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

Abstract

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.