journal article

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 Ångstrom 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 Ångstrom 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.
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