Ion-exchange membrane capacitive deionization: A new strategy for brackish water desalination

Membrane capacitive deionization (MCDI) integrates the advantages of capacitive deionization (CDI) and ion-exchange membrane technology and has shown great potential to improve the desalting efficiency. MCDI works based on the same working principle of CDI. In addition, ion-exchange membranes are introduced in front of the electrodes so that the charged ions can be selectively passed through the membrane layer and are subsequently adsorbed by the oppositely charged electrodes without interference of co-ions and therefore improve the salt removal efficiency as well as strengthen the regeneration. In this research, electrodes made from single walled carbon nanotubes (SWCNTs) were used together with cation- and anion-exchange membranes. The correlation between solution concentration and conductivity was calibrated prior to the experiment. Through bench scale batch mode desalination experiments, a salt removal efficiency of as high as 97% was achieved with initial conductivity of 110 μs/cm and electrical voltage of 1.2 V. This efficiency is much higher than the corresponding CDI without membrane whose salt removal efficiency is only about 60%. Further, the obtained adsorption rate constant as a result of adsorption kinetics clearly demonstrated that the ion-exchange membrane can help to achieve a faster ion transfer rate in the electrosorption process due to low co-ions expulsion effect.

Research Highlights
► Ion-exchange membranes are introduced in front of the electrodes in capacitive deionization process as MCDI.
► Electrodes made from single walled carbon nanotubes (SWCNTs) are used together with cation- and anion-exchange membranes.
► Charged ions can be selectively passed through the membrane and adsorbed without interference of co-ions.
► Much higher desalting efficiency is achieved by MCDI comparing to the process without membrane.
► The ion-exchange membrane can help to achieve faster ion transfer rate in the electrosorption process due to low co-ions expulsion effect.