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Capacitive deionization (CDI) and membrane capacitive deionization (MCDI) are the most common cell architectures in the use of CDI for water treatment. In this work, the Faradaic reactions occurring in batch-mode CDI and MCDI processes were compared by investigating the variation of H₂O₂ and dissolved oxygen (DO) concentrations, pH, conductivity and current during charging and discharging under different charging voltages. During charging, the H₂O₂ concentration in CDI increased rapidly and then decreased while almost no H₂O₂ was generated in MCDI due to the inability of oxygen to penetrate the ion exchange membrane. Chemical kinetic models were developed to quantitatively describe the variation of H₂O₂ concentration and found to present satisfactory descriptions of the experimental data. The pH drop during charging could be partially explained by Faradaic reactions with proton generation …