Selective transport of trivalent lanthanide in electrodialysis: Limitations due to concentration polarization

Extraction of rare-earth ions from mixtures with other cations is an important part of recovering rare-earth ions from recycling or other streams. This study examines whether highly selective Donnan partitioning into a cation-exchange membrane can lead to effective electrodialysis (ED) extraction of La3+ from solutions containing monovalent or divalent ions. In Nafion membranes, trivalent La3+ sorbs selectively over Na+ and Mg2+, and the La3+/Na+ partitioning selectivity increases to values > 100 with decreasing ionic strength and/or higher ratios of Na+ to La3+ in solution. However, the experimental La3+/Na+ and La3+/Mg2+ ED selectivities are <10. The low La3+ mobility in the membrane accounts for some of the difference between partitioning and transport selectivities. Equally important, concentration polarization due to different ion transport numbers in the solution and the membrane likely leads to significant diffusion of monovalent ions through the membrane. For example, although simulated La3+/Na+ electromigration selectivities reach 35 in dilute solutions, diffusion of Na+ across the membrane reduces the overall ED transport selectivity to <10. This work shows the importance of accounting for concentration polarization when considering applications of ED for ion separations.