The importance of ionic hydration in the transport of monovalent anions through nanofiltration membranes

To improve the solute-specific selectivity of nanofiltration (NF) membranes, a fundamental understanding of the transport mechanisms in these membranes is required. In this study, we explored the selectivity trends of common anionic pollutants (i.e., nitrate and perchlorate) compared to the common chloride anion and examined the underlying transport mechanisms for these trends in loose polyamide NF membranes. Permeation experiments show that nitrate and perchlorate, despite being polyatomic and larger, permeate the membrane faster than the monoatomic, spherical chloride, suggesting that other mechanisms beyond size and charge exclusion govern the separation. Significantly higher enthalpic barriers measured for the transport of chloride compared to nitrate and perchlorate elucidated the important role of ion dehydration in the selectivity observed. To further support the influence of dehydration, we systematically altered the ions’ hydration by introducing various organic aliphatic alcohols of different hydrophobicity into the feed solution. The inclusion of aliphatic alcohols intensified the chaotropic characteristics of nitrate and perchlorate, augmenting their capacity to dehydrate, as reflected by their enhanced permeation and reduced enthalpic barrier in the presence of alcohols. We also demonstrated that this effect is boosted when a strong kosmotropic anion like sulfate is added to the system due to its counter effect on water structuring. We conclude with proposing mechanisms for the anion behavior in water-alcohol solutions that highlight the critical importance of ionic hydration in transmembrane permeation.