Selective and continuous ion recovery using flow electrode capacitive deionization with polymer multilayers functionalized ion exchange membrane

Flow electrode capacitive deionization (FCDI) is a newly developed salt removal technology that overcomes the limitations of conventional CDI by eliminating the need for a discharging process, thus achieving much higher desalination capacity and a continuous desalination operation. In this study, we aimed to achieve selective and continuous sodium recovery from Na+/Ca2+ mixtures through FCDI by utilizing polymer multilayer functionalized cation-exchange membranes (CEM). A layer-by-layer method was used to deposit polymer multilayers consisting of PAH (poly allylamine hydrochloride) and PSS (poly styrene sulfonate) on the CEM. Changes in the concentration of feed electrolyte, number of coated polymer layers, and type of outermost layer were investigated in terms of their effect on the selective ion separation. The functionalized CEM switches from a divalent ion affinity to a monovalent ion affinity due to their different ion sizes and charge densities. As the number of polymer layers increased, the selectivity of Na+/Ca2+ increased, reaching up to 3.26 from 0.16 for pristine CEM. We believe that our approach can provide insight into the selective recovery of valuable materials including lithium-ion battery recycling, the recovery of noble metals, and the separation of toxic ions using the FCDI system. [Display omitted] •Selective ion recovery was realized by FCDI using a functionalized membrane.•PAH/PSS polymer multilayers were successfully deposited onto an ion exchange membrane.•The multilayer coating changed the affinity of the FCDI system from divalent to monovalent.•Na/Ca selectivity increased from 0.16 to 3.26 as a result of multilayer coating.