Thin polyimide film with high porosity as an alternative substrate for forward osmosis membranes

Enhancing the substrate properties of forward osmosis (FO) membranes to achieve high water flux can significantly improve the feasibility of FO-based water treatment. This study aimed to evaluate the potential of polyamide (PA) FO membranes fabricated with a thin, highly porous polyimide (PI) film. Among the variable concentrations of m -phenylenediamine (MPD) and trimesoyl chloride (TMC), MPD and TMC at 2.0 and 0.15 wt%, respectively, achieved a water flux of 17.2 L/m 2 h and reverse salt flux of 1.9 g/m 2 h when using a 1.0 M NaCl draw solution. Examination of the PA-selective layer formed on the PI substrate revealed cavities and folds with a thickness of 20–40 nm. Interestingly, increasing the hole diameter of the PI substrate from 80 to 300 nm reduced the water permeate flux from 17.2 to 9.8 L/m 2 h. The structural parameter of the FO membrane developed in this study (193 μm) was comparable to that of most other FO membranes reported in literature. When treating municipal raw sewage, the water flux and reverse salt flux of the developed FO membrane were approximately 60% higher than those of a commercial cellulose triacetate FO membrane. This study highlights the potential and limitations of PI-based FO membranes for wastewater treatment. • Polyamide (PA) FO membranes were fabricated using a polyimide (PI) substrate. • The PI substrate had a thin (25–40 nm) and highly porous (70%) structure. • The PA-selective layer formed on the PI substrate exhibited cavities and folds. • The PI substrate with 80 nm holes exhibited the best water and reverse salt fluxes. • It exhibited a 60% higher water flux than a commercial cellulose triacetate FO membrane.