Engineering soft nanochannels in isoporous block copolymer nanofiltration membranes for ion separation

Membranes with high ion selectivity are important for the separation of ionic species at the & Aring;ngstrom scale. Here, we report on positively charged isoporous block copolymer (BCP) nanofiltration membranes with designed soft nanochannels. The isoporous BCP membranes were obtained from the combination of BCP self-assembly and non-solvent-induced phase separation. The well-defined soft nanochannels were engineered via a versatile post-modification approach using a combination of di- and monofunctional alkyl halide. The water permeance and effective pore size of the membranes were fine-tuned within the nanofiltration regime. Both single- and binary-salt retentions demonstrate the capability of separating mono-/divalent cations based on the ionic size difference in & Aring;ngstrom scale, charge, and energy to strip away the hydration shells. Especially the real selectivity from the binary-salt systems is > 2 times higher than the selectivity obtained from the single-salt systems, e.g., the real selectivity of K+/Mg2+ is up to 15 for the 10 mM binary-salt mixture with a mole ratio of 1:1. Such enhancement arises from the competition effect regarding energy barrier, size of hydrated ion, and diffusivity. The Na+/Mg2+ binary-salt system with simulating the salt concentration ratio of the seawater illustrates the potential of the resulting membranes to recover the valuable ionic species from water, e.g. Mg, classified as a "critical raw material" in Europe. The Na+/Mg2+ real selectivity is up to 42 with a permeance of 15 L m(-2) h(-1) MPa-1 at a salt concentration of 10 mM.