High-performance thin-film-nanocomposite cation exchange membranes containing hydrophobic zeolitic imidazolate framework for monovalent selectivity

Abstract

Zeolitic imidazolate framework-8 (ZIF-8) offers good hydrothermal, chemical, and thermal stabilities, and is therefore of interest in membrane synthesis. In this work, an interfacial polymerization (IP) method was applied by anchoring ZIF-8 to the skin layer of thin-film nanocomposite (TFN) membranes in order to obtainmonovalent selectivity in electrodialysis. Organic trimesoyl chloride (TMC, 0.1 wt %) solutions and aqueous m-phenyl diamine (MPD, 2% w/v) solutions were used during the interfacial polymerization process. A range of polyamine (PA)/ZIF-8 based membranes was fabricated by varying the concentration of ZIF-8 in the organic solution. The properties of the primary and modifiedmembrane were characterized by scanning electronmicroscope (SEM), energy dispersive X-ray analysis (EDAX), atomic force microscopy (AFM), water uptake, ion exchange capacity, and contact angle measurements. No significant changes of the surface structure of the PA/ZIF-8 based membranes were observed. Nevertheless, the presence of ZIF-8 under the PA layer plays a key role in the separation process. For single salt solutions that were applied in electrodialysis (ED), faster transport of Na+ andMg2+ was obtained after introducing the ZIF-8 nanoparticles, however, the desalination efficiency remained constant. When the hybrid membranes were applied to electrodialysis for binary mixtures containing Na+ as well as Mg2+, it was demonstrated that the monovalent selectivity and Na+ flux were enhanced by a higher ZIF-8 loading.