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.
Ming Lin,Yihong Liu,Jinzhao Guan,Yunliang Li,Mouhamadou Aziz Diop,Dianbao Feng,Yikun Ren,Yixiao Kang,Shifang Ma,Yujun Li,Fenghui An,Xianwei Hu
The KF-ZrF4 molten salt system has emerged as a promising candidate solvent for fourth-generation reactor designs, where its physicochemical characteristics are fundamentally governed by the ionic structure within the molten fluoride matrix. A variety of complex Zr–F ions, including ZrF5–, ZrF62–, ZrF73–, and ZrF84–, were identified in KF-ZrF4 molten salts with different molar ratios through Raman spectroscopy and quantum chemical calculations. Furthermore, the Zr2F9– ion with a bridging F structure was discovered in KF-ZrF4 molten salts with molar ratios of 1 and 0.65, and the experimental Raman spectrum of this ion was obtained. At higher KF to ZrF4 molar ratios of 5.7 and 2.3, ZrF73– and ZrF84– dominated, while ZrF5– and ZrF84– were predominant at molar ratios of 1 and 1.5. At a lower ratio of 0.67, ZrF5– emerged as the primary species. As the KF to ZrF4 molar ratio decreased, the system favored the formation of less-coordinated Zr–F ions. Moreover, a dynamic mutual transformation equilibrium existed among these complex ions, and this equilibrium was influenced by the zirconium fluoride content in the molten salt. Quantum electrochemical analysis revealed that ZrF5– was mainly involved in the cathode reaction. Zr4+ was first released from ZrF5–, gaining three electrons to form Zr+. Subsequently, Zr+ gained one electron, transforming into metallic zirconium at the cathode interface. These findings deepen our understanding of the physicochemical properties of molten salt reactors and have significant implications for advanced applications in metal purification, spent nuclear fuel recovery, and zirconium alloy production, thereby contributing to improved nuclear materials management and energy sustainability.
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