Highly Selective Ion Separations Based on Counter-flow Electromigration in Isoporous Membranes

Highly selective ion separations are vital in producing raw materials for manufacturing lithium-ion batteries, high-flux magnets, catalytic converters, and many other functional products. This dissertation explores an ion-separation mechanism based on counter-flow electromigration in isoporous membranes. In this mechanism, movement of ions due to an electrical potential gradient opposes their advective flux in a solution flowing through membrane pores. For ions with especially high electrical mobilities, electromigration results in a net flux that approaches zero, whereas ions with lower mobilities pass through the membrane to create high selectivity.

Remarkably, in dilute solutions the electrical potential gradient needed for these separations arises spontaneously during flow through pores containing sufficient surface charge. Pressure-driven flow through negatively charged membranes yields a streaming potential that creates a cation electromigration flux that opposes advection and gives high selectivities among cations that have different electrical mobilities. In track-etched polycarbonate membranes with 30 nm pores, pressure-driven flow spontaneously yields Li+/K+ selectivities as high as 70 at around 50% Li+ passage. However, to achieve this selectivity the ionic strength in the feed solution needs to be low (0.2 mM), and significant concentration polarization can dramatically reduce selectivity.

Typical natural aqueous resources have ionic strengths that are orders of magnitude > 0.2 mM, and in such solutions the spontaneous streaming potentials are too weak to effectively separate cations. The use of electrodes to provide the counter-flow electric field, rather than relying on streaming potentials, expands the applicability of counter-flow electromigration to monovalent ion separations in 0.3 M ionic strength solutions and gives Li+/K+ selectivities above 100. However, the energy cost is high due to low current efficiency.

Most membrane-based ion separations exploit differences in the size, charge, dehydration energy, and chemical affinity of various ions. This dissertation demonstrates that opposing flow and electromigration is also a viable approach for separating ions with significantly different mobilities.