Organic solvent-free fabrication of thin film polyamide/zeolitic imidazolate framework membranes for removal of dyes from water

Thin film composite (TFC) polyamide (PA) membranes in combination with zeolitic imidazolate frameworks (ZIF), such as ZIF-8 and ZIF-93, have been fabricated using a novel approach based on vapor phase interfacial polymerization (VIP). These membranes have been investigated for nanofiltration (NF) of aqueous solutions of organic dyes, such as Rose Bengal (RB, 1017 Da), Sunset Yellow (SY, 452 Da) and Acridine Orange (AO, 265 Da). The preparation of the PA layer by VIP technique avoids the use of any organic solvent that is typically required in the conventional PA synthesis. Both ZIF-8 and ZIF-93 were initially synthesized using conventional procedures (methanol-based route) and incorporated to the PA layer in two different configurations: i) thin film nanocomposite with ZIF embedded within the PA film, and ii) bilayer membranes with a layer of ZIF between the support and the PA layer. The synthesis of ZIF-93 using a water-based route was developed for the bilayer configuration allowing for an organic solvent-free fabrication of PA/ZIF-93 bilayer VIP-TFC. These membranes showed the highest water permeances of 2.6, 3.4 and 2.2 L m-2h−1 bar−1 for RB, SY and AO, respectively. As compared to the control membrane VIP-TFC, this represents permeance increases of 69–117% and maintains high (greater than 99%) rejection of all the organic dyes. Membrane characterization demonstrated that the permeance enhancement for bilayer PA/ZIF-93 VIP-TFCs is related to an improved hydrophilicity, higher surface roughness and thinner selective layers. The prepared membranes demonstrated excellent cycle stability, maintaining their initial water permeance and dye rejection throughout 34 h of long-term operation, and being able to operate across the entire pH range studied (pH 3 to 9) without any detrimental effects on performance.