Catalytic electrochemical processes for chloride oxidation, ammonia removal, and nitrate reduction in water
Date
2023
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Publisher
University of Delaware
Abstract
Lack of access to clean and safe water remains a major challenge for humanity, but conventional water treatment methods consume large amounts of chemicals and energy, result in secondary pollution and could not achieve high efficiency at small scales. Electrochemical water purification has emerged as one of the most promising technologies for water treatment. It is highly effective in the removal of contaminants, has low energy consumption and does not require the addition of chemicals. In addition, the electrochemical system can be operated under ambient conditions with a high degree of automation and reduce the safety issues related to chemical storage and transportation. This study focuses on electrochemical chloride oxidation using multimetallic electrodes, ammonia removal in the presence of chloride and nitrate reduction using bimetallic electrodes. ☐ The electrodeless deposition method was used for fabricating Pt-Ni-Co-G electrode. Results showed that the molar ratio of Ni- and Co-oxides on graphite surface influenced surface structure and in-situ chlorine generation capacity. The best performing electrode, Pt0.01Ni0.24Co0.75-G, had a highly porous 3D structure, high oxygen deficit, lower onset chlorine evolution potential than oxygen, and smaller transfer coefficient (α). The Pt0.01Ni0.24Co0.75-G electrode also exhibited a high disinfection capability, which achieved 7.5-log disinfection of E. coli in 15 min in the presence of 0.01 M NaCl under 5 mA/cm2 current density at room temperature. The lipid peroxide, morphology changes and destruction of E. coli cell membrane were detected in the in-situ disinfection process, indicating that reactive species such as chlorine radical in addition to free chlorine played the main role of in-situ disinfection. ☐ Platinum, nickel, and cobalt oxides were coated on different electrode supports (graphite, stainless steel mesh, Ti foil and Ni form) by multistep electrode deposition method. Results showed that the deposition process, such as applied current and deposition time, and the substrates influenced electrode morphology and electrochemical chlorine generation capability. The graphite substrate electrode with 0.6A×20s×30 deposition condition was more favorable to chloride oxidation than other electrodes, which have the lowest chlorine evolution potential, the largest difference between water oxidation and chloride oxidation potential, and the smallest transfer coefficient (α). Moreover, a mechanism of chlorine generation has been proposed in this study, indicating that chlorine radicals were formed on electrode surface. ☐ Indirect ammonia oxidation in the presence of chlorine on electrode 0.6A×20s×30-G (Pt0.01Ni0.24Co0.75) was studied. The in-situ generation of reactive chlorine species enhanced ammonia removal efficiency. Compared to 40% ammonia removal efficiency at 4h in the absence of chloride, the ammonia removal efficiency reached up to 70% at 4 h with 10 mM initial chloride, and around 90% of the ammonia removal was achieved at 2 h with 30 mM initial chloride. The applied current density played the main role in the formation of nitrogen, and the nitrogen conversion rate increased from around 13% to around 58% as the current density increased from 3 to 15 mA/cm2 at 10 mM initial chloride concentration. ☐ The effects of the PdSn electrode fabrication process on the nitrate reaction were investigated, including calcination temperature, electrode support and quenching process. Results indicated that electrode fabrication process played a role in electrode surface morphology, chemical composition, and crystalline formation. High calcination temperature resulted in the oxidation of electrodes and decreased nitrate reduction capability and nitrogen selectivity. The order of nitrate reduction rate on different electrode supports is SS≈Ni>G>Ti. Quenched electrodes in ice water led to a decrease in nitrate reduction capability. The porous surface, less oxidation of electrode (higher proportion of Pd0 and Sn0), and Sn crystals were beneficial for nitrate removal and nitrogen selectivity. ☐ Overall, this dissertation provides an opportunity to design low-cost, high-electrochemical catalytic materials for water treatment processes and demonstrates the applicability of electrochemical water purification to various contaminants.
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Keywords
Ammonia removal, Catalytic materials, Chloride oxidation, Electrochemical Processes, Nitrate reduction, Water treatment