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

Abstract

Previous studies indicate that effective generation of hydrogen peroxide (H2O2) from cathodic oxygen (O2) 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., O3, H2O2, and H2O reductions) may occur in competition with O2 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 O2/O3 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 O2/O3 sparging, H2O and H2O2 reductions were usually negligible cathodic reactions. However, O3 can be preferentially reduced at much more positive potentials (ca. 0.9 V vs. SCE) than O2 (ca. −0.1 V vs. SCE) at the carbon cathode. Therefore, cathodic O2 reduction was inhibited when the process was operated under current limited conditions for cathodic O3 reduction. The inhibition of O2 reduction prevented the desired E-peroxone process (cathodic O2 reduction to H2O2 and ensuing reaction of H2O2 with O3 to OH) from occurring. In contrast, when cathodic O3 reduction was limited by O3 mass transfer to the cathode, cathodic O2 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 O3) 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 O2 reduction during the E-peroxone process.