Permselective Transport of Cu²⁺ through Polymer Networks Functionalized with Iminodiacetic Acid

Cost-effective technologies for recovering or recycling materials from waste streams are needed to support the growing demand for critical elements and materials. In this work, we investigate ligand-functionalized membranes with ion-specific selectivity toward Cu2+ over Ni2+ and Mg2+. We prepared a series of membranes functionalized with the iminodiacetic acid (IDA) functional group, which binds preferentially to Cu2+ over Ni2+ and Mg2+. The membranes varied in terms of IDA content and water content, and the salt sorption, diffusion, and permeation were quantified over a range of solution pH values, which impact the binding energy of the IDA group to divalent cations. We observed that IDA-functionalized membranes had strong sorption selectivity toward CuCl2 at a pH near 1, and the sorption selectivity increased with increasing IDA content. On the other hand, the membranes exhibited diffusivity-selectivity toward NiCl2 and MgCl2 over CuCl2. However, at a pH near 1, where sorption selectivity was highest, the membranes were permselective toward CuCl2 over NiCl2 and MgCl2. Furthermore, the permselectivity was influenced by the membrane water content, and permselectivity increased with decreasing water content. This reflects a permselectivity–permeability trade-off for ion-specific membranes, which has not been previously reported. This work provides insight into the design of membranes with ion-specific permeabilities and identifies important factors that impact sorption, diffusion, and permeability selectivity, including the ion-specific ligand content, water content, and solution pH. This work also demonstrates ligand-functionalized membranes permselective toward Cu2+ over Ni2+ and Mg2+, with permselectivity as high as 2.8:1. This work can lead to ion-selective separation processes for the recovery of Cu2+ and design strategies for the recovery of ions of interest.