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Previous investigations on membrane capacitive deionization (MCDI) cells focused on electrode and membrane materials; while little attention has been paid to the separator compartment. This study demonstrated that the MCDI's desalination rate can be enhanced by inserting activate carbon fibers (ACF) in the cell's separator compartment. For comparison, stainless steel fibers (SSF) and filter paper (FP) were also examined as the separator. The highest desalination rate [513.4 mg/(L·h)] was obtained when ACF was inserted, followed by SSF [374.1 mg/(L·h)] and FP [297.9 mg/(L·h)]. The pseudo-first-order kinetic model provided a good approximation ( R 2 > 0.99) to the measured adsorption data, and the model fitting results revealed that the MCDI with the ACF separator delivered not only the highest desalination rate but also the largest adsorption capacity. Electrochemical impedance spectrometry (EIS) analysis revealed that the ACF separator considerably reduced the MCDI's internal resistance and accordingly increased its desalination rate. Our findings highlight the need for revisiting and modifying the current designs of the MCDI's separator compartment. [ABSTRACT FROM AUTHOR]

Abstract: A capacitive deionization (CDI) cell was built with electrodes made of an inexpensive commercial activated carbon fiber (ACF), and then modified by incorporating ion-exchangers into the cell compartment. Three modified CDI designs were tested: MCDI – a CDI with electrodes covered by ion-exchange membranes (IEMs) of the same polarity, FCDI – a CDI with electrodes covered by ion-exchange felts (IEFs), and R-MCDI – an MCDI with cell chamber packed with ion-exchange resin (IER) granules. The cell was operated in the batch reactor mode with an initial salt concentration of 1000 mg/L NaCl, a typical level of domestic wastewater. The desalination tests involved investigations of two consecutive operation stages of CDIs: electrical adsorption (at an applied voltage of 1.2 V) and desorption [including short circuit (SC) desorption and discharge (DC) desorption]. The R-MCDI showed the highest electric adsorption as measured in the present study by desalination rate [670 ± 20 mg/(L h)] and salt removal efficiency (90 ± 1%) at 60 min, followed by the MCDI [440 ± 15 mg/(L h) and 60 ± 2%, respectively]. The superior desalination performance of the R-MCDI over other designs was also affirmed by its highest charge efficiency (110 ± 7%) and fastest desorption rates at both the SC [1960 ± 15 mg/(L·h)] and DC [3000 ± 20 mg/(L·h)] modes. The desalination rate and salt removal efficiency of the R-MCDI increased from ∼270 mg/(L h) and 83% to ∼650 mg/(L h) and 98% respectively when the applied voltage increased from 0.6 V to 1.4 V, while decreased slightly when lowering the salt water flow rate that fed into the cell. The packing of IER granules in the R-MCDI provided additional surface area for ions transfer; meanwhile, according to the results of electrochemical impedance spectroscopy (EIS) analysis, it substantially lower down the R-MCDI''s ohmic resistance, resulting in improved desalination performance. [Copyright &y& Elsevier]