A Three-Dimensional Framework for Rational Ion Separation with Polymeric Membrane Systems

This review treats selective ion separation in polymeric membrane systems as a design problem rather than as a fixed material property. To address the gap between laboratory performance and process-level relevance, a three-dimensional analytical framework that integrates transport mechanism, dynamic structural state, and separation-matrix complexity is proposed. Five research streams are compared: polymer inclusion membranes, ion-exchange membranes and electrodialysis, nanostructured and subnanometric frameworks, composite architectures, and polymer inclusion membrane - electrodialysis (PIM-ED) hybrids. The comparison shows that selectivity is governed not only by the nominal membrane mechanism, but also by hydration-driven structural changes and by the complexity of the operating matrix. Overall, the framework provides a common basis for interpreting selectivity, comparing membrane families, and guiding rational design for ion-separation processes.