D. Jassby, E. Hoek, J. Wang, A. Iddya
University of California, Los Angeles,
United States
Keywords: ion selective membranes, ion exchange, resource recovery
Summary:
Specific ion selectivity is a highly desirable feature for the next generation of membranes. However, existing membranes rely on differences in charge, size, and hydration energy, which limits their ability to target individual ion species. Here, we demonstrate a nanocomposite ion exchange membrane material that enables a reverse-selective transport mechanism that can selectively pass a single ion species. Selective transport is achieved through the blending of inorganic particles with specific interaction properties that enable selective binding and transport of target ions. We demonstrate this transport mechanism with two representative ions: phosphate (anions) and lithium (cations). For phosphorous (phosphate) recovery, we blend amorphous MnO2 particles into a cation exchange membrane (CEM). Phosphate ions can form specific outer-sphere interactions with the nanoparticles, which enables their passage across the negatively charged ion exchange polymer. Other anions, such as chloride, sulfate, and nitrate do not form these interactions and are denied passage. As a result, the membrane’s phosphate flux increased by a factor of 27 over an unmodified CEM, and the selectivity of phosphorous over sulfate, nitrate, and chloride reaches 47, 100 and 20, respectively. For lithium separation from brines, we blended specific particles into an anion exchange membrane (AEM). The unique crystal structure of the particles allows for specific interactions of lithium ions with these particles, which form a bridge across the AEM. These membranes demonstrate a Li/Mg and Li/Na selectivity of 10,000 and 250, respectively, with high lithium ion fluxes (0.5 M Li/m2/hr). These two examples of ion selective membranes serve as a template for other ion separation materials that can be formed through the tailored pairing of specific particles and polymers.