Enhanced monovalent over divalent cation selectivity with polyelectrolyte multilayers in membrane capacitive deionization via optimization of operational conditions
Sevil Sahin, Han Zuilhof, Rafael L. Zornitta, Louis C. P. M. de Smet
Abstract
In this work, we tuned the ion selectivity of a polyelectrolyte multilayer (PEM)-coated, cation-exchange membrane (CMX) in a membrane capacitive deionization (MCDI) process by carefully studying different operational modes, namely constant voltage (CV) and constant current (CC). The monovalent cation selectivity and its time-dependent behavior were monitored at different voltage and current values. Upon optimizing the current density (10 A/m2) and the number of polyelectrolyte bilayers (5.5) on the CMX membranes, a time-independent and nearly full monovalent cation selectivity was obtained for various feed solutions, provided a polycation-terminated PEM was applied. Furthermore, the selectivity values of several commercially available cation-exchange membranes were tested under the optimized conditions and compared with CMX and PEM-CMX, yielding the best performance for PEM-CMX, regardless the composition of feed solution. Before this optimization, this MCDI system showed a time-dependent selectivity, with a maximum of ρMgNa ≈3. The results were rationalized by applying an MCDI model based on the dynamic potential profile, describing the potential drops across the membrane and demonstrating a threshold for the current density.