Desalination Behavior And Performance Of Flow-electrode Capacitive Deionization Under Various Operational Modes

With the improvement of people's living standards and rapid development of economy,people's demand for clean water is increase.The shortage of fresh water resources and water pollution are the two most important environmental problems facing mankind in the 21 st century.This requires people to continue to search for environmentally friendly and cost-effective seawater desalination technologies.Flowing electrode capacitive deionization(FCDI)has attracted more and more attention due to its excellent desalination ability and uniterrupted operation.Researchers have been studying the important factors that affect the desalination performance of FCDI,including electrode materials,electrolyte solutions,etc.,and have continuously improved the design of FCDI devices,and a variety of operation modes of FCDI technology have emerged.According to different operation modes,an FCDI system has a total of 12 combinations of operation modes.However,up to now,there are few researches on it,such as constant voltage/short-circuited closed-cycle/single-pass,constant current/Isolated closed-cycle/single-pass,etc.Moreover,most of the operating modes studied are independent studies conducted under different experimental conditions,which makes it difficult to compare the desalination behavior and desalination performance of FCDI in this case.It is also an important issue to selecting the appropriate FCDI operational mode.In this study,the desalination behavior and performance of FCDI under various operational modes was systematically studied for the first time,and did a particular comparison of interrelated operations,which offer available information for the option of suitable FCDI operational mode.Five operating modes were studied under the condition of constant voltage and constant current by combining the flow mode of electrode slurry and feed water.It is isolated closed-cycle/batch-mode,isolated closed-cycle/single-pass,short-circuited closed-cycle/batch-mode,short-circuited closed-cycle/single-pass,and open cycle/single-pass.The results show that the conductivity of effluent decreases continuously in the batch mode during the adsorption stage,while the conductivity of the effluent in the single-pass mode decreases rapidly and then stabilizes.The single-pass mode is suitable for the application of constant effluent salt concentration.Under the isolated closed-cycle(ICC),short-circuit closed-cycle(SCC)and open cycle(OC)operational modes,the pH value of the flow electrode fluctuated greatly,while the pH value of the feed water remained stable in total running modes.The changes of current and voltage can be explained as the influence of the FCDI resistance change mainly caused by the salt concentration of the influent.From the three capability indicators of average salt removal rate,charge efficiency and removed salt normalized energy consumption,isolated closed-cycle/single-pass and short-circuit closed-cycle/single pass are the two optimal operational modes,followed by open cycle/single-pass,isolated closed-cycle/batch-mode and short-circuit closed-cycle/batch-mode.Further in view of the superiority of SCC mode mixing to neutralization and regeneration of carbon particles charge at the electrode,and the results show that SCC has better energy efficiency than ICC,so it can be considered that the short-circuit closed-cycle/single-pass operational mode is the optimal.