespread accessibility of FAC and other reagents. (C) 2021 Elsevier Ltd. All rights reserved.

Automated summary

The reaction of hypochlorous acid with superoxide radical can be a novel nonphotochemical advanced oxidation process, generating hydroxyl radical and various reactive chlorine species, with high radical exposures, simplicity, rapid time-scales, and potential for on-site superoxide generation.