While flow-electrode capacitive deionization (FCDI) is an emerging salted water treatment technology, water hardness is one subset of ionic contaminants that has so far received minimal attention. As a result, the treatment of influent containing both monovalent and divalent cations by using FCDI was carried out in this study with the flow electrodes operated in short-circuited closed-cycle (SCC) configuration. Physical discrepancies between the cations were observed as a selectivity of removal of Ca2+ over Na+, with the selectivity becoming dominant when FCDI operated at lower current densities and hydraulic retention time. Results showed that SCC operation of the FCDI was much more energy-efficient for brackish water softening compared to the isolated closed-cycle (ICC) mode with energy recovery implemented. This finding was largely ascribed to (i) the neutralization of charges on the flow electrodes in the SCC configuration and (ii) regeneration of the active materials to maintain the pseudo "infinite" capacity during electrosorption. In addition, mixing of the flow electrodes in SCC operation significantly inhibited pH excursion in the flow electrode with the formation of calcium precipitates on the carbon surface alleviated. Provided that the electrodes were individually recycled, most of the Ca2+ following migration across the cation exchange membrane would end up on the carbon surface in form of precipitates that might block the active sites and deteriorate the desalting performance of FCDI.

Short-circuited closed-cycle operation of flow-electrode CDI: Energy-saving applications for brackish water softening

256th National Meeting (2018)

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