While the detrimental effect of concentration polarization (CP) on water flux and solute rejection in pressure-driven membrane processes has been extensively explored, the impact of CP on the selectivity between solutes in these processes has been somewhat overlooked. Considering the growing interest in solute–solute selectivity, in this study, we explored the effect of CP on ion–ion selectivity in nanofiltration (NF) membranes. We first show and discuss the “reversed” observed rejection trend of monovalent cations in NF, which is opposite to the trend of the ions’ hydrated size and mobility in solution. Next, we apply the film theory using three independent approaches to evaluate the extent of CP in the boundary layer adjacent to the membrane surface, from which the real rejection of the ions can be calculated. Our calculated real rejections of monovalent cations, which were in higher correspondence with the ions’ hydration properties and mobility in solution, suggest that CP played a major role in the “reversed” selectivity observed. Last, we demonstrate how CP adversely affects the commonly pursued monovalent–divalent ion separation in NF. Overall, our results highlight the necessity to rigorously account for CP in future studies on NF and suggest minimizing CP as a primary step to improve the selectivity between solutes.

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