In this study, a flow-through ozone-electrolysis (O 3 -electrolysis) process was developed by combining ozonation with an electrolysis using a porous reactive electrochemical membrane (REM) cathode. Due to the convection-enhanced mass transport and fast radial diffusion inside the small pores of REM cathodes, the rate of cathodic O 3 reduction to ozonide radicals (O 3 •− ) was significantly enhanced, while the further cathodic O 3 •− reduction to oxygen was inhibited during the flow-through O 3 -electrolysis process compared to the conventional mixed-tank O 3 -electrolysis process. Consequently, more hydroxyl radicals (•OH) were formed from O 3 •− decay in water during the flow-through O 3 -electrolysis process than the mixed-tank O 3 -electrolysis process. Corresponding to the higher •OH yields from cathodic O 3 reduction, the flow-through O 3 -electrolysis process substantially enhanced the abatement kinetics and efficiency of para -benzoic acid ( p CBA, a model compound of ozone-resistant micropollutant) in a groundwater than conventional ozonation and the mixed-tank O 3 -electrolysis process. These results suggest that the flow-through O 3 -electrolysis process may provide a competitive treatment technology for micropollutant abatement in water treatment .
Previous studies indicate that effective generation of hydrogen peroxide (H 2 O 2 ) 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 , H 2 O 2 , and H 2 O 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, H 2 O and H 2 O 2 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 H 2 O 2 and ensuing reaction of H 2 O 2 with O 3 to OH) from occurring. In contrast, when cathodic O 3 reduction was limited by O 3 mass transfer to the cathode, cathodic O 2 reduction to H 2 O 2 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 H 2 O 2 in water treatment, reaction conditions should be carefully controlled to promote cathodic O 2 reduction during the E-peroxone process.
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