Permselective transport of Cu²⁺ through polymer networks functionalized with iminodiacetic acid

Cost-effective separation strategies are needed to recover or recycle precious elements and minerals. Ion-selective membranes potentially provide a cost-effective recovery approach, but we lack approaches to design membranes with high ion-specific permselectivity. Recent work has demonstrated that membranes functionalized with ion-specific chelating groups can provide perm-selective transport, but the underlying mechanisms for ion-selective transport and design criteria for enhancing selectivity remain unclear. Here, we report the design and develpment of a series of water-swollen polymer networkss containing iminodiacetic acid functional groups, which bind to divalent cations but bind most strongly to Cu2+. We quantified the sorption, diffusion, and permeation of a series of divalent cation salts through the membranes. We found that the sorption of CuCl₂, NiCl₂, and MgCl₂ increased strongly with increasing membrane IDA content, and that the membranes had strong sorption selectivity towards CuCl­₂ over NiCl₂ and MgCl₂. Conversely, the diffusivity of CuCl₂ was reduced relative to that of NiCl₂ and MgCl₂ due to the interactions with the IDA functional groups, and an analysis of sorption and diffusion over a range of pH showed a trade-off between these two. However, the overall permeation of CuCl₂ was greater than that of NiCl₂ and MgCl₂ in IDA-functionalized membranes, indicating that sorption selectivity was more important than diffusion selectivity in these membranes. Finally, reducing the water content of the membranes reduced permeability of all ions and enhanced the perm-selectivity towards CuCl₂. The permeability and selectivity data collected for this series of membranes reflects a permeability-selectivity trade-off for ion-specific transport. This work provides insight into the transport properties of membranes functionalized with ion-specific chelating groups and demonstrates that this is a promising strategy for achieving ion-specific permselectivity.