Self-Regulated Ion Permeation through Extraction Membranes.

Separation of rare earth compounds from water into an organic phase in practical cases requires the use of specific ion binding ligands in high concentrations. These tend to form complex liquid crystalline phases preferentially at ion-rich locations inside a pertraction membrane. They form a blocking layer above an ion concentration threshold, which is experimentally characterized. It is shown to limit the flux through the membrane, which is studied for the application of rare earth recycling, an example being the phase transfer of Nd from water into organic phase. This feedback leads to a stationary membrane permeation rate that can be modeled without any free parameters in very good agreement with experiment. The ion-specific formation and dissolution of the blocking layer, a feature found also in nature, and its control suggest further studies to enhance permeation as well as its selectivity control.