Synthesis and performance evaluation of zeolitic imidazolate framework-8 membranes deposited onto alumina hollow fiber for desalination

This work describes the development of zeolitic imidazolate framework-8 (ZIF-8) membranes on modified alumina hollow fiber for desalination by forward osmosis. Effects of different seeds (ZnO, NiO and PDA) and sodium formate on in-situ deposition of ZIF-8 were studied in relation to the membrane’s morphology and performance. XRD result shows that ZIF-8 was successfully synthesized in the presence of sodium formate. FESEM images showed PDA modified support was unsuccessful in producing well defined and dense ZIF-8 membrane layer even after another ZIF-8 re-deposition due to its minimal amount. The NiO modified support was also found unsuccessful, as ZIF-8 crystals were formed in clusters. On the contrary, dense ZIF-8 membrane was successfully prepared on ZnO modified support with SF-1 synthesis solution producing bigger ZIF-8 crystal and thinner ZIF-8 membrane than as of SF-2. Water flux performance in forward osmosis showed that NiO/ZIF-8, PDA/ZIF-8 and PDA/ZIF-8 (re-deposition) membranes gave negative water fluxes of -50 kg/m2·h, -5.2 kg/m2·h and -1.7 kg/m2·h with reverse solutes of 42.66 mol/m2·h, 27.42mol/m2·h and 3.22 mol/m2·h, respectively, indicating the solute from draw solution diffused into the feed solution. However, ZIF-8 membrane prepared using SF with molar ratio of 1, on the ZnO modified support had a water flux of 13.3 kg/m2·h, reverse solute of 0.95 kg/m2·h and salt rejection of 52.1%. When the SF ratio was increased to 2, the ZIF-8 membranes showed a water flux of 12.5 kg/m2·h, reverse solute of 1.64 kg/m2·h and salt rejection of 54.9%. The moderate salt rejection could be associated with defects in the ZIF-8 membranes due to poor grain boundaries.