Desalination behavior and performance of flow-electrode capacitive deionization under various operational modes

Flow-electrode capacitive deionization (FCDI) has attracted growing attention due to its superior desalination capacity and continuous operation. This study, for the first time, systematically investigated the desalination behaviors of FCDI under a variety of operational modes and made a detailed comparison of relevant operation to provide useful information for selecting appropriate FCDI operational mode. Five operational modes with respect to the flow of electrodes and feed water were studied with both constant voltage and constant current applied. Results revealed that the effluent conductivity during charging decreased continuously in batch mode while decreased quickly and then leveled off in single-pass mode. pH of flow-electrodes fluctuated differently for the operational modes of isolated closed-cycle (ICC), short-circuited closed-cycle (SCC) and open cycle (OC), while pH of the effluent stream maintained stable for all operational modes. The variation of current or voltage could be explained by the change in the resistance of FCDI mainly induced by the middle-chamber salt concentration. Based on the three performance indicators of average salt removal rate, charge efficiency and removed salt normalized energy consumption, ICC/single-pass and SCC/single-pass are the two most superior operational modes, followed by OC/single-pass, ICC/batch-mode and SCC/batch-mode. Further considering the advantage of SCC in the continuous charge neutralization and electrode regeneration, SCC/single-pass operational mode can be deemed optimal.