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

Abstract

The reaction of hypochlorous acid (HOCl) with superoxide radical (O2•–) – a source of hydroxyl radical (HO•) and various reactive chlorine species (RCS) – was investigated as the basis for a novel non-photochemical advanced oxidation process (AOP). Moderately stable (t1/2 ~ minutes) aqueous O2•– 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•/O2•–]0/[FAC]0 ~ 2, with ~0.8 mol HO• formed/mol FAC consumed, and HO• and RCS exposures reaching ~5×10−10 and ~10−9 M×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 O2•–/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 O2•– generation, and widespread accessibility of FAC and other reagents.