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.