The competition between cathodic oxygen and ozone reduction and its role in dictating the reaction mechanisms of an electro-peroxone process

Previous studies indicate that effective generation of hydrogen peroxide (H2O2) from cathodic oxygen (O-2) reduction is critical for the improved water treatment performance (e.g., enhanced pollutant degradation and reduced bromate formation) during the electro-peroxone (E-peroxone) process (a combined process of electrolysis and ozonation). However, undesired reactions (e.g., O-3, H2O2, and H2O reductions) may occur in competition with O-2 reduction at the cathode. To get a better understanding of how these side reactions would affect the process, this study investigated the cathodic reaction mechanisms during electrolysis with O-2/O-3 gas mixture sparging using various electrochemical techniques (e.g., linear sweep voltammetry and stepped-current chronopotentiometry). Results show that when a carbon brush cathode was used during electrolysis with O-2/O-3 sparging, H2O and H2O2 reductions were usually negligible cathodic reactions. However, O-3 can be preferentially reduced at much more positive potentials (ca. 0.9 V vs. SCE) than O-2 (ca. -0.1 V vs. SCE) at the carbon cathode. Therefore, cathodic O-2 reduction was inhibited when the process was operated under current limited conditions for cathodic O-3 reduction. The inhibition of O-2 reduction prevented the desired E-peroxone process (cathodic O-2 reduction to H2O2 and ensuing reaction of H2O2 with O-3 to (OH)-O-center dot) from occurring. In contrast, when cathodic O-3 reduction was limited by O-3 mass transfer to the cathode, cathodic O-2 reduction to H2O2 could occur, thus enabling the E-peroxone process to enhance pollutant degradation and mineralization. Many process and water parameters (applied current, ozone dose, and reactivity of water constituents with O-3) can cause fundamental changes in the cathodic reaction mechanisms, thus profoundly influencing water treatment performance during the E-peroxone process. To exploit the benefits of H2O2 in water treatment, reaction conditions should be carefully controlled to promote cathodic O-2 reduction during the E-peroxone process. (C) 2017 Elsevier Ltd. All rights reserved.